FIELD OF THE INVENTION

The present invention relates silica aerogels, particularly silica aerogels comprising one or more channels for use as liquid core waveguides in opto-microfluidic devices.

BACKGROUND OF THE INVENTION

Liquid core waveguides (LOW) that are capable of pairing fluid and light within channels are highly desirable for multiple applications such as an opto-microfluidic module to measure/detect bio-chemicals in fluids. Notably, a large refractive index contrast between the fluid and the LOW provides improved optical performance due to an increased numerical aperture of the waveguide.

The term Liquid Core Waveguide is used interchangeably with the term Liquid Capillary Waveguide throughout. A capillary can provide an optical cavity in a fluidic environment and the optofluidic architectures allow for light penetration and interaction with the fluid in the capillary.

Typical materials used for the manufacture of liquid core waveguides are quartz and Teflon AF (also commonly referred as PTFE (Polytetrafluoroethylene)).

The major drawback of these materials is their relatively high refractive index (Rl), e.g. around 1.3 for Teflon AF, which results in low Rl contrast with the fluid core, usually an aqueous based fluid with a Rl around 1.33, and thus reduces optical guidance efficiency (See for instance A. Datta et al., “Microfabrication and characterization of Teflon AF-coated liquid core waveguide channels in silicon,” IEEE Sens. J., vol. 3, no. 6, pp. 788-795, 2003.)

Recent effort has been made to develop new material with a lower Rl. Among attractive materials, aerogel has received increasing attention due to its low Rl, below 1.2, typically between 1.01 to 1.1. Its use within optofluidic systems as a coating material has been described in several documents. For instance W02000010044A1 (also as EP1119786B1) describes a light guide comprising a flexible elongated tube with an aerogel film affixed; i.e. coated; to the inner surface of the tube.

Similarly, DE102019131698B4 describes a system and a method for spectroscopic analysis of liquids comprising a conduit device cladded with an aerogel for conducting light and for guiding a liquid. In order to prevent the channel from clogging, its diameter should not be less than 0.5 mm. In these systems, the thickness of the cladding is of high importance and should be controlled. WO2019075274A1 discloses an aerogel where the gas permeability is modified using a removable organic scaffold, typically a nitrocellulose scaffold. However, in the process described in this document, two solvents are required: a first solvent in the preparation of the aerogel, and a second solvent to dissolve the organic scaffold.

More recently, several attempts were made to provide an aerogel comprising microchannels that could be used as a LCW. However, the channel formation remains challenging.

Xiao et al tried to form channels through removing optical fibers from pre-formed aerogel samples. Briefly the reactants were poured into a mould in which an optical fiber of desired diameter was previously placed and after the final step of the aerogel formation, i.e. drying, the fiber was removed. While channels were described as smooth, resulting in low optical loss (1.5 dB/cm) and diameter controlled at around 125 pm, the adhesion of the fibers caused frequent problems of fissures within the aerogel, inducing a low reproduction rate. In light of such fissures, the resultant channels may not be considered to have a smooth surface and have acceptable properties for use in devices such as waveguides and opto-microfluidic devices. Furthermore, this technique only allows for limited channel shaping and sizing capabilities limiting its industrial application (L. Xiao and T. A. Birks, “Optofluidic microchannels in aerogel,” Opt. Lett., vol. 36, no. 16, p. 3275, 2011).

Yalizay et al then used femtosecond-laser ablation to generate channels. However, this process generated non-uniform channels of around 500 pm diameters with rough surfaces. Indeed, the ablation resulted in the formation of silica particles that needed to be removed through the simultaneous use of high-pressure nitrogen flushing (around 4 atm). The presence of residual particles might explain the increased optical attenuation observed (10 dB/cm' ¹ ). Furthermore, due to the need of nitrogen flushing, the channel length is limited (B. Yalizay et al., “Versatile liquid-core optofluidic waveguides fabricated in hydrophobic silica aerogels by femtosecond-laser ablation,” Opt. Mater. (Amst)., vol. 47, pp. 478-483, 2015).

Manually drilled channel formation was further assessed, as described by Ozkabir et al. While optical attenuation was reduced, surface quality was also compromised during the drilling process and low reproductive success of sample formation limits this approach (Y. Ozbakir, A. Jonas, A. Kiraz, and C. Erkey, “Aerogels for Optofluidic Waveguides,” Micromachines, vol. 8, no. 4, 2017).

Eris et al used trifluoropropyl Polyhedral Oligomeric Silsesquioxane (T-POSS) polymer to form, after dissolution in supercritical carbon dioxide, a singular U-shaped channel within an aerogel sample. However, the T-POSS material is highly challenging to fabricate, involving freeze-melt formation and bending around the end of a spoon before cooling and inserting into the gel. Also, the complexity of the process, the potential lack of smooth surface induced by the quality of the mould and the limited formation possibilities remained problematic for this approach. (G. Eris et al., “Three-dimensional optofluidic waveguides in hydrophobic silica aerogels via supercritical fluid processing,” J. Supercrit. Fluids, vol. 73, pp. 28-33, 2013). Thus the aerogel channel formation techniques developed to date are summarized in Table 1 below:

Therefore, there remains a need to provide products suitable for forming LCWs with low refractive index, low optical loss and smooth channels providing optimal light and fluid interactions.

It is the aim of the invention to provide such a product as well a process of manufacturing it, wherein the process is typically of lower cost, highly reproducible and allows the formation of channels with different shapes and/or diameters.

It is also a goal of the invention to provide a method to sinter the material while maintaining the channel integrity/smoothness, to realise more robust modules for opto and microfluidic applications.

STATEMENT OF INVENTION

An aspect of the invention provides a silica aerogel product comprising one or more channels obtainable by a process comprising: i) preparing a sol from a silica precursor, ethanol, water and an acid and/or base catalyst; wherein the silica precursor comprises tetramethyl orthosilicate or tetraethyl orthosilicate; and wherein the molar ratio of silica precursor to water is from about 1 :2 to about 1 :5; and the molar ratio of silica precursor to ethanol is from about 1 :0.1 to about 1 :25; and ii) agitating the sol; iii) a) inserting one or more polymer inserts into the sol, wherein the polymer is suitable for complete dissolution in ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa but is not suitable for dissolution in liquid ethanol at room temperature and atmospheric pressure; b) allowing the sol to set to form a silica alcogel in a first sealed container; iv) contacting the silica alcogel containing the one or more polymer inserts formed in step (iii) with further ethanol in a second sealed container; v) contacting the product of step iv) with ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa, preferably about 280 °C and about 10 MPa; vi) reducing the pressure to allow release of ethanol from pores in the silica alcogel to form the silica aerogel, optionally to atmospheric pressure; and vii) optionally reducing the temperature to room temperature.

In a further aspect, the invention provides a process for preparing a silica aerogel product comprising one or more channels, the process comprising: i) preparing a sol from a silica precursor, ethanol, water and an acid and/or base catalyst; wherein the silica precursor comprises tetramethyl orthosilicate or tetraethyl orthosilicate; and wherein the molar ratio of silica precursor to water of from about 1 :2 to about 1 :5; and the molar ratio of silica precursor to ethanol is from about 1 :0.1 to about 1 :25; and ii) agitating the sol; iii) a) inserting one or more polymer inserts into the sol, wherein the polymer is suitable for complete dissolution in ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa but is not suitable for dissolution in liquid ethanol at room temperature and atmospheric pressure; b) allowing the sol to set to form a silica alcogel in a first sealed container; iv) contacting the silica alcogel containing the one or more polymer inserts formed in step (iii) with further ethanol in a second sealed container; v) contacting the product of step iv) with ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa, preferably about 280 °C and about 10 MPa; vi) reducing the pressure to allow release of ethanol from pores in the silica alcogel to form the silica aerogel, optionally to atmospheric pressure; and vii) optionally reducing the temperature to room temperature.

In an additional aspect, the invention provides a silica aerogel comprising one or more channels, each channel having a surface roughness of from about 0.1 to about 10 pm, an internal diameter in the range of from about 100 pm to about 2900 pm and a length of from about 5mm to about 200 mm.

The present invention enables the dissolution of the polymer insert within the aerogel material in step v) using the supercritical ethanol, i.e. the ethanol that has reached its supercritical point, leading to the formation of smooth, customisable channels that can therefore act as waveguides. The supercritical drying process acts to dissolve the polymer insert while also removing the supercritical phase ethanol following exchange for gas (air) in the aerogel pores. The process produces monolithic and substantially fissure free silica aerogels with customisable channels integrated within the material. This can enable a reliable, low-cost, and adaptable method for the preparation of optofluidic modules. Furthermore, the highly nano- porous nature of the material means that these modules enable diffusion of gaseous bubbles, resolving a frequent problem in microfluidic detection. The high contrast of refractive index between the fluid core and silica aerogel channel makes these materials ideal candidates for liquid core waveguiding (LCW) modules.

In particularly preferred embodiments of the above aspects of the invention the polymer insert is poly(ethylene terephthalate), poly(ethylene terephthalate) glycol or poly(lactic acid).

Typically, step i) comprises preparing a first solution of the silica precursor in ethanol; preparing a second solution comprising the acid and/or base catalyst, water and ethanol; and mixing the first and second solutions, optionally mixing equal volumes of the first and second solutions. Preferably, step i) comprises gradually mixing the first and second solutions, optionally for a period of about 20 to about 60 seconds, optionally about 30 seconds.

In one embodiment, the catalyst is a base.

In a preferred embodiment, the molar ratio of silica precursor to base is from about 1 :0.0001 to about 1 :0.1 , preferably 1 :0.01.

In one preferred embodiment, the silica precursor is tetramethyl orthosilicate.

In another preferred embodiment, the silica precursor is tetraethyl orthosilicate

In one embodiment the silica precursor is tetraethyl orthosilicate, and the catalyst comprises a base and an acid, optionally wherein the base is added to the second solution prior to mixing and the acid is added to the mixture of the first and second solutions. Typically step ii) comprises agitating the sol for a period of from about 10 to about 14 mins, optionally about 12 mins, optionally at about 200 to 300 rpm, optionally at about 250 rpm.

Typically in step iii) the sol sets for a period of from about 4 to about 10 hours.

In an embodiment of the invention, in step iii) and/or step iv) the sealed container is a sealed mould.

In an embodiment of the invention the first sealed container is formed of a polymer suitable for complete dissolution in ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa but is not suitable for dissolution in liquid ethanol at room temperature and atmospheric pressure, and wherein the first sealed container dissolves in step v).

In one embodiment, the first sealed container and the second sealed container are the same container.

Typically step iv) is for a period of from about 24 to about 72 hours.

In one embodiment the ethanol is renewed during step iv).

Preferably, step v) involves a gradual increase of temperature and pressure from room temperature and atmospheric pressure to a final temperature of from about 240 °C to about 320 °C and a final relative pressure of from about 6.3 MPa to about 12 MPa over a period of about 2 to about 5 hours, optionally about 4 hours, optionally the temperature is raised about 75 °C per hour to the final temperature.

In a preferred embodiment, in step i) the molar ratio of silica precursor to water is about 1 :4.

Typically the aerogel has a bulk density of from about 0.003 to about 0.5 g/cm ³ .

Typically the aerogel has a refractive index below 1.2, preferably from about 1.01 to about 1.10.

Typically the silica aerogel has a mean pore diameter of from about 20 nm to about 190 nm, optionally wherein the mean pore diameter of the silica aerogel is from about 20 to about 180nm when the silica precursor is tetramethyl orthosilicate or from about 60 to about 190nm when the silica precursor is tetraethyl orthosilicate.

Typically the aerogel has a porosity of from about 50% to about 99.8%, preferably from about 95% to about 99.8%.

Preferably the silica aerogel is substantially free of detectable fissures. In one embodiment the catalyst comprises or is a base catalyst, preferably ammonium hydroxide, sodium hydroxide, sodium carbonate, or potassium carbonate.

In one embodiment the catalyst comprises or is an acid catalyst, preferably ammonium fluoride, oxalic acid or nitric acid.

In one embodiment the silica precursor further comprises one or more of methyltrimethoxysilane, methyltriethoxysilane, dimethylchlorosilane, trimethylethoxysilane, ethyltriethoxysilane or phenyltriethoxysilane.

Typically the silica precursor is present in the mixture of step i) in an amount of from about 20 to about 80 wt%.

In one embodiment the base is present in the mixture of step i) in an amount of from about 0.003 to about 0.2 wt%.

Typically the ethanol is present in the mixture of step i) in an amount of from about 1 to 80 wt%.

Typically the water is present in the mixture of step i) in an amount of from about 5 to about 40 wt%.

In one embodiment the process further comprises a step of modifying the hydrophobicity of the aerogel by contacting the aerogel with hexamethyldisilazane. Typically this comprises vapour deposition of hexamethyldisilazane after step vii). Alternatively the ethanol used in step iv) comprises about 15 wt % to about 30 wt %, optionally about 20 wt % hexamethyldisilazane.

In one embodiment step v) occurs for a period of from about 6 to about 8 hours. Typically each of steps v) to vii) occur in an autoclave.

In one embodiment step vi) occurs for a period of from about 3 to about 4 hours.

In one embodiment in step vi) the temperature is maintained in the range of from about 240 °C to about 320 °C and the pressure gradually reduces from a relative pressure of from about 6.3 MPa to about 12 MPa to atmospheric pressure.

Typically the ethanol is 99% ethanol.

In preferred embodiments the insert is prepared by fused deposition modelling 3D printing or is formed from one or more polymer filaments.

If the one or more polymer inserts are prepared using 3D printing, the channel design can be obtained with a high level of reproducibility. 3D printing is suitable for scale up, and can be used to prepare products on an industrial scale. In addition, 3D printing can be used to prepare polymer inserts having complex geometries of any shape or size, leading to channels with correspondingly complex geometries.

In one embodiment a polymer filament has an outer diameter of from about 100 pm to about 2850 pm. In one embodiment a polymer filament is an extruded polymer filament having an outer diameter of from about 100 pm to about 1000 pm. In one embodiment a polymer filament is heated for a period of about 2 mins to straighten the filament prior to inserting the filament into the sol, optionally in an oven at about 70 to 90 °C, preferably 80 °C.

In one embodiment, the one or more polymer inserts of step iii) a) are inserted into the surface of the sol and wherein the process comprises a further bonding step viii) comprising activating the said surface of the product obtained after step vii) and a surface of a second silica aerogel and contacting the surface of the product obtained after step vii) with the surface of the second aerogel thereby obtaining the silica aerogel product.

Typically, the activation is performed by heating at a temperature of from about 900 to about 1100 °C, optionally wherein the heating is for at least 30 seconds, optionally wherein the heating is in a Rapid Thermal Annealing (RTA) furnace; or wherein the activation is performed using a radiofrequency plasma, optionally comprising low pressure Argon gas at a pressure of from about 1 to about 100 millitor, optionally for about 1 to about 5 minutes.

In one embodiment the process further comprises a sintering step ix) wherein the product of step vi), vii) or viii) is heated to a temperature of from about 900 to 1100 °C, optionally about 1000 °C for a period of from about 1 to about 6 hours, optionally about 2 hours. Typically the sintering step causes a 50% to 90% increase in bulk density.

In one embodiment step ix) converts the silica aerogel into a product which consists essentially of silica.

In an embodiment of the invention the internal diameter of the one or more channels can be controlled by altering the time period of step ix).

In one embodiment the one or more channels each have a hydraulic diameter in the range of from about 100 pm to about 2850 pm, preferably from about 200 pm to about 600 pm.

Typically the one or more channels each have a length in the range of from about 5mm to about 200 mm.

Typically the one or more channels each have a substantially circular, square or rectangular cross-section. In one embodiment the one or more channels each have a surface roughness of from about 0.1 to about 10 pm, optionally from about 0.1 to about 5 pm, optionally from about 0.1 to about 1 pm.

Typically the one or more channels each have an optical loss of less than or equal to about 1.5 dB/cm.

In an embodiment of the invention one or more of the channels is non-linear.

In an embodiment of the invention, the silica aerogel product further comprises one or more chambers or reservoirs.

Typically the one or more channels and/or one or more chambers or reservoirs are in fluid connection.

The one or more channels may comprise one or more fluid inlets and one or more fluid outlets. The one or more channels may comprise one or more junctions, optionally a T-junction, a cross-junction, an inter-junction or a Y-shape junction. One or more of the channels may be curved, U-shaped or a serpentine channel.

The channels are formed in the aerogel in step v) when the supercritical ethanol dissolves the one or more polymer inserts.

In a further aspect the invention provides a liquid core waveguide cell comprising the silica aerogel product as defined above. Typically, the one or more channels provide a path length of from about 0.5 cm to about 20 cm, optionally from about 1 cm to about 20 cm, optionally from about 1 to 15 cm, optionally from about 1 cm to about 10 cm.

In an additional aspect the invention provides an optofluidic device comprising: i) a liquid core waveguide cell as defined above; ii) a light source; iii) a light inlet enabling passage of light from the light source to the one or more channels of the liquid core waveguide cell; iv) one or more sample sources, wherein each sample is a fluid; v) one or more sample inlets; each sample inlet enabling passage of a sample from a sample source to the one or more channels of the liquid capillary waveguide cell; vi) one or more sample outlets; and vii) a detector for detecting light transmission or absorbance by the one or more samples in the liquid core waveguide cell. Typically the light source is a fiber optic cable, a laser, ultraviolet light, infrared light or a spectral light source.

In one embodiment the device measures absorbance of ultraviolet, visible or near infra-red light by the one or more samples.

The light may have a wavelength of from about 230 to about 3000 nm, optionally from about 230 to about 1400 nm, most preferably from about 230 to about 800 nm.

In a preferred embodiment the optofluidic device comprises two modules, wherein the liquid core waveguide cell (4) is located between the two modules, and wherein the modules are attached by means of a plurality of alignment bars (5), each module comprising a sample inlet (1) and a light inlet (2), and a sample outlet and a light outlet, wherein the light inlet is an optical fiber inlet which is sealed using a silica window (3), and wherein each module is attached to the liquid core waveguide cell by means of a waterproof seal. Typically the modules are formed of aluminium.

One embodiment of the optofluidic device comprises the product as defined above as a silica window.

In one embodiment the silica aerogel product is gas permeable to prevent formation of bubbles and/or eliminate bubbles formed in the fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic of the hydrolysis (1) and condensation (2 and 3) reactions that occur to form the silica network of an alcogel.

Figure 2(A) shows an illustration of a 3D printing filament that can then be embedded within the alcogel (photograph, Figure 2(B)). Figure 2(C) provides a schematic of aerogel channel formation through polymer insert dissolution. Figure 2(D) shows an aerogel channel formed through PET insert extraction.

Figure 3 shows (a) an illustration of the natural aerogel surface (hydrophilic) and (b) the reacted surface (hydrophobic) following liquid HMDS treatment.

Figure 4 provides a plot of temperature and pressure in the autoclave during the supercritical drying process.

Figure 5 shows a plot of temperature during the sintering (densification) step.

Figure 6 is a photograph of a sample containing a microfluidic channel A) prior to sintering and B) after sintering. Figure 7 is a plot of refractive index (n) mapped against rising TEOS aerogel density.

Figure 8 (a) and (b): drilled channel imaged through 3D profilometry; and (c) an (d): a channel formed through the thermoplastic polymer dissolution method of the invention.

Figure 9 is a plot of surface roughness versus lateral distance for a drilled channel according to a prior art method (significant variations in surface roughness) and a channel formed by the thermoplastic polymer dissolution method of the invention.

Figure 10 provides photographs of common 3D printing filaments, TPU (left), ABS (centre) and PLA (right). A) Prior to supercritical ethanol processing and B) following supercritical ethanol processing.

Figure 11 shows the manifold system used to integrate the aerogel module. Figure 11(A) is a photograph of the manifold, and Figure 11 (B) is a schematic of the manifold.

Figure 12 shows absorbance detection using the manifold on varying concentrations of blue food dye at a wavelength of 630nm.

Figure 13 shows various geometries of aerogel geometries obtained, including: A) a junction; B), C); aerogels comprising multiple channels; D), E); channels having complex geometries.

Figure 14 shows further possible channel geometries, including I) a complex geometry comprising multiple connected channels with complex shapes; II) a serpentine channel; III) droplet formation geometries that use T-junction (A), cross-junction (B) and inter-junction geometries (C).

Figure 15 shows further possible channel geometries, including a design with a long optical path (A), and a design with multiple fluid inlets and a single fluid outlet.

Figure 16 shows a schematic of a process according to an embodiment of the invention. Step 1) involves providing a silicon wafer with a thermal oxide layer (SiO2 in the range 5 - 50 nm, wafer thickness: 400 - 800 micrometers; diameter: 3 to 8 inches. The wafer undergoes a standard cleaning step without hydrofluoric acid. Step 2): Silica aerogel deposition by spin or dip coating. Targeted thickness : 20 to 100 micrometers. Step 2’: A second wafer is prepared using the same process. Step 3 : Channel formation in aerogel by stamping. Stamps are typically made with a 3D printer. Step 4: Layer bonding: Layers from step 2 and 2’ in contact and thermal annealing, typically 1000°C for 30seconds (bond activation without strain between layers of the different materials). Step 5: Dicing.

DETAILED DESCRIPTION OF THE INVENTION An aspect of the invention provides a silica aerogel product comprising one or more channels obtainable by a process comprising: i) preparing a sol from a silica precursor, ethanol, water and an acid and/or base catalyst; wherein the silica precursor comprises tetramethyl orthosilicate or tetraethyl orthosilicate; and wherein the molar ratio of silica precursor to water is from about 1 :2 to about 1 :5; and the molar ratio of silica precursor to ethanol is from about 1 :0.1 to about 1 :25; and ii) agitating the sol; iii) a) inserting one or more polymer inserts into the sol, wherein the polymer is suitable for complete dissolution in at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa but is not suitable for dissolution in liquid ethanol at room temperature and atmospheric pressure; b) allowing the sol to set to form a silica alcogel in a first sealed container; iv) contacting the silica alcogel containing the one or more polymer inserts formed in step (iii) with further ethanol in a second sealed container; v) contacting the product of step iv) with ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa, preferably about 280 °C and about 10 MPa; vi) reducing the pressure to allow release of ethanol from pores in the silica alcogel to form the silica aerogel, optionally to atmospheric pressure; and vii) optionally reducing the temperature to room temperature.

In step i) a sol is prepared from a silica precursor, ethanol, water and an acid and/or base catalyst.

Typically, the silica aerogel product is formed without the use of any further solvents other than those mentioned in steps i) to vii) above, e.g. the only components required are the silica precursor, ethanol, water and an acid and/or base catalyst (as well as the one or more polymer inserts). In particular, ethanol is used both in the formation of the aerogel and at critical temperature and pressure to dissolve the polymer inserts. This is in contrast to the process described in, for example, WO2019075274A1 where different solvents are required for the formation of the aerogel and for the dissolution of the insert/scaffold.

A silica precursor is an alkyl orthosilicate which is used as a reactant to form a silica aerogel when reacted with water, ethanol and an acid and/or base catalyst. In the present invention, the silica precursor comprises or is tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS). “orthosilicate” means SiO '. TMOS is also referred to as tetramethoxysilane and TEOS is also referred to as tetraethoxysilane, having the chemical formulae Si(OCH ₃ )4 and Si(OCH ₂ CH ₃ ) ₄ respectively. The silicon precursors are also referred to as silicon alkoxides.

A sol is a type of colloid. A colloid is a mixture in which at least two different phases are intimately mixed at the nanolevel. The term “phase” generally refers to a solid, liquid, or gas form of some substance. A colloid typically has a continuous phase in which something else with a different phase is dispersed (the “dispersed phase”). Different phases can still be the same phase of matter, for example, two different phases could both be liquids, just not miscible liquids. Colloids are different from homogeneous solutions, in which a substance is dissolved or mixed with another substance and does not separate out, in that the components of colloids are nanoparticles or macromolecules.

A sol is a type of colloid in which the continuous phase is a liquid and the dispersed phase is a solid. The difference between a sol and a non-colloidal liquid is that solid nanoparticles are dispersed throughout the liquid in a sol. If you put a sol in a centrifuge, you can force the nanoparticles dispersed in the liquid to precipitate out. This will not happen for a non-colloidal liquid solution, for example, salt dissolved in water.

During steps i) to iii) the components of the sol react to form a silica alcogel. This process involves hydrolysis and condensation reactions (see Figure 1). The sol may be agitated. Agitation may be achieved by any method known to a person skilled in the art, for example by stirring or shaking the sol. The sol sets in a sealed container, the “first sealed container,” to prevent evaporation of the ethanol. The sealed container may comprise one or more entry ports or holes for insertion of the polymer insert.

A hydrolysis reaction occurs in which silicon alkoxide (Si-OR) groups react with water, forming a silanol group (silicon-OH). This reaction produces the reactive groups that are capable of polymerisation. Once silanol groups form they can condense through two different reactions to form connective silicon-oxygen-silicon bridges. A condensation reaction occurs when two silanol groups (Si-OH + HO-Si) combine to form a siloxane bridge (Si-O-Si). The reaction forms one water molecule. Another condensation reaction also occurs when a silanol group reacts with a methoxysilanol group (for TMOS) or an ethoxysilanol group (for TEOS) to form a siloxane bridge (Si-O-Si), but this time releasing one methanol or ethanol molecule rather than water.

The kinetics of the above reaction are impractically slow at room temperature, often requiring several days to reach completion. For this reason, acid and/or base catalysts are added to the formulation. A sol is a solution of the reactants (a silica precursor, ethanol, water and an acid and/or base catalyst) that are undergoing hydrolysis and condensation reactions. The molecular weight of the oxide species produced continuously increases. As these species grow, they may begin to link together in a three-dimensional network. The gel point is the time at which the network of linked oxide particles spans the container holding the sol. At the gel point the sol becomes a rigid substance called an alcogel. An alcogel consists of two parts, a solid phase and a liquid phase. An alcogel is a wet gel that has alcohol within the pores of the gel. The alcogel can be removed from its original container and can stand on its own. The continuous phase is a solid phase formed by the three-dimensional network of linked silicon oxide particles and the dispersed phase is a liquid (the original solvent of the sol) with by-products of the reaction (in this case water) which fills the free space surrounding the solid part. Gels tend to be mostly liquid in composition and typically exhibit the density of a liquid as result but have cohesiveness like a solid.

In step iv) the alcogel is contacted with further ethanol to ensure that water and other reactants that remain within the pores of the alcogel are exchanged with ethanol. This step typically involves submerging the alcogel in an ethanol bath. This step also occurs in a sealed container (“the second sealed container”) to avoid evaporation of ethanol. The ethanol bath may be renewed several times with fresh ethanol. In an embodiment of the invention step iv) comprises one or more heating steps wherein the alcogel is heated to about 40°C and then allowed to cool. The one or more heating steps increase the ethanol exchange rate, reducing the time required to exchange the alcohol.

Step v) comprises contacting the product of step iv) with ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa, preferably about 280 °C and about 10 MPa. When ethanol is at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa it is a supercritical fluid. Therefore, step v) comprises contacting the product of step iv) with supercritical ethanol. A supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist, but below the pressure required to compress it into a solid. It is a highly compressed fluid that combines the properties, such as density, viscosity or diffusivity of gases and liquids. Ethanol with a temperature and pressure above its critical point is referred to as “supercritical ethanol” throughout. The supercritical ethanol is present in the pores of the gel. Subsequently, in step vi) the pressure is reduced back to atmospheric pressure leading to displacement of ethanol in the pores of the gel by air, followed in step vii) by a return to room temperature. This process is called supercritical drying and leads to the formation of the final silica aerogel product. An aerogel is what remains when the liquid part of an alcogel is removed without damaging the solid part (achieved by supercritical extraction with supercritical ethanol). The wet alcogel is dried. If made correctly, the aerogel retains the original shape of the alcogel and at least 85% (typically greater than 95%) of the alcogel’s volume.

An aerogel is solid with air pockets dispersed throughout. Aerogels are synthetic, porous, ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas, without significant collapse of the gel structure. The result is a solid with extremely low density and extremely low thermal conductivity. Aerogels are typically composed of more than 95% air, and up to 99.8% air, and thus have a very low refractive index close to the refractive index of air, typically below 1.2.

Despite the name, aerogels are solid, rigid, and dry materials that do not resemble a gel in their physical properties: the name comes from the fact that they are made from gels.

Aerogels may be inorganic, organic or inorganic-organic hybrids but the present invention relates to silica aerogels formed from tetramethyl Orthosilicate (TMOS) or Tetraethyl Orthosilicate (TEOS). TMOS is more reactive and can be formed through a single step basic reaction whereas TEOS generally requires a longer formation period through a 2-step (acid/basic) catalysis reaction. Through the control of the molar ratio of reactants during formation, the physical characteristics of the gel such as transparency can be controlled.

Each polymer insert comprises or consists of a polymer which is suitable for complete dissolution in ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa but is not suitable for dissolution in liquid ethanol at room temperature and atmospheric pressure, typically poly(ethylene terephthalate) (PET), poly(ethylene terephthalate) glycol (PET-G) or poly(lactic acid) (PLA). The polymer is thermoplastic. The steps of this aspect of the invention are entirely the same as the steps set out in claim 1 except one or more polymer inserts are inserted into the sol in step iii). The polymer insert is solid in step iii). The process may comprise inserting a plurality of polymer inserts into the sol.

The term “complete dissolution” means that the polymer will substantially dissolve in ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa, i.e. at least 90 wt%, preferably at least 95 wt%, more preferably at least 99 wt%, most preferably 100 wt% of the polymer dissolves, i.e. the solid polymer degrades and leaves behind a void corresponding to its three-dimensional shape. These polymers remain intact during the ethanol exchange process during step iv) and are then dissolved during the supercritical ethanol drying stage of steps v) and vi) leading to aerogel formation. These polymers are easy to manipulate to form 3D geometries, remain smooth (for improved channel surface quality) and are dissolvable in supercritical ethanol. The polymers, including poly(ethylene terephthalate) derivatives, need to exist as polymer filaments for 3D-printing and to be dissolvable in supercritical ethanol but not in liquid ethanol.

A single polymer insert or multiple polymer inserts can be used. A single polymer insert can be used to form a single channel or can be used to provide a plurality of channels depending on the three dimensional structure of the insert.

The polymer insert can also be referred to as a mould, cast, template or form. Each polymer is inserted into the sol before it sets, i.e. while the sol is in a liquid state. The sol then sets to form a silica alcogel comprising the polymer insert. The polymer insert remains intact whilst the silica alcogel is in contact with liquid ethanol in step iv). However, when the silica alcogel comprising the polymer insert is subjected to ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa, i.e. supercritical ethanol, in step v) the polymer insert dissolves, leaving behind a three dimensional cavity (for example a channel) corresponding to the three dimensional shape of the polymer insert.

PET (Polyethylene Terephthalate) became a potential candidate due to research performed on using supercritical ethanol to dissolve PET for recycling purposes (De Castro et al “Depolymerization of poly(ethylene terephthalate) wastes using ethanol and ethanol/water in supercritical conditions,” J. Appl. Polym. Sci., vol. 101 , no. 3, pp. 2009-2016, 2006, also described in BRPI0402976A2).

Thermoplastic filaments of PET, PET-G or PLA can be commonly printed with most low-cost fused deposition modelling (FDM) 3D printers. This ease of manufacturing allows the formation of polymer inserts or moulds with the desired shape and size. Alternatively, for simpler geometries, unprocessed PET-G filament or extruded PET, PET-G or PLA filament can be directly used. The absence of crystallinity of the PET derivative induced by the addition of glycol increases its flexibility and makes it less prone to be broken.

In a further aspect, the invention provides a process for preparing a silica aerogel product comprising one or more channels, the process comprising: i) preparing a sol from a silica precursor, ethanol, water and an acid and/or base catalyst; wherein the silica precursor comprises tetramethyl orthosilicate or tetraethyl orthosilicate; and wherein the molar ratio of silica precursor to water of from about 1 :2 to about 1 :5; and the molar ratio of silica precursor to ethanol is from about 1 :0.1 to about 1 :25; and ii) agitating the sol; iii) a) inserting one or more polymer inserts into the sol, wherein the polymer is suitable for complete dissolution in ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa but is not suitable for dissolution in liquid ethanol at room temperature and atmospheric pressure; b) allowing the sol to set to form a silica alcogel in a first sealed container; iv) contacting the silica alcogel containing the one or more polymer inserts formed in step (iii) with further ethanol in a second sealed container; v) contacting the product of step iv) with ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa, preferably about 280 °C and about 10 MPa; vi) reducing the pressure to allow release of ethanol from pores in the silica alcogel to form the silica aerogel, optionally to atmospheric pressure; and vii) optionally reducing the temperature to room temperature.

Typically, the silica aerogel product is formed without the use of any further solvents other than those mentioned in steps i) to vii) above, e.g. the only components required are the silica precursor, ethanol, water and an acid and/or base catalyst (as well as the one or more polymer inserts). In particular, ethanol is used both in the formation of the aerogel and at critical temperature and pressure to dissolve the polymer inserts. This is in contrast to the process described in, for example, WO2019075274A1 where different solvents are required for the formation of the aerogel and for the dissolution of the insert/scaffold.

Typically, step i) comprises preparing a first solution of the silica precursor in ethanol; preparing a second solution comprising the acid and/or base catalyst, water and ethanol; and mixing the first and second solutions, optionally mixing equal volumes, or approximately equal volumes, of the first and second solutions. Preferably, step i) comprises gradually mixing the first and second solutions, optionally for a period of about 20 to about 60 seconds, optionally about 30 seconds.

In one embodiment, the catalyst is a base.

In a preferred embodiment, the molar ratio of silica precursor to base is from about 1 :0.0001 to about 1 :0.1 , preferably 1 :0.01 . In one preferred embodiment, the silica precursor is tetramethyl orthosilicate.

In another preferred embodiment, the silica precursor the silica precursor is tetraethyl orthosilicate

In one embodiment the silica precursor is tetraethyl orthosilicate, and the catalyst comprises a base and an acid, optionally wherein the base is added to the second solution prior to mixing and the acid is added to the mixture of the first and second solutions.

Typically step ii) comprises agitating the sol for a period of from about 10 to about 14 mins, optionally about 12 mins, optionally at about 200 to 300 rpm, optionally at about 250 rpm.

Typically in step iii) the sol sets for a period of from about 4 to about 10 hours. Typically the sol sets in a mould provided within the first sealed container. The mould may be formed from a plastic syringe which has had the end tip removed. Inserting a plunger (for example the plunger of the modified syringe) allows removal of the alcogel. The alcogel can then be placed in the second sealed container.

Alternatively, the first sealed container mould could also be made by 3D printing using PET, PET-G or PLA. The resulting printed mould could then be dissolved using supercritical ethanol in step v) thereby avoiding the need to remove the alcogel from the mould before step iv). If not done correctly, this step might indeed weaken the alcogel which might result in unwanted fissures within the resulting aerogel.

In an embodiment of the invention, in step iii) and/or step iv) the sealed container is a sealed mould.

In one embodiment, the first sealed container and the second sealed container are the same container.

Typically step iv) is for a period of from about 24 to about 72 hours.

In one embodiment the ethanol is renewed during step iv). This means that the ethanol is removed from the second sealed container and replaced with new (fresh) ethanol. The ethanol may be replaced every 24 hours.

Preferably, step v) involves a gradual increase of temperature and pressure from room temperature and atmospheric pressure to a final temperature of from about 240 °C to about 320 °C and a final relative pressure of from about 6.3 MPa to about 12 MPa over a period of about 2 to about 5 hours, optionally about 4 hours, optionally the temperature is raised about 75 °C per hour to the final temperature. In a preferred embodiment, in step i) the molar ratio of silica precursor to water is about 1 :4.

Typically the aerogel has a bulk density of from about 0.003 to about 0.5 g/cm3.

Typically the aerogel has a refractive index below 1.2, preferably from about 1.01 to about 1.10. The critical angle of reflection can be obtained using a He-Ne laser apparatus (Y. Ozbakir, A. Jonas, A. Kiraz, and C. Erkey, “Total internal reflection-based optofluidic waveguides fabricated in aerogels,” J. Sol-Gel Sci. Technol., vol. 84, no. 3, pp. 522-534, 2017, doi: 10.1007/S10971-017-4426-8).

The critical angle can then be used to calculate the refractive index of the silica aerogel.

Typically the silica aerogel has a mean pore diameter of from about 20 nm to about 190 nm, optionally wherein the mean pore diameter of the silica aerogel is from about 20 to about 180nm when the silica precursor is tetramethyl orthosilicate or from about 60 to about 190nm when the silica precursor is tetraethyl orthosilicate.

Typically, the aerogel has a porosity of from about 50% to about 99.8%, preferably from about 95% to about 99.8%.

The percentage of porosity of the aerogel can be calculated using the following formula:

Porosity = 100

Where the bulk density is obtained by dividing the sample mass (in g) by the sample volume (in cm ³ ) and the solid density is considered as equal to the density of SiC>2, i.e, 2.65 g.cnr ³ .

Preferably the silica aerogel is substantially free of detectable fissures. Fissures can be detected by optical microscopy. The term “substantially free of detectable fissures” means that no or very few fissures are observable by optical microscopy. Being free of detectable fissures is very important as it prevents fissure propagation that results in material degradation.

In one embodiment the catalyst comprises or is a base catalyst, preferably ammonium hydroxide, sodium hydroxide, sodium carbonate, or potassium carbonate.

In one embodiment the catalyst comprises or is an acid catalyst, preferably ammonium fluoride, oxalic acid or nitric acid.

In one embodiment the silica precursor further comprises one or more of methyltrimethoxysilane, methyltriethoxysilane, dimethylchlorosilane, trimethylethoxysilane, ethyltriethoxysilane or phenyltriethoxysilane. Typically the silica precursor is present in the mixture of step i) in an amount of from about 10 to about 80 wt%, preferably of from about 20 to about 60 wt%, more preferably of about 40%.

In one embodiment the base is present in the mixture of step i) in an amount of from about 0.003 to about 0.2 wt%, preferably of from about 0.05 to about 0.15 wt%, more preferably of about 0.093%.

Typically the ethanol is present in the mixture of step i) in an amount of from about 1 to 86 wt%, preferably of from about 20 to about 60 wt%, more preferably of about 40%.

Typically the water is present in the mixture of step i) in an amount of from about 5 to about 40 wt%, preferably of from about 30 to about 40 wt%, more preferably of about 20%.

In one embodiment the process further comprises a step of modifying the hydrophobicity of the aerogel by contacting the aerogel with hexamethyldisilazane. Typically this comprises vapour deposition of hexamethyldisilazane after step vii). Alternatively the ethanol used in step iv) comprises about 15 wt % to about 30 wt %, optionally about 20 wt % hexamethyldisilazane. Both methods act to replace the silanol groups on the aerogel surface with non-polar groups to render the material hydrophobic.

Heating the samples and gentle agitation increases the surface chemical exchange rate and therefore hydrophobic performance of the liquid deposition method.

In one embodiment step v) occurs for a period of from about 4 to about 8 hours, preferably 4 to 6. Typically step v) occurs in an autoclave.

In one embodiment step vi) occurs for a period of from about 3 to about 4 hours.

In one embodiment in step vi) the temperature is maintained in the range of from about 240 °C to about 320 °C and the pressure gradually reduces from a relative pressure of from about 63 bar to about 120 bar to atmospheric pressure. Step vi) typically occurs in an autoclave.

Typically the ethanol is 99% ethanol.

In preferred embodiments the insert is prepared by fused deposition modelling (FDM) 3D printing or is formed from one or more polymer filaments. For the FDM process, a hot-end extruder is used to extrude the thermoplastic material, where deposition is guided using X, Y and Z axis control to enable 3D thermoplastic design formation. The fused deposition modelling 3D printing process is not especially limited provided the polymer is capable of dissolution in ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa but is not capable of dissolution in liquid ethanol at room temperature and atmospheric pressure, for example PET, PET-G or PLA, and the final printed polymer insert has the required shape and dimensions. PET and PET-G are available from a commercial source such as Ooznest materials. PLA is also available from a commercial source. Typically, PET-G is printed at a temperature of from about 230 to about 250°C, preferably 240°C. The temperature of the heated bed is from about 60 to about 80 °C, typically about 70°C. For PET, the extruder temperature is generally about 215°C and the bed about 70°C, compared to about 190°C and about 60°C respectively for PLA. Recommended printing parameters (extruder and bed temperature) are generally provided by the material supplier. Any commercially available 3D printer can be used, such as Creality Ender 3. Typically, the 3D printer is used with a selection of nozzle sizes (ranging from 100pm to 1 mm). Inserts are typically printed horizontally, with low printing speed and 7mm/s retraction. Use of an extruded thermoplastic polymer filament means that the filament insert is already straight for insertion into the alcogel and provides a wide-range of channel sizes.

In one embodiment a polymer filament has an outer diameter of from about 100 pm to about 2850 pm. In one embodiment a polymer filament is an extruded polymer filament having an outer diameter of from about 100 pm to about 1000 pm. In one embodiment a polymer filament is heated for a period of about 2 mins to straighten the filament prior to inserting the filament into the sol, optionally in an oven at about 70 to 90 °C, preferably 80 °C.

In one embodiment, the one or more polymer inserts of step iii) a) are inserted into the surface of the sol and wherein the process comprises a further bonding step viii) comprising activating the said surface of the product obtained after step vii) and a surface of a second silica aerogel and contacting the surface of the product obtained after step vii) with the surface of the second aerogel thereby obtaining the silica aerogel product.

A schematic of the process of this embodiment of the invention is shown in Figure 16.

In this process, a first silica aerogel product is formed including one or more channels formed in the surface of the silica aerogel product. A second silica aerogel is formed, typically via the same process as described above according to steps i) to vii) but without step iii)a) i.e. no polymer inserts are inserted into the sol, and therefore no channels are typically formed in the second aerogel. The surface of the first aerogel comprising the one or more channels and a surface of the second aerogel are then activated and contacted.

Typically, the activation is performed by heating at a temperature of from about 900 to about 1100 °C, optionally wherein the heating is for at least 30 seconds, optionally wherein the heating is in a Rapid Thermal Annealing (RTA) furnace. Alternatively, the activation is performed using a radiofrequency plasma, optionally comprising low pressure Argon gas at a pressure of from about 1 to about 100 millitorr, optionally for about 1 to about 5 minutes.

Radiofrequency plasmas are formed in a flow of gas by an externally applied radio frequency field. In this embodiment both surfaces are exposed to a RF plasma, typically formed from a low pressure Argon gas. Just after exposure the surfaces are brought close together.

Plasma ions break chemical bonds in each surface and surface reorganization occurs when the surfaces are in contact. Chemical bonding is therefore obtained between the surface of the first aerogel comprising the one or more channels and the surface of the second aerogel.

In one embodiment the process further comprises a sintering step ix) wherein the product of step vi), vii) or viii) is heated to a temperature of from about 900 to 1100 °C, optionally about 1000 °C for a period of from about 1 to about 6 hours, optionally about 2 hours. Typically the sintering step causes a 50% to 90% increase in bulk density.

In one embodiment step ix) converts the silica aerogel into a product which consists essentially of silica.

In an embodiment of the invention the internal diameter of the one or more channels can be controlled by altering the time period of step ix).

To compare the internal diameter of both circular and non-circular channels, hydraulic diameter can be used. This is calculated by dividing (4 x the area of internal channel crosssection) by the channel perimeter. As such, the hydraulic diameter of circular channels is the actual diameter and for rectangular channels, this provides a means to treat non-circular channels as circular.

Hydraulic diameter (D _H ) is calculated as follows: where

A is the cross-sectional area of the flow, P is the wetted perimeter of the cross-section.

A, the cross-sectional area of the flow, is the area of a cross-section of a channel taken perpendicular to the direction of flow not including the area of the channel walls. P, the wetted perimeter is the portion of the cross-sectional perimeter of a container that is in contact with fluid.

For example, if the channel has a circular cross section A is calculated as follows:

A= nr ² wherein r is the internal radius of the channel

The wetted perimeter of the cross-section P is calculated as follows:

P = 2TTr wherein r is the internal radius of the channel.

It can therefore be seen that when the cross-section is circular, the hydraulic diameter is the same as the internal diameter:

DH = 4rrr ² /2rrr

= 2r

= d

If the channel has a rectangular cross-section the hydraulic diameter can be calculated as

D _H = 4 a b / (2 (a + b))

= 2 a b / (a + b) where a = width/height of the duct (internal dimension) b = height/width of the duct (internal dimension)

In one embodiment the one or more channels each have a hydraulic diameter in the range of from about 100 pm to about 2900 pm, or from about 100 pm to about 2850 pm, preferably from about 200 pm to about 600 pm.

Typically the one or more channels each have a length in the range of from about 5mm to about 200 mm.

Typically, the one or more channels each have a circular, square or rectangular cross-section or a substantially circular, oval, square or rectangular cross-section. The term “substantially” means that the cross-section of the channel may have some minor deviations from the exact geometric shape. For example, the square or rectangle may have rounded corners.

In one embodiment the one or more channels each have a surface roughness of from about 0.1 to about 10 pm, optionally from about 0.1 to about 5 pm, optionally from about 0.1 to about 1 pm.

Typically the one or more channels each have an optical loss of less than or equal to about 1.5 dB/cm. The optical loss corresponds to the loss of signal intensity and is using a logarithm scale. An optical loss of 3 dB/cm corresponds to a decrease of 50% of the incident signal and a loss of 1.5 dB/cm corresponds to a decrease of ca. 20% of the incident signal.

In an embodiment of the invention one or more of the channels is non-linear. This provides a great advantage of the products and processes of the invention versus the products of the prior art which are formed via drilling or other processes where it is difficult to form non-linear channels, for example curved channels or channels with complex shapes. Non-linear channels and channels with complex geometries are easy to form with the present process because the channels are formed by dissolving the polymer insert using supercritical ethanol.

In an embodiment of the invention, the silica aerogel product further comprises one or more chambers or reservoirs. The one or more chambers or reservoirs may be for storing, reacting or mixing one or more fluids.

Typically the one or more channels and/or one or more chambers or reservoirs are in fluid connection.

The one or more channels may comprise one or more fluid inlets and one or more fluid outlets. The one or more channels may comprise one or more junctions, optionally a T-junction, a cross-junction, an inter-junction or a Y-shape junction. One or more of the channels may be curved, U-shaped or a serpentine channel. Examples of possible channel geometries are provided in Figures 13 to 15.

The channels are formed in the aerogel in step v) when the supercritical ethanol dissolves the one or more polymer inserts.

In a further aspect, the invention provides a silica aerogel comprising one or more channels, each channel having a surface roughness of from about 0.1 to about 10 pm, an internal diameter in the range of from about 100 pm to about 2900 pm and a length of from about 5mm to about 200 mm. Typically, the silica aerogel is obtainable by any process as defined above.

Surface roughness is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small, the surface is smooth. Typically surface roughness is measured using a contact profilometer. A contact profilometer has a stylus, typically a diamond stylus, that contacts the surface. As the stylus is drawn across the surface a force is applied so that the stylus maintains contact with the surface and measures deviations from a mean surface depth. Surface roughness is measured perpendicular to the lay direction. Typically the surface roughness measured is the arithmetical mean roughness value (Ra) which is the arithmetical mean of the absolute values of the profile deviations (Zi) from the mean line of the roughness profile. In a preferred embodiment the contact profilometer used is a Dektak® profilometer.

In a further aspect the invention provides a liquid core waveguide cell comprising the silica aerogel product as defined above. Typically the one or more channels provide a path length of from about 0.5 cm to about 20 cm, optionally from about 1 cm to about 20 cm, optionally from about 1 cm to about 10 cm.

In an additional aspect the invention provides an optofluidic device comprising: i) a liquid core waveguide cell as defined above; ii) a light source; iii) a light inlet enabling passage of light from the light source to the one or more channels of the liquid core waveguide cell; iv) one or more sample sources, wherein each sample is a fluid; v) one or more sample inlets; each sample inlet enabling passage of a sample from a sample source to the one or more channels of the liquid core waveguide cell; vi) one or more sample outlets; and vii) a detector for detecting light transmission or absorbance by the one or more samples in the liquid core waveguide cell.

Optofluidics is a term used to describe the synergistic integration of photonics and microfluidics within systems. It is a recent analytical field that provides a number of unique characteristics for enhancing the sensing performance and simplifying the design of microsystems (Fan et al. “Optofluidic microsystems for Chemical and Biological Analysis” Nat Photonics. 5(10): 591-597; 2011).

Typically the light source is a fiber optic cable, a laser, ultraviolet light, infrared light or a spectral light source.

In one embodiment the device measures absorbance of ultraviolet, visible or near infra-red light by the one or more samples.

The light may have a wavelength of from about 230 to about 3000 nm, optionally from about 230 to about 1400 nm, most preferably from about 230 to about 800 nm.

In a preferred embodiment the optofluidic device comprises two modules, wherein the liquid core waveguide cell (4) is located between the two modules, and wherein the modules are attached by means of a plurality of alignment bars (5), each module comprising a sample inlet (1) and a light inlet (2), and a sample outlet and a light outlet, wherein the light inlet is an optical fiber inlet which is sealed using a silica window (3), and wherein each module is attached to the liquid core waveguide cell by means of a waterproof seal. Typically the modules are formed of aluminium. Typically, the device comprises four alignment bars.

One embodiment of the optofluidic device comprises the product as defined above as a silica window.

In one embodiment the silica aerogel product is gas permeable to prevent formation of bubbles and/or eliminate bubbles formed in the fluid.

EXAMPLES

Example 1 : Process of Silica Aerogel Formation

The sol-gel process of gel formation.

Solution A contains the TMOS precursor and Ethanol (ethanol 98%). Whereas B contains distilled water (mixed with a dilute concentration of ammonia (NH4OH) solution) and ethanol. TMOS was purchased from Sigma Aldrich.

The basic catalyst (NH4OH) was added to the distilled water with a concentration of 0,486g in 100ml. The mass (g) of each component was calculated using the molar ratio of 1 :4:3.2 of TMOS, water and ethanol. Altering the molar ratios provides a means to alter gel properties such as to increase sample transparency ( A. Venkateswara Rao, G. M. Pajonk, and N. N. Parvathy, “Influence of molar ratios of precursor, catalyst, solvent and water on monolithicity and physical properties of TMOS silica aerogels,” J. Sol-Gel Sci. Technol., vol. 3, no. 3, pp. 205-217, 1994, doi: 10.1007/BF00486559.), therefore ratios can be altered within the claimed range. Furthermore, aerogel density can be controlled through TMOS precursor weight percentage (wt%) alteration within the solution.

Solution B was then slowly poured into solution A over a period of 30 seconds while under continual agitation at around 250 rpm. The agitation continued for a further 12 minutes, where a single step base catalysis reaction was undergone for the sol-gel reaction to take place. At this point the gel was still in a liquid phase and could be poured into an appropriate mould where a gelation period and ageing could take place.

The mould

The mould used was highly smooth in order to prevent the solid gels fissuring following mould removal. A plastic syringe was used that had been cut at their base to expose a flat open surface. The sol-gel mixture was poured into the mould. Parafilm was used in order to seal the exposed mould and ensure a constant environment of alcohol vapour. In this way, evaporation was avoided that can cause fissures in the gel prior to removal from the mould. Following the required gelation period (around 4 hours), the gels could be easily removed through placing pressure on the plunger of the syringe into a bath of ethanol.

Polymer insert incorporation

It is essential to place the preformed polymer insert, for example a PET-G filament within the liquid phase, prior to alcogel formation. The liquid phase lasts for around 15 minutes, depending on the duration and speed of the solution agitation, before alcogel formation occurs.

Therefore, it is advisable to have the polymer inserts pre-formed, so they are ready for insertion. For singular and multiple linear channel formation, direct use of commercially available PET-G filament is possible (diameter either 1 ,75mm or 2,85mm), cut to desired lengths. Trimming of both sample ends is required for a flat surface to be inserted into the gels. Also cut sections of PET-G are generally bent in shape and therefore require straightening prior to use. This is completed using a heat treatment of 80°C in an oven for 1 minute. This process reverts the filament sections into their straight form, which remains fixed following the return to ambient temperature.

Alternatively, extrusion of the thermoplastic polymer material can be performed using any commercially available 3D printer, such as the Creality Ender 3 with a selection of nozzle sizes (ranging from 100pm to 1mm). Inserts were printed horizontally, with low printing speed and 7mm/s retraction in order to improve the print quality. Use of extruded polymer means that the polymer insert is already straight for insertion into the alcogel and provides a wide-ranging channel size. Polymer insert formation can be achieved using 3D printing for more complex forms (Figure 2).

As the gels still required a parafilm cover to prevent alcohol evaporation, a hole was made in the parafilm cover which could be used to guide the PET-G filament into the sample while also holding the polymer mould in place during gelation.

During the mould removal process, care was taken to pierce the parafilm next to the thermoplastic polymer filament in order to ease the removal of the parafilm cover. Once removed, the entire gel containing the PET-G insert could be submerged in the Ethanol bath.

The ageing process 1 This process is completed to ensure water and other reactants that remain within the gel are exchanged with ethanol through submerging in an ethanol bath. Fresh ethanol was exchanged every 24 hours. The amount of ethanol required is 5 times the volume of the gels that are undergoing the ageing process.

As the sample volumes remained low, this exchange period could take place in around 3 days. This process could be accelerated (to around 1 day) through a series of heating steps (heating the samples at 40°C to increase the ethanol exchange rate, then allowing to cool, changing the ethanol and repeating 3 to 4 times).

HMDS Deposition

An approach that can be used to render the gels hydrophobic is through the submersion of the gels within a bath containing a solution of 20 wt% of HMDS in ethanol. An increased temperature of 40°C for 24 hours with mild agitation assists the chemical surface group exchange between external -OH groups to trimethysilyl groups, as seen in Figure 3.

Sample submersion in ethanol for 24 hours is then used to remove unreacted HMDS from the samples prior to supercritical drying.

Supercritical drying

The supercritical drying process is typically performed using an autoclave.

The heater surrounding the reactor containing the gels is used to gradually increase the temperature to above that of the supercritical point of the ethanol that is contained within the reactor (J. L. Gurav, I. K. Jung, H. H. Park, E. S. Kang, and D. Y. Nadargi, “Silica aerogel: Synthesis and applications,” J. Nanomater., vol. 2010, 2010, doi: 10.1155/2010/409310). The required temperature and pressure for the supercritical point of ethanol is 243°C and 6.3 Mpa (63 bar) (T. Woignier, J. Phalippou, F. Despetis, and S. Calas-Etienne, “Aerogel Processing,” Handb. Sol-Gel Sci. Technol., vol. 8, no. 1 , pp. 1-27, 2016, doi: 10.1007/978-3-319-19454- 7_27-1) , therefore technically any temperature/pressure above this point should be successful. This temperature was surpassed to around 300°C to ensure the removal of any other potential reactants that have remained in the gel.

Once the temperature and pressure have reached their desired value, a period of 30 minutes to 1 hour of stabilisation was maintained to ensure supercritical ethanol exchange. Following this, a depressurisation process was begun through opening the autoclave tap to allow the gradual evacuation and condensation of the supercritical ethanol. This process was completed over 4 hours to reduce the stress on the samples that can introduce fissures in the gels. Following controlled pressure reduction to atmospheric pressure while maintaining the constant high temperature of the heater (around 310°C), the autoclave taps were then then left completely open to maximise ethanol evacuation for around 15 minutes. At which point the heater was stopped and the autoclave was left overnight to cool. This prevents condensation formation on the samples that can dramatically damage the finished samples. Alternatively, to increase the speed of formation, the autoclave could be opened directly following depressurisation. However, care must be taken as the reactor remains around 280°C. Figure 4 shows a plot of the temperature and pressure in the autoclave during the supercritical drying process.

The samples could then be removed from the autoclave.

Alternative HMDS vapour treatment

If the silica aerogels have been formed without performing the liquid phase HMDS deposition, they will remain hydrophilic. An alternative approach for surface modification can be performed through placing the gels in a sealed container along with 4ml of liquid HMDS. The gels were placed on a mesh to prevent direct contact with the liquid HMDS.

The container was then heated to 120°C for 2 hours, following by 1 hour of continued heating with the cover removed in order to evaporate unreacted HMDS from the gel surface. Initial attempts suggested an increased level of hydrophobicity (140° compared to 120° from the liquid HMDS deposition approach). These differing approaches may provide a method to tailor the hydrophobicity of the eventual device for a given application.

Densifi cation

Aerogel samples were placed into an oven at 1000°C to undergo sintering, a process to densify the samples. A plot of the temperature in the oven during the densification process is shown in Figure 5. The densification of the samples means further reducing channel dimensions that result in more robust optofluidic modules (Figure 6). Importantly, while pore size is generally reduced, the refractive index remains lower than the fluidic core. In addition, further sintering can be carried out to render the material pure silica, enabling complex 3D geometries of microfluidic channels to be integrated into the sample. Microfluidic systems formed in pure silica would present multiple advantages such as high chemical resistance and temperature stability. In addition, pure silica windows can be prepared which are transparent and leak resistant.

Example 2: Characterisation of Channels Refractive index

The refractive index of aerogels remains extremely low throughout the range of aerogel densities due to the continuous high level of porosity of the material. This is demonstrated in Figure 7.

As a result, even with increased silica gel densities that are resultantly more robust, the refractive index remains considerably lower than that of the fluid core (n = 1.33). The effective refractive index can be accurately estimated using literature values. If required, measurements of the critical angle of reflection can be obtained using a He-Ne laser set-up (Y. Ozbakir, A. Jonas, A. Kiraz, and C. Erkey, “Total internal reflection-based optofluidic waveguides fabricated in aerogels,” J. Sol-Gel Sci. Technol., vol. 84, no. 3, pp. 522-534, 2017, doi: 10.1007/sl 0971-017-4426-8).

The critical angle can then be used to calculate the effective refractive index of the sample.

Channel surface roughness

A significant advantage of the present invention is the improved channel smoothness compared to other channel formation methods in aerogel (such as manual drilling). To quantify this initially, a 3D visualisation was used using a digital microscope, comparing a drilled and PET-G formed channel, as seen in Figure 8.

As can be seen in Figure 8 a) and b), the high level of rugosity in the drilled channel provided sufficient scattering for optical detection of the channel, where the sample rugosity could be visualised. However, the samples formed using PET-G (Figure 8 c) and d), were too smooth to be able to visualise. To better quantify the improved channel smoothness, a Surface Roughness Meter (contact profilometer) was then used on both samples.

3 measurements of 1mm were taken for both formation methods. The average of the 3 was then calculated and plotted. These measures were then treated to compensate for discrepancies due to misalignment with the horizontal sample position. This approach has demonstrated fluctuations around 12pm for the drilled samples compared to 0.4pm for channels formed by the process of the invention involving dissolution of a PET-G insert, therefore demonstrating a quantified improvement in surface channel smoothness.

Example 3: Materials for the polymer insert

Initial experiments were carried out using a polymer insert consisting of poly(ethylene terephthalate) (PET), but options for alternative polymer inserts were explored. A successful alternative insert material would require:

1) stability in ethanol (98%) at room temperature

2) dissolution in supercritical ethanol.

The present inventors have also identified that it is preferable to have a low crystallisation peak as measured by Differential Scanning Calorimetry (DSC) when heating to avoid structural changes that could fissure the aerogel samples during the supercritical drying process.

In the literature, a possible material that has been highlighted is polycarbonate (H. Jie, H. Ke, Z. Qing, C. Lei, W. Yongqiang, and Z. Zibin, “Study on depolymerization of polycarbonate in supercritical ethanol,” Polym. Degrad. Stab., vol. 91 , no. 10, pp. 2307-2314, 2006, doi: 10.1016/j.polymdegradstab.2006.04.012). In this study, Jie et al show that at 290°C for 30 minutes in supercritical ethanol, polycarbonate can be completely degraded. However, as a material, this approach remains more expensive and requires specialist 3D printers to attain the high extrusion and bed temperatures required to print successfully with polycarbonate.

Nylon 6 (M. Goto, “Chemical recycling of plastics using sub- and supercritical fluids,” J. Supercrit. Fluids, vol. 47, no. 3, pp. 500-507, 2009, doi: 10.1016/j. supflu.2008.10.011) has also been demonstrated to be soluble in supercritical water with a 100% yield at 290°C for 60 minutes. Furthermore, the DSC (differential scanning calorimetry) of both polycarbonate and nylon have very low levels of crystallisation which again make them good potential candidates for the insert.

Phenol resin (M. Goto, “Chemical recycling of plastics using sub- and supercritical fluids,” J. Supercrit. Fluids, vol. 47, no. 3, pp. 500-507, 2009, doi: 10.1016/j. supflu.2008.10.011) is another possibility due to research showing degradation in supercritical water. The presence of Na2COs catalyst resulted in monomer yield of 90%.

However, in the first instance polymers that are commonly used in 3D printers were considered, namely:

1) PLA (Polylactic Acid)

2) TPU (Thermoplastic polyurethane)

3) ABS (Acrylonitrile Butadiene Styrene)

The filaments remained stable in ethanol at room temperature, therefore fulfilling the first criteria. The solubility of these three filament types was then tested by taking 3 samples of each (2cm by 1 ,75mm diameter) and performing the same supercritical drying process as for the aerogels. In this way, increasing the temperature and pressure gradually over 4h to 110 bar and 300°C. The mass of the filaments was measured prior to the supercritical processing and then the samples were visually assessed and weighed in order to determine the effect of supercritical processing. The results of which can be seen below in Figure 10.

As can be seen in Figure 10, the supercritical ethanol process was unsuccessful in the dissolution of both the ABS and TPU polymers. However, the dissolution of PLA has occurred leaving no trace of the PLA within the tube (Figure 10). Therefore, inserts can be prepared from PLA as an alternative to PET and derivatives thereof, typically by 3D printing. PLA is a highly common, low-cost 3D printing filament.

Example 4: Aerogel as LCW in optofluidic module

A novel manifold system was developed to integrate and use the silica aerogel samples for detection purposes as shown in Figure 11. This system uses a dual Z-fold design in order to direct both fluid and light (through optical fibre integration) to pass through the LCW modules. The dual integration of fluid and light enables UV-Vis detection spectroscopy.

The path length and channel design/form can be easily altered in order to modify/im prove the detection performance.

The manifold system is typically composed of two identical modules formed in aluminium with fluid (1) and optical fibre (2) inputs and outputs. The optical fibre inputs are sealed using quartz windows (3) to enable high rates of optical transmission while controlling fluid flow through the system. Aerogel samples (4) can be placed between the two modules, that can then be fixed together using the alignment bars (5) to ensure optical and fluidic channel alignment. Waterproof seals (for example O-rings) are used on both modules to ensure leak-free connections, while remaining elastic to prevent damaging the aerogel samples. A fibre optic cable connected to the optical output (6) is used to detect the light that is transmitted within the system.

It was possible to measure absorbance of a blue food dye using the aerogel manifold at varying concentrations of blue food dye.

UV-Vis absorbance spectroscopy was performed using the manifold with integrated aerogel samples. Firstly, the sample was placed within the manifold and held in place by fixing the modules on each side. Waterproof seals were used on both ends to ensure leak-free connections with the aerogel sample. Fluid was then passed through the channel of the hydrophobic aerogel. The optical transmission (%) of the system was determined using multiple solutions of varying blue dye concentration (g/L). The transmission of each measurement can then be used to calculate the absorbance through using the equation (A = 2 * LN(10 - LN(T%)). The absorbance peak of each curve was then transposed onto a calibration curve graph as seen in Figure 12.

The present invention will now be explained further with reference to the following clauses:

1. A silica aerogel obtainable by a process comprising: i) preparing a sol from a silica precursor, ethanol, water and an acid and/or base catalyst; wherein the silica precursor comprises tetramethyl orthosilicate or tetraethyl orthosilicate; and wherein the molar ratio of silica precursor to water is from about 1 :2 to about 1 :5; and the molar ratio of silica precursor to ethanol is from about 1:0.1 to about 1 :25; ii) agitating the sol; iii) allowing the sol to set to form a silica alcogel in a first sealed container; iv) contacting the set silica alcogel formed in step iii) with further ethanol in a second sealed container; v) contacting the product of step iv) with ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa, preferably about 280 °C and about 10 MPa; vi) reducing the pressure to allow release of ethanol from pores in the silica alcogel to form the silica aerogel, optionally to atmospheric pressure; and vii) optionally reducing the temperature to room temperature.

2. A silica aerogel comprising one or more channels, each channel having a surface roughness of from about 0.1 to about 10 pm, an internal diameter in the range of from about 100 pm to about 2900 pm and a length of from about 5 mm to about 200 mm.

3. A silica aerogel product comprising one or more channels obtainable by a process comprising: i) preparing a sol from a silica precursor, ethanol, water and an acid and/or base catalyst; wherein the silica precursor comprises tetramethyl orthosilicate or tetraethyl orthosilicate; and wherein the molar ratio of silica precursor to water is from about 1 :2 to about 1 :5; and the molar ratio of silica precursor to ethanol is from about 1:0.1 to about 1 :25; and ii) agitating the sol; iii) a) inserting one or more polymer inserts into the sol, wherein the polymer is suitable for complete dissolution in ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa, but is not suitable for dissolution in liquid ethanol at room temperature and atmospheric pressure; b) allowing the sol to set to form a silica alcogel in a first sealed container; iv) contacting the silica alcogel containing the one or more polymer inserts formed in step (iii) with further ethanol in a second sealed container; v) contacting the product of step iv) with ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa, preferably about 280 °C and about 10 MPa; vi) reducing the pressure to allow release of ethanol from pores in the silica alcogel to form the silica aerogel, optionally to atmospheric pressure; and vii) optionally reducing the temperature to room temperature.

4. A process for preparing a silica aerogel product comprising one or more channels, the process comprising: i) preparing a sol from a silica precursor, ethanol, water and an acid and/or base catalyst; wherein the silica precursor comprises tetramethyl orthosilicate or tetraethyl orthosilicate; and wherein the molar ratio of silica precursor to water of from about 1 :2 to about 1 :5; and the molar ratio of silica precursor to ethanol is from about 1:0.1 to about 1:25; and ii) agitating the sol; iii) a) inserting one or more polymer inserts into the sol, wherein the polymer is suitable for complete dissolution in ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa but is not suitable for dissolution in liquid ethanol at room temperature and atmospheric pressure; b) allowing the sol to set to form a silica alcogel in a first sealed container; iv) contacting the silica alcogel containing the one or more polymer inserts formed in step (iii) with further ethanol in a second sealed container; v) contacting the product of step iv) with ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa, preferably about 280 °C and about 10 MPa; vi) reducing the pressure to allow release of ethanol from pores in the silica alcogel to form the silica aerogel, optionally to atmospheric pressure; and vii) optionally reducing the temperature to room temperature.

5. A silica aerogel product or process according to any preceding clause wherein the polymer insert is poly(ethylene terephthalate), poly(ethylene terephthalate) glycol or poly(lactic acid). A silica aerogel, silica aerogel product or process according to any preceding clause wherein step i) comprises preparing a first solution of the silica precursor in ethanol; preparing a second solution comprising the acid and/or base catalyst, water and ethanol; and mixing the first and second solutions, optionally mixing equal volumes of the first and second solutions. A silica aerogel, silica aerogel product or process according to clause 6 wherein step i) comprises gradually mixing the first and second solutions, optionally for a period of about 20 to about 60 seconds, optionally about 30 seconds. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the catalyst is a base. A silica aerogel, silica aerogel product or process according to clause 8 wherein the molar ratio of silica precursor to base is from about 1:0.0001 to about 1 :0.1, preferably about 1:0.01. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the silica precursor is tetramethyl orthosilicate. A silica aerogel, silica aerogel product or process according to any of clauses 1 to 7 wherein the silica precursor is tetraethyl orthosilicate, and wherein the catalyst comprises a base and an acid, optionally wherein the base is added to the second solution prior to mixing and the acid is added to the mixture of the first and second solutions. A silica aerogel, silica aerogel product or process according to any preceding clause wherein step ii) comprises agitating the sol for a period of from about 10 to about 14 mins, optionally about 12 mins, optionally at about 200 to 300 rpm, optionally at about 250 rpm. A silica aerogel, silica aerogel product or process according to any preceding clause wherein in step iii) the sol sets for a period of from about 4 to about 10 hours. A silica aerogel, silica aerogel product or process according to any preceding clause wherein in step iii) and/or step iv) the sealed container is a sealed mould. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the first sealed container is formed of a polymer suitable for complete dissolution in ethanol at a temperature of from about 240 °C to about 320 °C and a relative pressure of from about 6.3 MPa to about 12 MPa but is not suitable for dissolution in liquid ethanol at room temperature and atmospheric pressure, and wherein the first sealed container dissolves in step v). A silica aerogel, silica aerogel product or process according to any preceding clause wherein the first sealed container and the second sealed container are the same container. A silica aerogel, silica aerogel product or process according to any preceding clause wherein step iv) is for a period of from about 24 to about 72 hours. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the ethanol is renewed during step iv). A silica aerogel, silica aerogel product or process according to any preceding clause wherein step v) involves a gradual increase of temperature and pressure from room temperature and atmospheric pressure to a final temperature of from about 240 °C to about 320 °C and a final relative pressure of from about 6.3 MPa to about 12 MPa over a period of about 2 to about 5 hours, optionally about 4 hours, optionally the temperature is raised about 75 °C per hour to the final temperature. A silica aerogel, silica aerogel product or process according to any preceding clause wherein in step i) the molar ratio of silica precursor to water is about 1:4. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the aerogel has a bulk density of from about 0.003 to about 0.5 g/cm ³ . A silica aerogel, silica aerogel product or process according to any preceding clause wherein the aerogel has a refractive index below 1.2, preferably from about 1.01 to about 1.10. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the silica aerogel has a mean pore diameter of from about 20 nm to about 190 nm, optionally wherein the mean pore diameter of the silica aerogel is from about 20 to about 180nm when the silica precursor is tetramethyl orthosilicate or from about 60 to about 190nm when the silica precursor is tetraethyl orthosilicate. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the aerogel has a porosity of from about 50% to about 99.8%, preferably from about 95% to about 99.8%. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the silica aerogel is substantially free of detectable fissures. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the catalyst comprises or is a base catalyst, preferably ammonium hydroxide, sodium hydroxide, sodium carbonate, or potassium carbonate. A silica aerogel, silica aerogel product or process according to any of clauses 1 to 7 and 10 to 26 wherein the catalyst comprises or is an acid catalyst, preferably ammonium fluoride, oxalic acid or nitric acid. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the silica precursor further comprises one or more of methyltrimethoxysilane, methyltriethoxysilane, dimethylchlorosilane, trimethylethoxysilane, ethyltriethoxysilane or phenyltriethoxysilane. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the silica precursor is present in the mixture of step i) in an amount of from about 20 to about 80 wt%. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the base is present in the mixture of step i) in an amount of from about 0.003 to about 0.2 wt%. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the ethanol is present in the mixture of step i) in an amount of from about 1 to 80 wt%. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the water is present in the mixture of step i) in an amount of from about 5 to about 40 wt%. A silica aerogel, silica aerogel product or process according to any preceding clause further comprising a step of modifying the hydrophobicity of the aerogel by contacting the aerogel with hexamethyldisilazane. A silica aerogel, silica aerogel product or process according to clause 33, which comprises vapour deposition of hexamethyldisilazane after step vii). A silica aerogel, silica aerogel product or process according to clause 33 wherein the ethanol used in step iv) comprises about 15 wt % to about 30 wt %, optionally about 20 wt % hexamethyldisilazane. A silica aerogel, silica aerogel product or process according to any preceding clause wherein step v) occurs for a period of from about 6 to about 8 hours. A silica aerogel, silica aerogel product or process according to any preceding clause wherein step v) occurs in an autoclave. A silica aerogel, silica aerogel product or process according to any preceding clause wherein step vi) occurs for a period of from about 3 to about 4 hours. A silica aerogel, silica aerogel product or process according to any preceding clause wherein in step vi) the temperature is maintained in the range of from about 240 °C to about 320 °C and the pressure gradually reduces from a relative pressure of from about 6.3 MPa to about 12 MPa to atmospheric pressure. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the ethanol is 99% ethanol. A silica aerogel product or process according to any of clauses 2 to 40 wherein the insert is prepared by fused deposition modelling 3D printing or is formed from one or more polymer filaments. A silica aerogel product or process according to clause 41 , wherein the polymer filament has an outer diameter of from about 100 pm to about 2850 pm. A silica aerogel product or process according to clause 41 , wherein the polymer filament is an extruded polymer filament having an outer diameter of from about 100 pm to about 1000 pm. A silica aerogel product or process according to clauses 41 or 42, wherein the polymer filament is heated for a period of about 2 mins to straighten the filament prior to inserting the filament into the sol, optionally in an oven at about 70 to 90 °C, preferably 80 °C. A silica aerogel, silica aerogel product or process according to any preceding clause wherein the process further comprises a sintering step viii) wherein the product of step vi) or step vii) is heated to a temperature of from about 900 to 1100 °C, optionally about 1000 °C for a period of from about 1 to about 6 hours, optionally about 2 hours. A silica aerogel, silica aerogel product or process according to clause 45 wherein the sintering step causes a 50% to 90% increase in bulk density. A silica aerogel, silica aerogel product or process according to clause 45 or clause 46 wherein step viii) converts the silica aerogel into a product which consists essentially of silica. A silica aerogel product or process according to any of clauses 45 to 47 wherein the internal diameter of the one or more channels can be controlled by altering the time period of step viii). A silica aerogel product or process according to any of clauses 2 to 48 wherein the one or more channels each have a hydraulic diameter in the range of from about 100 pm to about 2850 pm, preferably from about 200 pm to about 600 pm. A silica aerogel product or process according to any of clauses 2 to 49 wherein the one or more channels each have a length in the range of from about 5 mm to about 200 mm. A silica aerogel product or process according to any of clauses 2 to 50 wherein the one or more channels each have a substantially circular, square or rectangular crosssection. A silica aerogel product or process according to any of clauses 2 to 51 wherein the one or more channels each have a surface roughness of from about 0.1 to about 10 pm, optionally from about 0.1 to about 5 pm, optionally from about 0.1 to about 1 pm. A silica aerogel product or process according to any of clauses 2 to 52 wherein the one or more channels each have an optical loss of less than or equal to about 1 .5 dB/cm. A silica aerogel product or process according to any of clauses 2 to 53 wherein one or more of the channels is non-linear. 55. A silica aerogel product or process according to any of clauses 2 to 54 wherein the aerogel product further comprises one or more chambers or reservoirs.

56. A silica aerogel product or process according to any of clauses 2 to 55 wherein the one or more channels and/or one or more chambers or reservoirs are in fluid connection.

57. A silica aerogel product or process according to any of clauses 2 to 56 wherein the one or more channels comprise one or more fluid inlets and one or more fluid outlets.

58. A silica aerogel product or process according to any of clauses 2 to 57 wherein the one or more channels comprise one or more junctions, optionally a T-junction, a cross-junction, an inter-junction or a Y-shape junction.

59. A silica aerogel product or process according to any of clauses 2 to 58 wherein one or more of the channels is curved, U-shaped or a serpentine channel.

60. A silica aerogel product or process according to any of clauses 2 to 59 wherein the channels are formed in the aerogel in step v) when the ethanol reaches a supercritical point and dissolves the polymer insert.

61. A liquid core waveguide cell comprising the silica aerogel product as defined in any of clauses 2, 3 or 5 to 60.

62. A liquid core waveguide cell according to clause 61 wherein the one or more channels provide a pathlength of from about 0.5 cm to about 20 cm, optionally from about 1 cm to about 20 cm, optionally from about 1 cm to about 15 cm, optionally from about 1 cm to about 10 cm.

63. An optofluidic device comprising: i) a liquid core waveguide cell as defined in clause 61 or clause 62; ii) a light source; iii) a light inlet enabling passage of light from the light source to the one or more channels of the liquid core waveguide cell; iv) one or more sample sources, wherein each sample is a fluid; v) one or more sample inlets; each sample inlet enabling passage of a sample from a sample source to the one or more channels of the liquid core waveguide cell; vi) one or more sample outlets; and vii) a detector for detecting light transmission or absorbance by the one or more samples in the liquid core waveguide cell.

64. An optofluidic device according to clause 63 wherein the light source is a fiber optic cable, a laser, ultraviolet light, infrared light or a spectral light source.

65. An optofluidic device according to clause 63 or clause 64 wherein the device measures absorbance of ultraviolet, visible or near infra-red light by the one or more samples. An optofluidic device according to any of clauses 63 to 65 wherein the light has a wavelength of from about 230 to about 3000 nm, optionally from about 230 to about 1400 nm, most preferably from about 230 to about 800 nm. An optofluidic device according to any of clauses 63 to 66 comprising two modules, wherein the liquid core waveguide cell (4) is located between the two modules, and wherein the modules are attached by means of a plurality of alignment bars (5), each module comprising a sample inlet (1) and a light inlet (2), and a sample outlet and a light outlet, wherein the light inlet is an optical fiber inlet which is sealed using a silica window (3), and wherein each module is attached to the liquid core waveguide cell by means of a waterproof seal. An optofluidic device according to any of clauses 63 to 67 comprising the product as defined in any of clauses 45 to 48 as a silica window. A liquid core waveguide cell according to clause 61 or clause 62 or an optofluidic device according to any of clauses 63 to 68 wherein the silica aerogel product is gas permeable to prevent formation of bubbles and/or eliminate bubbles formed in the core waveguide cell or optofluidic device.