MONITORING OF IN VITRO PROTEIN SYNTHESIS FIELD OF THE INVENTION Provided herein are methods, and compositions for the on-device detection of protein synthesis. The methods are applicable to monitoring on a microfluidic device. BACKGROUND TO THE INVENTION When performing cell-free protein synthesis at microfluidic scale in a microfluidic device, such as a digital microfluidic device, it is useful to detect in real-time the proteins that are synthesized from said cell-free protein synthesis reaction. However, it is difficult to perform real-time detection of proteins in a cell-free protein synthesis reaction environment. The reaction contains many other proteins and biomolecules at high concentration, making non- specific protein detection via standard protein staining methods difficult (e.g., Coomassie Brilliant Blue G-250, SYPRO ^TM Ruby, Silver staining). Immunostaining or affinity-based purification followed by non-specific proteins staining are equally unhelpful as significant washing on a solid support must be performed to prevent background interference. As washing is known to be difficult in microfluidic and digital microfluidic devices, background interference may become debilitating. Currently existing luminescent complementation approaches cannot achieve prolonged real- time detection in cell-free protein synthesis reactions, which often extend beyond 16 hours. This limit is due to a combination of reasons including O2 consumption by the luminescence- generating enzyme that competes with cell-free protein synthesis O2 requirements and temporary or permanent exhaustion of luminescent substrate over 3 – 24 hours of recombinant protein expression detection. Proteins of interest may also be expressed as a fusion to a fluorescent protein, such as green fluorescent protein (GFP). However, GFP is a 26.9 kDa protein, which is the typical size for most fluorescent proteins. Tags of this size increase the total size of the protein of interest, especially if the protein of interest must be tagged with other large fusion proteins such as maltose-binding protein (MBP), which is 42.5 kDa. Given the average size of a human protein is ~52 kDa and the average size of an E. coli protein is ~35 kDa (Kim, Y. E. et al. Annu. Rev. Biochem. 2013. 82:323-355), the addition of a comparably sized fluorescent protein tag can significantly change the biological function and biophysical characteristics of a protein. Many pieces of prior art disclose the use of sub-component tags for monitoring expression in a cellular system. For example US 7,666,606 discloses protein-protein interaction detection systems using microdomains. Schinn et al. Biotechnol. Bioeng 114 10 October 2017 2412-2417. (https://onlinelibrary.wiley.com/doi/10.1002/bit.26305) discloses Rapid in vitro screening for the location-dependent effects of unnatural amino acids on protein expression and activity - Schinn - 2017 - Biotechnology and Bioengineering - Wiley Online Library. Split green fluorescent protein (GFP) has been used in a variety of applications. ccGFP is a split green fluorescent protein engineered from a tetrameric GFP found in Corynactis californica, a bright red colonial anthozoan similar to sea anemones and scleractinian stony corals, as described in Nguyen et al; Nature Scientific Reports volume 11, Article number: 18440 (2021). The inventors herein have optimised the use of ccGFP for use in cell-free protein synthesis. SUMMARY Here, we report the surprising discovery that a split peptide system can be engineered to perform in situ, fluorescence-based monitoring of the expression of a protein of interest in cell-free protein synthesis reactions. The monitoring can be performed on device during the course of the expression, so can be used in real-time or as an end-point measurement. Disclosed herein is a method for the real-time monitoring of in vitro protein synthesis comprising a. in vitro transcription and translation of a protein of interest fused to a peptide tag; and b. monitoring the presence of the peptide tag using a further polypeptide which in the presence of the peptide tag produces a detectable signal. Disclosed herein is a method for the monitoring of cell-free protein synthesis in a droplet on a digital microfluidic device comprising a. cell-free transcription and translation of a protein of interest fused to a peptide tag; and b. monitoring the presence of the peptide tag using a further polypeptide which in the presence of the peptide tag produces a detectable signal. Disclosed herein is a method for the monitoring of cell-free protein synthesis in a droplet on a digital microfluidic device comprising a. cell-free transcription and translation of a protein of interest fused to a ccGFP ₁₁ peptide tag; and b. monitoring the presence of the peptide tag using a further ccGFP _1-10 polypeptide which in the presence of the ccGFP ₁₁ peptide tag produces a detectable signal. The use of the terms “in vitro” and “cell-free” may be used interchangeably herein. The detectable signal may be for example fluorescence or luminescence. The detectable signal may also be caused by the binding of a ligand to the complemented oligopeptide, peptide, or polypeptide tag fused to the protein of interest. The detectable signal may also be caused by the binding of the polypeptide to the protein of interest fused to a His-tag. Any in vitro transcription and translation may be used, for example extract-based systems derived from rabbit reticulocyte lysate, human lysate, Chinese Hamster Ovary lysate, a wheat germ, HEK293 lysate, E. coli lysate, yeast lysate. Alternatively the in vitro transcription and translation may be assembled from purified components, for example a system of purified recombinant elements (PURE). The in vitro transcription and translation may be coupled or uncoupled. The peptide tag may be one component of a fluorescent protein and the further polypeptide a complementary portion of the fluorescent protein. The fluorescent protein could include sfGFP, GFP, eGFP, deGFP, frGFP, eYFP, eBFP, eCFP, Citrine, Venus, Cerulean, Dronpa, DsRED, mKate, mCherry, mRFP, FAST, SmURFP, miRFP670nano. For example the peptide tag may be GFP ₁₁ and the further polypeptide GFP _1-10 . The peptide tag may be one component of sfCherry. The peptide tag may be sfCherry ₁₁ and the further polypeptide sfCherry _1-10 . The peptide tag may be CFAST ₁₁ or CFAST ₁₀ and the further polypeptide NFAST in the presence of a hydroxybenzylidene rhodanine analog. The peptide tag may be ccGFP ₁₁ and the further polypeptide ccGFP _1-10 . For example, the GFP _1-10 polypeptide amino acid sequence could be derived from sfGFP: SEQ ID NO: 1 Alternatively, the GFP _1-10 polypeptide amino acid sequence could be further mutated from the sequence above to become brighter more quickly upon complementation. The sequence may have a greater than 90 % homology to any sequence mentioned herein. The sequence may have a greater than 95 % homology to any sequence mentioned herein. SEQ ID NO: 2 The GFP _1-10 polypeptide amino acid sequence could also be derived from ccGFP, having a greater than 90 or 95% homology to: SEQ ID NO: 3 (ccGFP _1-11 ) SEQ ID NO: 4 (ccGFP _1-10 ) SEQ ID NO 5 (ccGFP _1-10 ) Nucleic acid sequence to express seq ID No 5 ccGFP _1-10 ; SEQ ID NO: 6 SEQ ID NO 7 Nucleic acid sequence to express seq ID No 7 ccGFP _1-10 ; SEQ ID NO: 8 The complementary GFP ₁₁ peptide amino acid sequence could be the following: or a truncated version thereof. Truncations may involve a shortening of up to 5 amino acids from the N terminus, the C terminus or a combination thereof. GFP ₁₁ or GFP _1-10 can be fused to the protein of interest through an amino acid linker. In one embodiment, the oligopeptide, peptide, or polypeptide linker can be 0 – 50 amino acids. Also disclosed are nucleic acid sequences for expressing particular tags. Nucleic acid sequences include SEQ ID NO: 16 SEQ ID NO: 17 or a truncated version thereof. These sequences may be repeated one or more times to produce a protein having multiple GFP ₁₁ domains. For example, the sfCherry _1-10 polypeptide amino acid sequence could be: SEQ ID NO: 18 The complementary sfCherry11 peptide amino acid sequence could be: SEQ ID NO: 19 sfCherry11 or sfCherry _1-10 can be fused to the protein of interest through an amino acid linker. In one embodiment, the oligopeptide, peptide, or polypeptide linker can be 0 – 50 amino acids. For example, the NFAST polypeptide amino acid sequence could be: SEQ IS NO: 20 The complementary CFAST11 peptide amino acid sequence could be: SEQ ID NO: 21 Or the complementary CFAST10 peptide amino acid sequence could be: SEQ ID NO: 22 NFAST, CFAST11, and/or CFAST10 can be fused to the protein of interest through an amino acid linker. In one embodiment, the oligopeptide, peptide, or polypeptide linker can be 0 – 50 amino acids. The peptide tag may also be one component of a protein that forms a detectable substrate, such as a luminescent or colorigenic substrate. The protein could include beta-galactosidase, beta-lactamase, or luciferase. The protein may be fused to multiple tags. For example the protein may be fused to multiple GFP ₁₁ peptide tags and the synthesis occurs in the presence of multiple GFP _1-10 polypeptides. The GFP may be ccGFP or sfGFP. For example the protein may be fused to multiple ccGFP ₁₁ peptide tags and the synthesis occurs in the presence of multiple ccGFP _1-10 polypeptides. The protein of interest may be fused to one or more sfCherry ₁₁ peptide tags and one or more GFP ₁₁ peptide tags and the synthesis occurs in the presence of one or more GFP _1-10 polypeptides and one or more sfCherry _1-10 polypeptides. Any protein of interest may be synthesised. The protein may be an enzyme, for example a terminal deoxynucleotidyl transferase (TdT) enzyme or a truncated version thereof or the homologous amino acid sequence of a terminal deoxynucleotidyl transferase (TdT) enzyme in other species or the homologous amino acid sequence of Polμ, Polβ, Polλ, and Polθ of any species or the homologous amino acid sequence of X family polymerases of any species. The synthesis may be performed in a microfluidic device, for example an electrowetting-on- dielectric (EWoD) device. Alternatively the synthesis may be performed in a microtitre plate format. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows schematically one embodiment of the invention. The cell-free protein synthesis reaction contains a nucleic acid template containing the expression cassette for the gene of interest fused to a detectable tag, which is then expressed into the protein of interest through coupled or uncoupled in vitro transcription and in vitro translation. The protein of interest is thus fused to a detectable peptide tag at the N- or C-termini. The nature of the detectable peptide tag is that it can be complemented with a complementary polypeptide resulting in a protein that is fluorescent. Figure 2 shows real-time detection of the expression of a protein of interest in a cell-free protein synthesis reaction under blue light illumination. A sfGFP ₁₁ _TdT was cloned into the p70a vector and added (2 µl, 30 nM) to MyTXTL Sigma70 CFPS Master Mix (10 µl; Arbor Biosciences) resulting in a 12 µl total reaction volume. A sfGFP _1-10 solution (0.9 mg/ml; 364 mM urea) was subsequently added to the CFPS reaction at 0, 1, 2, 3, 4, 6, and 12 µl resulting in a final CFPS urea concentration of 0, 28, 51, 72, 90, 120, and 180 mM, respectively. The reactions were then incubated in a 200 µl PCR tube at 29 °C. Photographs were taken under 470 nm blue light illuminator using a 580 nm filter. Figure 3 shows graphically the real-time detection of the expression of a protein of interest in a cell-free protein synthesis reaction of Figure 2. Fluorescence measurements were taken at indicated timepoints using an excitation wavelength of 485 nm and an emission wavelength of 520 nm. A negative control CFPS reaction containing sfGFP _1-10 solution but containing no sfGFP ₁₁ _TdT p70a plasmid was used to subtract background fluorescence. Figure 4 shows (A) droplets on a digital microfluidic device containing cell-free protein synthesis lysate. The droplets annotated with a white arrow have been merged with recombinant sfGFP _1-10 detector polypeptide (t = 0). The top two rows additionally contain a DNA construct for the expression of a protein with a sfGFP ₁₁ peptide tag (solid white arrow). (B) Fluorescence image showing the same droplets as in panel (A) after six hours have elapsed. Only the droplets that contain both the expressed protein with sfGFP ₁₁ peptide tag and recombinant sfGFP _1-10 detector polypeptide (i.e. solid arrows) show a significant increase in fluorescence. Drops without DNA construct (hollow arrows) or with no sfGFP _1-10 detector (no arrows) are not fluorescent. (C) Fluorescence quantification of drops in panel (B). Only the droplets with lysate, DNA construct, and sfGFP _1-10 detector polypeptide show a significant increase in fluorescence, indicating protein expression. The negative controls, i.e. bottom row of drops in (B) contain no DNA construct and so low fluorescence, even in the presence of sfGFP _1-10 detector. Numbering of drops in (B) and bars in (C) match. Figure 5 Images extracted from a time course experiment whereby droplets of cell-free protein synthesis (CFPS) lysate, optionally with a DNA construct for a protein of interest (POI – which here is maltose binding protein (MBP) tagged with a sfGFP ₁₁ peptide) and/or sfGFP _1-10 polypeptide, were incubated on a digital microfluidic device for 4 hours and imaged periodically. Only the droplets containing lysate, DNA construct, and sfGFP _1-10 polypeptide show a significant increase in fluorescence over the course of the experiment, as seen in the right-hand column of images. Figure 6 A chart showing the real-time fluorescence increase seen in the droplets present in Figure 5. Quantification of fluorescence was performed in Image J and the values presented have been subject to normalisation by subtracting the background fluorescence seen in droplets of CFPS lysate and sfGFP _1-10 (i.e. no DNA construct so no protein of interest, POI, expressed). Only the droplets containing all components – lysate, DNA construct for POI, and sfGFP _1-10 polypeptide – generate fluorescence over background. Figure 7 In this experiment, the recombinantly purified sfGFP _1-10 detector was added to the cell-free lysate at the same time as the DNA construct encoding for a sfGFP ₁₁ -tagged protein. The fluorescent signal was monitored over time in a plate reader. A chart showing fluorescence signal increases over time from plate reader measurements of CFPS reactions. sfGFP _1-10 detector is present in all three reactions from the start, enabling real-time detection of protein expression. In the two conditions which have a protein of interest fused to a sfGFP ₁₁ tag, fluorescence increases compared to the negative control condition where there is no sfGFP ₁₁ tagged protein. POI1 is an engineered terminal transferase and POI2 is SARS-COV- NL63-Mpro. Both had a 3xsfGFP ₁₁ tag at the N terminus. Figures 5-7 demonstrate real-time detection of protein expression. sfGFP _1-10 detector polypeptide is present right from the start of the experiment. Fluorescent signal increases as sfGFP ₁₁ tags are expressed. These experiments were are performed in a base fluid comprising 0.2% Span 85 in dodecamethylpentasiloxane rather than Tween20 in aqueous and no surfactant in dodecane. Figure 8 Demonstration that the complementation of the sfGFP ₁₁ tag with recombinant GFP _1-10 detector is inhibited in the presence of 0.1% w/v Tween20 surfactant. The complementation assay was performed in TNG Buffer (50 mM Tris, pH 7.4, 0.1 M NaCl, 10% v/v glycerol). Fluorescence was measured after 24 hours incubation at 29°C. The first four pairs are controls: sfGFP and deGFP are complete fluorescent proteins, while MBP-GFP ₁₁ and GFP _1-10 are partial fluorescent proteins (tag and detector respectively) hence show no fluorescence. The ten sample pairs have the same quantity of MBP-GFP ₁₁ but increasing molar excesses of recombinant GFP _1-10 detector polypeptide. The data also shows that increasing the molar excess of GFP _1-10 detector polypeptide over the GFP ₁₁ peptide tag leads to enhanced fluorescence signal. Figure 9 Demonstration that the complementation of the sfGFP ₁₁ tag with recombinant sfGFP _1- 10 detector is inhibited in the presence of 0.1% w/v Tween20 surfactant. The complementation assay was performed in a cell-free lysate (σ70 Linear Master Mix, Arbor Bioscience). Fluorescence was measured after 24 hours incubation using a plate reader. The first four pairs are controls: sfGFP and deGFP are complete fluorescent proteins, while MBP-GFP ₁₁ and GFP _1-10 are partial fluorescent proteins (tag and detector respectively) hence show no fluorescence. The three sample pairs have the same quantity of MBP-GFP ₁₁ but increasing molar excesses of recombinant GFP _1-10 detector polypeptide. Figure 10 Images extracted from a time course experiment whereby droplets of cell-free protein synthesis (CFPS) lysate, optionally with a DNA construct for a protein of interest (POI – which here is either a variant of TdT or a viral Mpro protease tagged with a GFP ₁₁ peptide) and/or with a DNA construct for GFP _1-10 polypeptide, were incubated on a digital microfluidic device for 6 hours and imaged periodically. Only the droplets containing lysate, DNA constructs for the GFP ₁₁ tagged POI and the GFP _1-10 polypeptide show a significant increase in fluorescence over the course of the experiment, as seen in the right-hand column of images. Lysates which had DNA for only GFP _1-10 or the POI alone did not show an increase in fluorescence. Figure 11: A chart showing the real-time fluorescence increase seen in the droplets present in Figure 10 (as well as a T=1 hour time point, which is similar to T=0 hour and was not included in Figure 10). Quantification of fluorescence was performed in ImageJ and the values presented have been subject to normalisation by subtracting the background fluorescence seen in droplets of CFPS lysate and GFP _1-10 (i.e. no POI DNA construct so no protein of interest, POI, expressed). Only the droplets containing all components – lysate, DNA constructs for both POI and GFP _1-10 polypeptide – generate fluorescence over background. Figure 12: A chart showing the real-time fluorescence of protein expression measured in a plate reader. DNA constructs for three different POIs (1: a TdT variant with asf GFP ₁₁ peptide tag, 2: a TdT variant with a different GFP ₁₁ peptide tag, 3: a viral Mpro protease with a sfGFP ₁₁ peptide tag) were co-expressed with DNA constructs for sfGFP _1-10 in cell-free protein synthesis (CFPS) lysate. Only the lysates which had DNA constructs for both the GFP ₁₁ -tagged POI and the GFP _1-10 showed an increase in fluorescence over time, whereas the lysates with DNA for only sfGFP _1-10 or sfGFP ₁₁ -tagged POI did not. Figure 13: The dispensing and merging of split ccGFP reagents into 384 droplet complementation array (white light images). Panel 1 shows 384-droplet MBP-ccGFP_S ₁₁ dispense (from edge 1, 3 and 4 reservoirs). Panel 2 shows ccGFP _1-10 dispensing, forming second offset 384 droplet array (768 droplets in total; from edge 2 reservoirs). Panel 3 depicts the final merged 384 droplet array (complementation incubation); A = 1 mg/mL FL ccGFP, B = TNG buffer, C = 1 mg/mL, D = 2.5 mg/mL ccGFP _1-10 . Figure 14: The developing fluorescent image due to ccGFP _1-10 :MBP-ccGFP_S ₁₁ complementation (blue light images). The image in panel 1 is taken at t = 0 min, the image in panel 2 is taken at t = 40 min and image 3 is taken at t = 110 min. On panel 3, A = 1 mg/mL FL ccGFP, B = TNG buffer, C = 1 mg/mL, D = 2.5 mg/mL ccGFP _1-10 . All images are taken at 400 ms exposure. Figure 15: The ccGFP _1-10 :MBP-ccGFP_S ₁₁ eDrop complementation time course. Figure 16: The expression array dispense and incubation (white light images). Panel 1 shows a 192 droplet array after edge 1 component dispense. Panel 2 shows a 384 droplet array after edge 1, 3 and 4 component dispense. Panel 3 shows a 192 droplet array after expression array merge. Panel 4 shows a 384 droplet array after expression array split. Figure 17: The expression array dispense and incubation (blue light image). Figure 18: The complementation array dispense and incubation. Panel 1 shows a 768 droplet complementation array after edge 2 component dispense (white light image). Panel 2 shows a 384 droplet array after complementation array merge and incubation (white light image). Panel 3 shows a 384 droplet array after complementation incubation end-point (blue light image). Figure 19: The eDrop complementation end point fluorescence. Panel A shows lysate in-situ complementation of recombinant MBP-ccGFP_S ₁₁ with recombinant ccGFP _1-10 . Panel B shows lysate in-situ complementation of expressed MBP-ccGFP_S ₁₁ (LEC_576) with recombinant ccGFP _1-10 . Figure 20: shows the sequence and domain structures for ccGFP _1-11 . Figure 21: The effect of truncating the sequence of the ccGFP ₁₁ binding domain on fluorescent signal. Figure 22: The effect of truncating the sequence of the ccGFP ₁₁ binding domain on fluorescent signal. Figure 23: The experimental result from 24 different proteins expressed in a reconstituted cell- free protein synthesis system in droplets on an electrowetting on dielectric (EWoD) device. Each construct contains a GFP ₁₁ tag. In the rows marked Screen, the GFP _1-10 detector species is present from the start of expression. The rows marked Endpoint shows the fluorescence signal from 10 hours expression in the absence of GFP _1-10 detector species followed by 5 hours complementation with the GFP _1-10 detector species. This experiment showed significant differences between expression/complementation in Screen BioInk compared to Endpoint detection. Figure 24: Normalised data for ccGFP _1-10 constructs and sfGFP _1-10 constructs added to CFPS generated MBP_GFP ₁₁ (sf or cc). All ccGFP _1-10 constructs have quicker maturation times and high max fluorescence. DETAILED DESCRIPTION OF THE INVENTION Described herein the use of split protein ccGFP for protein expression in electrowetting devices. Applicants have appreciated that ccGFP provides several benefits over other split fluorescent proteins, including brighter signal per molecule, faster complementation to produce fluorescent signal, greater solubility and ease of preparation. ccGFP is largely tolerant of buffer conditions and denaturants such as urea as the protein structure is highly stable. Other fluorescent proteins needs to be refolded from inclusion bodies, and are therefore sensitive to the presence of denaturants such as urea. The split ccGFP system is therefore particularly advantageous for use in protein expression applications on electrowetting devices. When compared to other split fluorescent proteins, for example sfGFP, ccGFP constructs have quicker maturation times and high max fluorescence for a normalised concentration, along with a lower variability between different buffer compositions due to the high folding stability. Disclosed herein is a method for the real-time monitoring of in vitro protein synthesis comprising a. in vitro transcription and translation of a protein of interest fused to a peptide tag; and b. monitoring the presence of the peptide tag using a further polypeptide which in the presence of the peptide tag produces a detectable signal. Disclosed herein is a method for the monitoring of cell-free protein synthesis in a droplet on a digital microfluidic device comprising a. cell-free transcription and translation of a protein of interest fused to a peptide tag; and b. monitoring the presence of the peptide tag using a further polypeptide which in the presence of the peptide tag produces a detectable signal. The peptide tag can be a ccGFP ₁₁ tag. The sequence of the tag can be selected from or a truncated version thereof. The tag should contain at least the region LQEHAVAK. The tag should be at least 13 amino acids in length. The nucleic acid sequence for expression of the tag can be a truncated version of The tag sequence LQEHAVAK can be expressed using a sequence Disclosed is a method for the cell-free expression of peptides or proteins in a digital microfluidic device. The droplets having the components required for cell-free protein synthesis (CFPS), otherwise known as in vitro protein synthesis, can be manipulated by electrokinesis in order to effect and improve protein expression. Electrowetting is the modification of the wetting properties of a surface (which is typically hydrophobic) with an applied electric field. Microfluidic devices for manipulating droplets or magnetic beads based on electrowetting have been extensively described. In the case of droplets in channels this can be achieved by causing the droplets, for example in the presence of an immiscible carrier fluid, to travel through a microfluidic channel defined by the walls of a cartridge or microfluidic tubing. Embedded in the walls of the cartridge or tubing are electrodes covered with a dielectric layer each of which are connected to an A/C biasing circuit capable of being switched on and off rapidly at intervals to modify the electrowetting field characteristics of the layer. This gives rise to the ability to steer the droplet along a given path. As an alternative to microfluidic channel systems, droplets can also be generated and manipulated on planar surfaces using digital microfluidics (DMF). In contrast to channel based microfluidics, DMF utilizes alternating currents on an electrode array for moving fluid on the surface of the array. Liquids can thus be moved on an open-plan device by electrowetting. Digital microfluidics allows precise control over the droplet movements including droplet fusion and separation. Cell-free protein synthesis, also known as in vitro protein synthesis or CFPS, is the production of peptides or proteins using biological machinery in a cell-free system, that is, without the use of living cells. The in vitro protein synthesis environment is not constrained within a cell wall or limited by conditions necessary to maintain cell viability, and enables the rapid production of any desired protein from a nucleic acid template, usually plasmid DNA or RNA from an in vitro transcription. CFPS has been known for decades, and many commercial systems are available. Cell-free protein synthesis encompasses systems based on crude lysate (Cold Spring Harb Perspect Biol. 2016 Dec; 8(12): a023853) and systems based on reconstituted, purified molecular reagents, such as the PURE system for protein production (Methods Mol Biol.2014; 1118: 275–284). CFPS requires significant concentrations of biomacromolecules, including DNA, RNA, proteins, polysaccharides, molecular crowding agents, and more (Febs Letters 2013, 2, 58, 261-268). To date, digital microfluidics, electrowetting-on-dielectric (EWoD), and electrokinesis in general have only found limited uses in cell-free biological-based applications, mostly due to biofouling, where biological components such as proteins, nucleic acids, crude cell extracts and other bioproducts adsorb and/or denature to hydrophobic surfaces. Biofouling is well known in the art to limit the ability of EWoD devices to manipulate droplets containing biomacromolecules. Wheeler and colleagues report that the maximum actuation time for droplets on EWoD devices containing biological media is 30 min before biofouling inhibits EWoD-based droplet actuation (Langmuir 2011, 27, 13, 8586-8594). Digital microfluidics can be carried out in an air-filled system where the liquid drops are manipulated on the surface in air. However, at elevated temperatures or over prolonged periods, the volatile aqueous droplets simply dry onto the surface by evaporation. This issue is compounded by the high surface area to volume ratio of nanoliter and microliter sized drops. Hence air-filled systems are generally not suitable for protein expression where the temperature of the system needs to be maintained at a temperature suitable for enzyme activity and the duration of the synthesis needs to be prolonged for synthesized proteins levels to be detectable. Protein expression typically requires an ample supply of oxygen. The most convenient and high yielding way to power CFPS is via oxidative phosphorylation where O ₂ serves as the final electron acceptor; however, there are other ways that involve replenishing with energy molecules not involved in oxidative phosphorylation. In a confined microfluidic or digital microfluidic system of droplets, insufficient oxygen is available to enable efficient protein synthesis. Described herein are improved methods allowing for the cell-free expression of peptides or proteins in a digital microfluidic device. Included is a method for the cell-free expression of peptides or proteins in a microfluidic device wherein the method comprises one or more droplets containing a nucleic acid template (i.e., DNA or RNA) and a cell-free system having components for protein expression in an oil-filled environment, and moving said droplets using electrokinesis. The components for the cell-free protein synthesis droplet can be pre-mixed prior to introduction to or mixed on the digital microfluidic device. The droplet can be repeatedly moved for at least a period of 30 minutes whilst the protein is expressed. The droplet can be repeatedly moved for at least a period of two hours whilst the protein is expressed. The droplet can be repeatedly moved for at least a period of twelve hours whilst the protein is expressed. The act of moving the droplet allows oxygen to be supplied to the droplet and dispersed throughout the droplet. The act of moving improves the level of protein expression over a droplet which remains static. The droplet can be moved using any means of electrokinesis. The droplet can be moved using electrowetting-on-dielectric (EWoD). The electrical signal on the EWoD or optical EWoD device can be delivered through segmented electrodes, active-matrix thin-film transistors, or digital micromirrors. The oil in the device can be any water immiscible liquid. The oil can be mineral oil, silicone oil, an alkyl-based solvent such as decane or dodecane, or a fluorinated oil. The oil can be oxygenated prior to or during the expression process. Alternatively, the device can be an air- filled device where droplets containing cell-free protein synthesis reagents are rapidly moved into position and fixed into an array under a humidified gas to prevent evaporation. Humidification can be achieved by enclosing or sealing the digital microfluidic device and providing on-board reagent reservoirs. Additionally, humidification can be achieved by connecting an aqueous reservoir to an enclosed or sealed digital microfluidic device. The aqueous reservoir can have a defined temperature or solute concentration in order to provide specific relative humidities (e.g., a saturated potassium sulfate solution at 30 °C). A source of supplemental oxygen can be supplied to the droplets. For example droplets or gas bubbles containing gaseous or dissolved oxygen can be merged with the droplets during the protein expression. Additionally, a source of supplemental oxygen can be found by oxygenating the oil that is used as the filler medium. It is well-known in the art that oils such as hexadecane, HFE-7500, and others can be oxygenated to support the oxygen requirements of cell growth, especially E. coli cell growth (RSC Adv., 2017, 7, 40990-40995). Oxygenation can be achieved by aerating the oil with pure oxygen or atmospheric air. The droplets can be formed before entering the microfluidic device and flowed into the device. Alternatively the droplets can be merged on the device. Included is a method comprising merging a first droplet containing a nucleic acid template such as a plasmid with a second droplet containing a cell-free extract having the components for protein expression to form a combined droplet capable of cell-free protein synthesis. The droplets can be split on the device either before or after expression. Included herein is a method further comprising splitting the aqueous droplet into multiple droplets. If desired the split droplets can be screened with further additives. Included is a method wherein one or more of the split droplets are merged with additive droplets for screening. The cell-free expression of peptides or proteins can use a cell lysate having the reagents to enable protein expression. Common components of a cell-free reaction include an energy source, a supply of amino acids, cofactors such as magnesium, and the relevant enzymes. A cell extract is obtained by lysing the cell of interest and removing the cell walls, DNA genome, and other debris by centrifugation. The remains are the cell machinery including ribosomes, aminoacyl-tRNA synthetases, translation initiation and elongation factors, nucleases, etc. Once a suitable nucleic acid template is added, the nucleic acid template can be expressed as a peptide or protein using the cell derived expression machinery. Any particular nucleic acid template can be expressed using the system described herein. Three types of nucleic acid templates used in CFPS include plasmids, linear expression templates (LETs), and mRNA. Plasmids are circular templates, which can be produced either in cells or synthetically. LETs can be made via PCR. While LETs are easier and faster to make, plasmid yields are usually higher in CFPS. mRNA can be produced through in vitro transcription systems. The methods use a single nucleic acid template per droplet. The methods can use multiple droplets having a different nucleic acid template per droplet. An energy source is an important part of a cell-free reaction. Usually, a separate mixture containing the needed energy source, along with a supply of amino acids, is added to the extract for the reaction. Common sources are phosphoenolpyruvate, acetyl phosphate, and creatine phosphate. The energy source can be replenished during the expression process by adding further reagents to the droplet during the process. The cell-free extract having the components for protein expression includes everything required for protein expression apart from the nucleic acid template. Thus the term includes all the relevant ribosomes, enzymes, initiation factors, nucleotide monomers, amino acid monomers, metal ions and energy sources. Once the nucleic acid template is added, protein expression is initiated without further reagents being required. Thus the cell-lysate can be supplemented with additional reagents prior to the template being added. The cell-free extract having the components for protein expression would typically be produced as a bulk reagent or ‘master mix’ which can be formulated into many identical droplets prior to the distinct template being separately added to separate droplets. Common cell extracts in use today are made from E. coli (ECE), rabbit reticulocytes (RRL), wheat germ (WGE), insect cells (ICE) and Yeast Kluyveromyces (the D2P system). All of these extracts are commercially available. Rather than originating from a cell extract, the cell-free system can be assembled from the required reagents. Systems based on reconstituted, purified molecular reagents are commercially available, for example the PURE system for protein production, and can be used as supplied. The PURE system is composed of all the enzymes that are involved in transcription and translation, as well as highly purified 70S ribosomes. The protein synthesis reaction of the PURE system lacks proteases and ribonucleases, which are often present as undesired molecules in cell extracts. The term digital microfluidic device refers to a device having a two-dimensional array of planar microelectrodes. The term excludes any devices simply having droplets in a flow of oil in a channel. The droplets are moved over the surface by electrokinetic forces by activation of particular electrodes. Upon activation of the electrodes the dielectric layer becomes less hydrophobic, thus causing the droplet to spread onto the surface. A digital microfluidic (DMF) device set-up is known in the art, and depends on the substrates used, the electrodes, the configuration of those electrodes, the use of a dielectric material, the thickness of that dielectric material, the hydrophobic layers, and the applied voltage. Once the CFPS reagents have been enclosed in the droplets, additional reagents can be supplied by merging the original droplet with a second droplet. The second droplet can carry any desired additional reagents, including for example oxygen or ‘power’ sources, or test reagents to which it is desired to expose to the expressed protein. Disclosed is a method wherein a first droplet on the device contains reagents for cell-free transcription and translation of a protein of interest fused to a ccGFP ₁₁ peptide tag; and a second droplet on the device contains a ccGFP _1-10 polypeptide. The droplets can be merged on the device and monitored for the presence of the detectable signal occurs once the first and second droplets are merged on the device. The signal indicates the amount of expressed protein via the ccGFP ₁₁ peptide tag. The merging can be before or after the expression has taken place, thus a real-time or end-point measurement. Disclosed is a method wherein the second droplet is added to the first droplet after expression of the protein of interest. Disclosed is a method wherein the second droplet is added to the first droplet before expression of the protein of interest. The droplets can be aqueous droplets. The droplets can contain an oil immiscible organic solvent such as for example DMSO. The droplets can be a mixture of water and solvent, providing the droplets do not dissolve into the bulk oil. The droplets can be in a bulk oil phase. A dry gaseous environment simply dries the bubbles onto the surface during the expression process, leaving comet type smears of dried material by evaporation. Thus the device is filled with liquid for the expression process. Alternatively, the aqueous droplets can be in a humidified gaseous environment. A device filled with air can be sealed and humidified in order to provide an environment that reduces evaporation of CFPS droplets. The droplets containing the cell-free extract having the components for protein expression will therefore typically be in the oil filled environment before the nucleic acid templates are added to the droplets. The templates can be added by merging droplets on the microfluidic device. Alternatively, the templates can be added to the droplets outside the device and then flowed into the device for the expression process. For example the expression process can be initiated on the device by increasing the temperature. The expression system typically operates optimally at temperatures above standard room temperatures, for example at or above 29 °C. The expression process typically takes many hours. Thus the process should be left for at least 30 minutes or 1 hour, typically at least 2 hours. Expression can be left for at least 12 hours. During the process of expression the droplets should be moved within the device. The moving improves the process by mixing the reagents and ensuring sufficient oxygen is available within the droplet. The moving can be continuous, or can be repeated with intervening periods of non-movement. Thus the aqueous droplet can be repeatedly moved for at least a period of 30 minutes or one hour whilst the protein is expressed. The aqueous droplet can be repeatedly moved for at least a period of two hours whilst the protein is expressed. The aqueous droplet can be repeatedly moved for at least a period of twelve hours whilst the protein is expressed. The act of moving the droplet allows mixing within the droplet, and allows oxygen or other reagents to be supplied to the droplet. The act of moving improves the level of protein expression over a droplet which remains static. Disclosed is a method comprising transcription of a nucleic acid sequence containing a sequence of SEQ ID 16 or 17: SEQ ID NO: 16 SEQ ID NO: 17 The sequence can be repeated to express multiple ccGFP amino acid tags. The transcription can be performed in droplets on a digital microfluidics device. A fluorescent signal is obtained when the expressed protein containing the ccGFP ₁₁ tag(s) complements ccGFP _1-10 protein. Digital microfluidics (DMF) refers to a two-dimensional planar surface platform for lab-on-a- chip systems that is based upon the manipulation of microdroplets. Droplets can be dispensed, moved, stored, mixed, reacted, or analyzed on a platform with a set of insulated electrodes. Digital microfluidics can be used together with analytical analysis procedures such as mass spectrometry, colorimetry, electrochemical, and electrochemiluminescense. The droplet can be moved using any means of electrokinesis. The aqueous droplet can be moved using electrowetting-on-dielectric (EWoD). Electrowetting on a dielectric (EWoD) is a variant of the electrowetting phenomenon that is based on dielectric materials. During EWoD, a droplet of a conducting liquid is placed on a dielectric layer with insulating and hydrophobic properties. Upon activation of the electrodes the dielectric layer becomes less hydrophobic, thus causing the droplet to spread onto the surface. The electrical signal on the EWoD or optically-activated amorphous silicon (a-Si) EWoD device can be delivered through segmented electrodes, active-matrix thin-film transistors or digital micromirrors. Optically-activated s-Si EWoD devices are well known in the art for actuating droplets (J. Adhes. Sci. Technol., 2012, 26, 1747-1771). The oil in the device can be any water immiscible or hydrophobic liquid. The oil can be mineral oil, silicone oil, an alkyl-based solvent such as decane or dodecane, or a fluorinated oil. The air in the device can be any humidified gas. A source of supplemental oxygen can be supplied to the droplets. For example droplets or gas bubbles containing gaseous or dissolved oxygen can be merged with the aqueous droplets during the protein expression. Alternatively the source of oxygen can be a molecular source which releases oxygen. Alternatively the droplets can be moved to an air/liquid boundary to enable increased diffusion of oxygen from a gaseous environment. Alternatively the oil can be oxygenated. Alternatively the droplets can be presented in a humidified air filled device. The droplet can be formed before entering the microfluidic device and flowed into the device. Alternatively the droplets can be merged on the device. Included is a method comprising merging a first droplet containing a nucleic acid template such as a plasmid with a second droplet containing a cell-free system having the components for protein expression to form the droplet. The droplets can be split on the device either before, during or after expression. Included herein is a method further comprising splitting the droplet into multiple droplets. If desired the split droplets can be screened with further additives. Included is a method wherein one of more of the split droplets are merged with additive droplets for screening. Through an affinity tag, such as a FLAG-tag, HIS-tag, GST-tag, MBP-tag, STREP-tag, or other form of affinity tag, CFPS-expressed proteins can be immobilized to a solid-support affinity resin and fresh batches of CFPS reagent can be delivered over the said resin. Thus, renewed reagents can be used to carry out protein synthesis, closely mimicking industrial methods of continuous flow (CF) and continuous exchange (CE) CFPS. By mimicking CF- and CE-CFPS, users can scale up their CFPS production methods. The droplets can be actuated on a hydrophobic surface on the digital microfluidic device (ACS Nano 2018, 12, 6, 6050-6058). The hydrophobic surface can be a hydrophobic surface such as polytetrafluoroethylene (PTFE), Teflon AF (DuPont Inc), CYTOP (AGC Chemicals Inc), or FluoroPel (Cytonix LLC). The hydrophobic surface may be modified in such a way to reduce biofouling, especially biofouling resulting from exposure to CFPS reagents or nucleic acid reagents. The hydrophobic surface may also be superhydrophobic, such as NeverWet (NeverWet LLC) or Ultra-Ever Dry (Flotech Performance Systems Ltd). Superhydrophobic surfaces prevent biofouling compared with typical fluorocarbon-based hydrophobic surfaces. Superhydrophobic surfaces thus prolong the capability of digital microfluidic devices to move CFPS droplets and general solutions containing biopolymers (RSC Adv., 2017, 7, 49633-49648). The hydrophobic surface can also be a slippery liquid infused porous surface (SLIPS), which can be formed by infusing Krtox-103 oil (DuPont) with porous PTFE film (Lab Chip, 2019, 19, 2275). Droplets can also contain additives to reduce the effects of biofouling on digital microfluidic surfaces. Specifically, droplets containing CFPS components can also contain additives such as surfactants or detergents to reduce the effects of biofouling on the hydrophobic or superhydrophobic surface of a digital microfluidic device (Langmuir 2011, 27, 13, 8586-8594). Such droplets may use antifouling additives such as TWEEN 20, Triton X-100, and/or Pluronic F127. Specifically, droplets containing CFPS components may contain TWEEN 20 at 0.1% v/v, Triton X-100 at 0.1% v/v, and/or Pluronic F127 at 0.08% w/v. For electrowetting on dielectrics (EWoD), the change in contact angle of reagent upon the application of electric potential is an inverse function of surface tension. Thus, for low voltage EWoD operations, reduction in surface tension is achieved by addition of surfactants to reagents, which for CFPS reactions means to the lysate and to the DNA. This results in a dilution of the lysate, and it has been seen, in experiments, that diluting or otherwise adulterating the lysate results in a decrease in expression level of the protein of interest. Thus performing CFPS on DMF where the surfactants are added to the solutions being moved will necessarily result in a dilution and adulteration of the lysate and thus a decrease in the level of protein expression. In addition to being a problem in its own right, this further complicates extrapolation of on-DMF results to in-tube predictions of protein yield. An additional detriment of having to add surfactants to the samples is that this increases the time required for sample preparation, as well as increasing the potential for inconsistent results due to ‘user error,’ as there is more handling of reagents. An additional detriment of having to add surfactants to the samples is that certain downstream operations are hindered. For example, if a protein of interest is expressed in a cell-free system with a GFP ₁₁ (or similar) peptide tag, it’s downstream complementation with a GFP _1-10 (or similar) detector polypeptide is hindered in the presence of surfactant. This is shown in Figures 8 and 9. Removal of the surfactant from the aqueous phase is therefore advantageous. Rather than adding surfactants to the aqueous sample, it is instead possible to add surfactant, such as a sorbitan ester such as Span85 (e.g. Sorbitan trioleate, Sigma Aldrich, SKU 8401240025), to the oil. This has the advantages of enabling CFPS reactions to proceed on- DMF without dilution or adulteration. Additionally, it simplifies the sample preparation procedure for setting up the reactions, increasing the ease of use and the consistency of results. Using 1% w/w Span85 in dodecane allows for dilution-free CFPS reactions on-DMF, as well as dilution-free detection of the expressed non-fluorescent proteins. Other surfactants besides Span85, and oils other than dodecane could be used. A range of concentrations of Span85 could be used. Surfactants could be nonionic, anionic, cationic, amphoteric or a mixture thereof. Oils could be mineral oils or synthetic oils, including silicone oils, petroleum oils, and perfluorinated oils. Surfactants can have a detrimental effect on (1) the CFPS reactions and (2) the efficiency of the detection system (if the detection system involves complementation of a tag and detector). For example, by performing the CFPS reaction on- DMF with oil-surfactant mix, the detection of the expressed protein can also proceed without dilution and without adding aqueous surfactant. It has been shown that surfactants reduce the efficiency of some detection systems, including but not limited to the Split ccGFP (e.g. ccGFP ₁₁ /ccGFP _1-10 ) system, so removing surfactants from the reagent mix and instead adding them to the oil can be beneficial. The peptide tag can be attached to the C or N terminus of the protein. The peptide tag may be one component of a green fluorescent protein (ccGFP). For example the peptide tag may be ccGFP ₁₁ and the further polypeptide ccGFP _1-10 . The peptide tag may be one component of sfCherry. The peptide tag may be sfCherry ₁₁ and the further polypeptide sfCherry _1-10 . The protein may be fused to multiple tags. For example the protein may be fused to multiple ccGFP ₁₁ peptide tags and the synthesis occurs in the presence of multiple ccGFP _1-10 polypeptides. For example the protein may be fused to multiple sfCherry ₁₁ peptide tags and the synthesis occurs in the presence of multiple sfCherry _1-10 polypeptides. The protein of interest may be fused to one or more sfCherry ₁₁ peptide tags and one or more ccGFP ₁₁ peptide tags and the synthesis occurs in the presence of one or more ccGFP _1-10 polypeptides and one or more sfCherry _1-10 polypeptides. After expression the proteins can be purified on the device. Disclosed is a method comprising a. taking a digital microfluidic device having a planar array of electrodes; b. synthesising a protein of interest having a binding tag in droplets on the device; c. capturing the proteins via the binding tags, thereby immobilising the proteins; d. moving the droplets using the electrodes, thereby removing the synthesised proteins from the droplet; e. optionally washing the immobilised proteins; and f. optionally releasing the proteins into further droplets. The binding moiety tag can be a sub-component of a fluorescent protein. Thus the fully assembled protein becomes fluorescent. For example if the immobilised material contains ccGFP _1-10 and the tag contains a ccGFP ₁₁ peptide, complementation forms immobilised fluorescent ccGFP, allowing simultaneous monitoring and purification. The immobilised material can be washed and then eluted by disrupting the complemented split ccGFP, e.g. through the use of salt or temperature. Disclosed is a method comprising a. taking a digital microfluidic device having a planar array of electrodes; b. synthesising a protein of interest having one of more ccGFP ₁₁ tags in droplets on the device; c. capturing the proteins via the ccGFP ₁₁ tags, thereby immobilising the proteins; d. moving the droplets using the electrodes, thereby removing the synthesised proteins from the droplet; e. optionally washing the immobilised proteins; and f. optionally releasing the proteins into further droplets. Immobilisation can be performed using immobilised ccGFP _1-10. Once immobilised via complementation, the proteins become fluorescent. Therefore the correctly expressed proteins can be both monitored and purified by complexing with the immobilised ccGFP _1-10 . Devices The manipulation of droplets by the application of electrical potential can be achieved on electrodes covered with an insulator or a dielectric or a series of insulators or dielectrics. Droplet manipulation as a result of an applied electrical potential is known as electrowetting. Electrokinesis occurs as result of a non-uniform electric field that influences the hydrostatic equilibrium of a dielectric liquid (dielectrophoresis or DEP) or a change in the contact angle of the liquid on solid surface (electrowetting-on-dielectric or EWoD). DEP can also be used to create forces on polarizable particles to induce their movement. The electrical signal can be transmitted to a discrete electrode, a transistor, an array of transistors, or a sheet of semi- conductor film whose electrical properties can be modulated by an optical signal. EWoD phenomena occur when droplets are actuated between two parallel electrodes covered with a hydrophobic insulator or dielectric. The electric field at the electrode-electrolyte interface induces a change in the surface tension, which results in droplet motion as a result of a change in droplet contact angle. The electrowetting effect can be quantitatively treated using Young-Lippmann equation: cos θ - cos θ ₀ = (1/2γLG) c.V ² where θ ₀ is the contact angle when the electric field across the interfacial layer is zero, γLG is the liquid-gas tension, c is the specific capacitance (given as ε _r . ε ₀ /t, where ε _r is dielectric constant of the insulator/dielectric, ε ₀ is permittivity of vacuum, t is thickness) and V is the applied voltage or electrical potential. The change in contact angle (inducing droplet movement) is thus a function of surface tension, electrical potential, dielectric thickness, and dielectric constant. When a droplet is actuated by EWoD, there are two opposing sets of forces that act upon it: an electrowetting force induced by electric field and resistant forces that include the drag forces resulting from the interaction of the droplet with filler medium and the contact line friction (ref). The minimum voltage applied to balance the electrowetting force with the sum of all drag forces (threshold voltage) is variably determined by the thickness-to-dielectric contact ratio of the insulator/dielectric, (t/ε _r ) ^1/2 . Thus, to reduce actuation voltage, it is required to reduce (t/ε _r ) ^1/2 (i.e., increase dielectric constant or decrease insulator/dielectric thickness). To achieve low voltage actuation, thin insulator/dielectric layers must be used. However, the deposition of high quality thin insulator/dielectric layers is a technical challenge, and these thin layers are easily damaged before the desired electrowetting contact angle is large enough to drive the droplet is achieved. Most academic studies thus report the use of much higher voltages >100V on easily fabricated, thick dielectric films (>3 µm) to effect electrowetting. High voltage EWoD-based devices with thick dielectric films, however, have limited industrial applicability largely due to their limited droplet multiplexing capability. The use of low voltage devices including thin-film transistors (TFT) and optically-activated amorphous silicon layers (a- Si) have paved the way for the industrial adoption of EWoD-based devices due to their greater flexibility in addressing electrical signals in a highly multiplex fashion. The driving voltage for TFTs or optically-activated a-Si are low (typically <15 V). The bottleneck for fabrication and thus adoption of low voltage devices has been the technical challenge of depositing high quality, thin film insulators/dielectrics. Hence there has been a particular need for improving the fabrication and composition of thin film insulator/dielectric devices. Typically, the electrodes (or the array elements) used for EWoD are covered with (i) a hydrophilic insulator/dielectric and a hydrophobic coating or (ii) a hydrophobic insulator/dielectric. Commonly used hydrophobic coatings comprise of fluoropolymers such as Teflon AF 1600 or CYTOP. The thickness of this material as a hydrophobic coating on the dielectric is typically <100 nm and can have defects in the form of pinholes or a porous structure; hence, it is particularly important that the insulator/dielectric is pinhole free to avoid electrical shorting. Teflon has also been used as an insulator/dielectric, but it has higher voltage requirements due to its low dielectric constant and the thickness required to make it pinhole free. Other hydrophobic insulator/dielectric materials can include polymer-based dielectrics such as those based on siloxane, epoxy (e.g. SU-8), or parylene (e.g., parylene N, parylene C, parylene D, or parylene HT). Due to minimal contact angle hysteresis and a higher contact angle with aqueous solutions, Teflon is still used as a hydrophobic topcoat on these insulator/dielectric polymers. However, there are difficulties in reliably producing <1 micron pinhole-free coatings of parylene or SU-8; thus, the thickness of these materials is typically kept at a 2-5 microns at the cost of increased voltage requirements for electrowetting. It has also been reported that traditional EWoD devices with parylene C are easily broken and unstable for repeated droplet manipulation with cell culture medium. Multi-layer insulator devices deposited with metal-oxide and parylene C films have been used to produce a more robust insulator/dielectric and enable operations with lower applied voltages. Inorganic materials, such metal oxides and semiconductor oxides, commonly used in the CMOS industry as “gate dielectrics”, have been used as insulator/dielectric for EWoD devices. They offer the advantage of utilizing standard cleanroom processes for thin film depositions (<100 nm). These materials are inherently hydrophilic, requiring an additional hydrophobic coating, and can be prone to pinhole formation as a result of thin film layer deposition process. Together with the need for lower voltage operations of EWoD, recent developmental work has focused on (1) using materials with improved dielectric properties (e.g., using high-dielectric constant insulators/dielectrics), (2) optimizing the fabrication process to make the insulator/dielectric pinhole free to avoid dielectric breakdown. Operation of EWoD devices suffers from contact angle saturation and hysteresis, which is believed to be brought about by either one or combination of these phenomena: (1) entrapment of charges in the hydrophobic film or insulator/dielectric interface, (2) adsorption of ions, (3) thermodynamic contact angle instabilities, (4) dielectric breakdown of dielectric layer, (5) the electrode-electrode-insulator interface capacitance (arising from the double layer effect), and (6) fouling of the surface (such as by biomacromolecules). One of the adverse effects of this hysteresis is reduced operational lifetime of the EWoD-based device. Contact angle hysteresis is believed to be a result of charge accumulation at the interface or within the hydrophobic insulator after several operations. The required actuation voltage increases due to this charging phenomenon resulting in eventual catastrophic dielectric breakdown. The most probable explanation is that pinholes at the insulator/dielectric may allow the liquid to come into contact with the electrode causing electrolysis. Electrolysis is further facilitated by pinhole-prone or porous hydrophobic insulators. Most of the studies to understand contact angle hysteresis on EWoD have been conducted on short time scales and with low conductivity solutions. Long duration actuations (e.g., >1 hour) and high conductivity solutions (e.g., 1 M NaCl) could produce several effects other than electrolysis. The ions in solution can permeate through the hydrophobic coat (under the applied electric field) and interact with the underlying insulator/dielectric. Ion permeation can result in (1) change in dielectric constant due to charge entrapment (which is different from interfacial charging) and (2) change in surface potential of a pH sensitive metal oxide. Both can result in reduction of electrowetting forces to manipulate aqueous droplets, leading to contact angle hysteresis. The inventors have previously found that the damage from high conductivity solutions reduces or disables electrowetting on electrodes by inhibiting the modulation of contact angle when an electric field is applied. An electrokinetic device includes a first substrate having a matrix of electrodes, wherein each of the matrix electrodes is coupled to a thin film transistor, and wherein the matrix electrodes are overcoated with a functional coating comprising: a dielectric layer in contact with the matrix electrodes, a conformal layer in contact with the dielectric layer, and a hydrophobic layer in contact with the conformal layer; a second substrate comprising a top electrode; a spacer disposed between the first substrate and the second substrate and defining an electrokinetic workspace; and a voltage source operatively coupled to the matrix electrodes. The dielectric layer may comprise silicon dioxide, silicon oxynitride, silicon nitride, hafnium oxide, yttrium oxide, lanthanum oxide, titanium dioxide, aluminum oxide, tantalum oxide, hafnium silicate, zirconium oxide, zirconium silicate, barium titanate, lead zirconate titanate, strontium titanate, or barium strontium titanate. The dielectric layer may be between 10 nm and 100 µm thick. Combinations of more than one material may be used, and the dielectric layer may comprise more than one sublayer that may be of different materials. The conformal layer may comprise a parylene, a siloxane, or an epoxy. It may be a thin protective parylene coating in between the insulating dielectric and the hydrophobic coating. Typically, parylene is used as a dielectric layer on simple devices. In this invention, the rationale for deposition of parylene is not to improve insulation/dielectric properties such as reduction in pinholes, but rather to act as a conformal layer between the dielectric and hydrophobic layers. The inventors find that parylene, as opposed to other similar insulating coatings of the same thickness such as PDMS (polydimethylsiloxane), prevent contact angle hysteresis caused by high conductivity solutions or solutions deviating from neutral pH for extended hours. The conformal layer may be between 10 nm and 100 µm thick. The hydrophobic layer may comprise a fluoropolymer coating, fluorinated silane coating, manganese oxide polystyrene nanocomposite, zinc oxide polystyrene nanocomposite, precipitated calcium carbonate, carbon nanotube structure, silica nanocoating, or slippery liquid-infused porous coating. The elements may comprise one or more of a plurality of array elements, each element containing an element circuit; discrete electrodes; a thin film semiconductor in which the electrical properties can be modulated by incident light; and a thin film photoconductor whose properties can be modulated by incident light. The functional coating may include a dielectric layer comprising silicon nitride, a conformal layer comprising parylene, and a hydrophobic layer comprising an amorphous fluoropolymer. This has been found to be a particularly advantageous combination. The electrokinetic device may include a controller to regulate a voltage provided to the individual matrix electrodes. The electrokinetic device may include a plurality of scan lines and a plurality of gate lines, wherein each of the thin film transistors is coupled to a scan line and a gate line, and the plurality of gate lines are operatively connected to the controller. This allows all the individual elements to be individually controlled. The second substrate may also comprise a second hydrophobic layer disposed on the second electrode. The first and second substrates may be disposed so that the hydrophobic layer and the second hydrophobic layer face each other, thereby defining the electrokinetic workspace between the hydrophobic layers. The method is particularly suitable for aqueous droplets with a volume of 1 µL or smaller. The EWoD-based devices shown and described below are active matrix thin film transistor devices containing a thin film dielectric coating with a Teflon hydrophobic top coat. These devices are based on devices described in the E Ink Corp patent filing on “Digital microfluidic devices including dual substrate with thin-film transistors and capacitive sensing”, US patent application no 2019/0111433, incorporated herein by reference. Described herein are electrokinetic devices, including: a first substrate having a matrix of electrodes, wherein each of the matrix electrodes is coupled to a thin film transistor, and wherein the matrix electrodes are overcoated with a functional coating comprising: a dielectric layer in contact with the matrix electrodes, a conformal layer in contact with the dielectric layer, and a hydrophobic layer in contact with the conformal layer; a second substrate comprising a top electrode; a spacer disposed between the first substrate and the second substrate and defining an electrokinetic workspace; and a voltage source operatively coupled to the matrix electrodes; Described herein is an electrokinetic device, including: a first substrate having a matrix of electrodes, wherein each of the matrix electrodes is coupled to a thin film transistor, and wherein the matrix electrodes are overcoated with a functional coating comprising: one or more dielectric layer(s) comprising silicon nitride, hafnium oxide or aluminum oxide in contact with the matrix electrodes, a conformal layer comprising parylene in contact with the dielectric layer, and a hydrophobic layer in contact with the conformal layer; a second substrate comprising a top electrode; a spacer disposed between the first substrate and the second substrate and defining an electrokinetic workspace; and a voltage source operatively coupled to the matrix electrodes; The electrokinetic devices as described may be used with other elements, such as for example devices for heating and cooling the device or reagent cartridges for the introduction of reagents as needed. EXAMPLE Example 1. A cDNA sequence encoding for a sfGFP ₁₁ -terminal deoxynucleotidyl transferase (sfGFP ₁₁ _TdT) fusion protein was created. GFP ₁₁ -terminal deoxynucleotidyl transferase (GFP ₁₁ _TdT) fusion protein (SEQ ID NO: 1): The sfGFP ₁₁ -terminal deoxynucleotidyl transferase (GFP ₁₁ _TdT) fusion protein comprised a 376 aa engineered TdT linked via a 9 aa linker to a 15 aa N-terminal GFP ₁₁ peptide tag. The GFP ₁₁ _TdT cDNA sequence was cloned into a p70a vector and added (2 µl, 30 nM) to MyTXTL Sigma70 CFPS Master Mix (10 µl; Arbor Biosciences) resulting in a 12 µl total reaction volume. A GFP _1-10 solution (0.9 mg/ml; 364 mM urea) was subsequently added to the CFPS reaction at 0, 1, 2, 3, 4, 6, and 12 µl resulting in a final CFPS urea concentration of 0, 28, 51, 72, 90, 120, and 180 mM, respectively. The reactions were then incubated in a 200 µl PCR tube at 29 °C. Photographs were taken under 470 nm blue light illuminator using a 580 nm filter. Figure 2 shows the real-time fluorescent detection of sfGFP ₁₁ _TdT in a CFPS reaction under blue light illumination. Figure 2 optimises the amount of sfGFP _1-10 solution, which is a trade-off between increasing the concentration of the sfGFP _1-10 protein and CFPS inactivation through increasing concentrations of urea. Figure 3 quantifies the background subtracted, real-time fluorescent detection of sfGFP ₁₁ _TdT in a CFPS reaction. Fluorescence measurements were taken at indicated timepoints using an excitation wavelength of 485 nm and an emission wavelength of 520 nm. A negative control CFPS reaction containing sfGFP _1-10 solution but containing no GFP ₁₁ _TdT p70a plasmid was used to subtract background fluorescence. Figure 4 shows measurements of expression in droplets on a microfluidic device. Droplets with tagged protein (maltose binding protein, MBP, fused to sfGFP ₁₁ , i.e. MBP-GFP ₁₁ ) expressed in CFPS on eDrop can be seen in figure 4A. The first two rows contain CFPS lysate with protein expressed, while the third row contains a CPFS lysate control (no protein expressed). The image of figure 4A is taken under darkfield illumination. Droplets where GFP _1-10 has been added to some of the droplets with protein expressed, T=6 hours, shown in figure 4B. The larger droplets have had concentrated, recombinant sfGFP _1-10 added to the CFPS lysate. This image is taken under fluorescent light. Only the droplets which have both the expressed tagged protein and the complementing polypeptide added show fluorescence. Figures 5-7 demonstrate real-time detection of protein expression. GFP _1-10 detector polypeptide is present right from the start of the experiment. Fluorescent signal increases as sfGFP ₁₁ tags are expressed. These experiments were performed in a base fluid comprising 0.2% Span 85 in dodecamethylpentasiloxane rather than Tween20 in aqueous and no surfactant in dodecane. Figures 8 and 9 show the deleterious effect of aqueous surfactant. Figures 10-12 show real time protein expression detection through co-expression of the detector (sfGFP _1-10 ) with the sfGFP ₁₁ -tagged protein of interest, in tubes and in droplets. EXAMPLE 2: Complementation of recombinant ccGFP _1-10 with MBP_ccGFP_S ₁₁ proteins An ACA eDrop device was pre-loaded with surfactant base fluid and then loaded onto the TTR prototype instrument and equilibrated at 29 ºC. Proteins (MBP-ccGFP_S ₁₁ 1 mg/mL; ccGFP _1-10 1 mg/mL and 2.5 mg/mL; FL ccGFP 1 mg/mL all in TNG buffer) were loaded onto the eDrop device by pipetting, in combination with script-driven actuation of TFT-pixels under the reagent port, to form aqueous reagent reservoirs within the surfactant base fluid, in the device (denoted in Figure 13). Figure 13 shows the dispensing and merging of split ccGFP reagents into 384 droplet complementation array (white light images). Panel 1 shows 384-droplet MBP-ccGFP_S ₁₁ dispense (from edge 1, 3 and 4 reservoirs). Panel 2 shows ccGFP _1-10 dispensing, forming second offset 384 droplet array (768 droplets in total; from edge 2 reservoirs). Panel 3 depicts the final merged 384 droplet array (complementation incubation); A = 1 mg/mL FL ccGFP, B = TNG buffer, C = 1 mg/mL, D = 2.5 mg/mL ccGFP _1-10 . Scripts were loaded that executed the following droplet operations: 1) Dispense 25 px ² droplets from ccGFP_S ₁₁ reservoirs to form a 32x24 (n=384) droplet array (Figure 13, panel 1) 2) Dispense 25 px ² droplets from ccGFP _1-10 reservoirs to form a second 32x24 (n=768) droplet array, offset from the ccGFP_S ₁₁ droplet array (Figure 13, panel 2). 3) Merge ccGFP_S ₁₁ droplet array with ccGFP _1-10 droplet array to form a 32x24 (n=384) 50 px ² mixed droplet array (Figure 13, panel 3). 4) Incubate the mixed droplet array at 29 ºC for 4 hrs with periodic droplet mixing to enable complementation of the split ccGFP detection system to proceed (Figure 14). Figure 14 shows the developing fluorescent image due to ccGFP _1-10 :MBP-ccGFP_S ₁₁ complementation (blue light images). The image in panel 1 is taken at t = 0 min, the image in panel 2 is taken at t = 40 min and image 3 is taken at t = 110 min. On panel 3, A = 1 mg/mL FL ccGFP, B = TNG buffer, C = 1 mg/mL, D = 2.5 mg/mL ccGFP _1-10 . All images are taken at 400 ms exposure. Endpoint fluorescence was extracted from droplets and used to plot the [ccGFP _1-10 ] dependence complementation time courses shown in Figure 15. Under conditions tested, split ccGFP complementation was demonstrated on eDrop and completed within 4 hours. Figure 15 shows the ccGFP _1-10 :MBP-ccGFP_S ₁₁ eDrop complementation time course. EXAMPLE 3: Lysate in-situ complementation of recombinant and expressed ccGFP_S ₁₁ POI and recombinant ccGFP _1-10 protein on eDrop (e.g. N22R183) This example experiment demonstrates the ability to complement recombinant, and in vitro expressed, ccGFP _1-10 with MBP-ccGFP_S ₁₁ in the presence of in vitro transcription/translation reagents (E coli lysate, PURExpress), to mimic split-GFP detection in the full eDrop workflow. An ACA eDrop device was pre-loaded with surfactant base fluid and then loaded onto the TTR prototype instrument and equilibrated at 29 ºC. Edge reservoirs were loaded as described in example 2 with the layout shown in Table 1.

Scripts were loaded that executed the following droplet operations: 1) Dispense 25 px ² droplets from edge 1 reservoirs to form a 12x16 (n=192) droplet array (Figure 16, panel 1). 2) Dispense 16 px ² droplets from edge 3 and 4 reservoirs to form a second 12x16 (n=192) droplet array, offset from the first droplet array. Total number of droplets in array = 384 (Figure 16, panel 2) 3) Merge the two 192-droplet arrays to form a new 12x16 (n=192) 64 px ² mixed droplet expression array. Mixed (Figure 16, panel 3). 4) Split array into 24x16 (n=384) 36 px2 mixed droplet expression array (Figure 16, panel 4). 5) Incubate the 384-droplet mixed droplet array at 29 ºC for 5 hrs with periodic droplet mixing to allow expression to proceed (LEC_575, Figure 17). 6) Dispense 36 px ² droplets from edge 2 reservoirs to form a new offset 24x16 (n=384) droplet array. Total number of droplets in array = 768 (Figure 18, panel 1). 7) Merge the two 384-droplet arrays to form a new 24x16 (n=384) 81 px ² mixed droplet complementation array (Figure 18, panel 2). 8) The device was incubated with periodic mixing for 5 hours to allow split ccGFP complementation to proceed (Figure 18, panel 3). Figure 16 shows the expression array dispense and incubation (white light images). Panel 1 shows a 192 droplet array after edge 1 component dispense. Panel 2 shows a 384 droplet array after edge 1, 3 and 4 component dispense. Panel 3 shows a 192 droplet array after expression array merge. Panel 4 shows a 384 droplet array after expression array split. Figure 17 shows the expression array dispense and incubation (blue light image). Figure 18 shows the complementation array dispense and incubation. Panel 1 shows a 768 droplet complementation array after edge 2 component dispense (white light image). Panel 2 shows a 384 droplet array after complementation array merge and incubation (white light image). Panel 3 shows a 384 droplet array after complementation incubation end-point (blue light image). Under conditions tested, split ccGFP complementation was demonstrated on eDrop in the presence of in vitro transcription/translation reagents Lys_01 (E coli cell-free lysate) and Lys_24 (PURExpress) and completed within 4 hours for both recombinant MBP-ccGFP_S ₁₁ and expressed MBP-ccGFP_S ₁₁ (Figure 19). Figure 19 shows on eDrop complementation end point fluorescence. Panel A shows lysate in- situ complementation of recombinant MBP-ccGFP_S ₁₁ with recombinant ccGFP _1-10 . Panel B shows lysate in-situ complementation of expressed MBP-ccGFP_S ₁₁ (LEC_576) with recombinant ccGFP _1-10 . Example 3 shows truncations of the ccGFP ₁₁ tag. The purified DNA of the full length and 18 shortened MBP-ccGFP ₁₁ (v1) variants were used in a reconstituted in vitro transcription/translation reaction together with a negative (-DNA) control and a positive (ccGFP ₁₁ (v1) purified protein) control. Aliquots of the samples were analyzed and relative MBP-ccGFP ₁₁ (v1) expression quantified by PAGE before complementation with ccGFP _1-10 . Figures 21 and 22 show the effect of shortening the ccGFP ₁₁ tag. The region GETIQ can be removed, leaving The region YFTE can be removed, leaving Up to three amino acids can be removed from each end, leaving IQLQEHAVAKY Figure 23 shows the experimental result from 24 different proteins expressed in a reconstituted cell-free protein synthesis system in droplets on an electrowetting on dielectric (EWoD) device. Each construct contains a GFP ₁₁ tag. In the rows marked Screen, the GFP _1-10 detector species is present from the start of expression. The rows marked Endpoint shows the fluorescence signal from 10 hours expression in the absence of GFP _1-10 detector species followed by 5 hours complementation with the GFP _1-10 detector species. This experiment showed significant differences between expression/complementation in Screen BioInk compared to Endpoint detection. The detected protein clusters formed after expression only with endpoint detection mean that the protein aggregated after expression, thereby lowering the soluble yield. It is evident from the image the presence of speckles for several constructs, indicating the likely presence of protein aggregates. The level of aggregation enables identification of conditions which are worthy of further testing, and identification of conditions having high level of aggregated protein from which further purification is unlikely to give material. Example 4 Comparison of ccGFP vs sfGFP ccGFP _1-10 and sfGFP _1-10 sequences were expressed and exposed to the corresponding gfp ₁₁ binding partner at the same normalised concentrations ( 0.93mg/ml). ccGFP _1-10 constructs to test: Dilutions to get all to 0.93 mg/ml stock: CFPS reactions in 1.5 ml eppendorf tubes and incubated for 20h @ 29°C at 5 nM final LEC DNA: LS70 lysate 9 μl T7 HP 0.5 μl LEC DNA (25nM) 2.5 μl Total 12 μl For CFPS reactions 1-14 take 6 μl and add 54 μl GFP _1-10 or TNG buffer as detailed in the table above - incubate @ 29°C in the plate reader and read fluorescence of sample every 30mins for 12h. Plates sealed to reduce evaporation. The intensity reaches a plateau after 6 hours. Purified ccGFP _1-10 shows stronger overall fluorescent signals with quicker onset times and maturation rates, as shown in Figure 24.