PROJECTS FINAL REPORT Call round: - UKAN Pilot project proposal call 2 Project Title: Patterned ionogel based acoustic sensor (PI-BASe) PI: Dr. Andy Reid Research Organisation: University of Strathclyde Department: EEE Start Date: 01/03/2023 Duration: 6 months Cost of award (80%): £47,824.15 Value of co-investment: £6,651 In kind: £6,651 (PI time and materials) Cash: Co-I and associated RO: Acoustic Research themes: Sensor material (UKAN Sense) Collaborations & Partnerships involved in project: Tell us about bi-lateral or multi-lateral partnerships/participation by the PI or research team in a network, consortium, multi-centre study N/A Project Partners: N/A Value and details of in-kind co-investment: - £3,636 PI salary costs £715 Licencing costs £2300 (approx.) materials costs for polymer preparation Value and details of cash co-investment: - Summary: This project explored the use of ionogels as acoustic sensors. Ionogels are room-temperature stable ionic liquids captured in a polymer matrix. Where an ionogel contacts an electrode, a strong interfacial capacitance is formed as the ions from the gel accumulate on the counter charged electrode. This is the electrical double-layer effect, which creates an extremely high capacitance over a length scale of nanometres. The project aimed to exploit this effect by patterning the surface of an ionogel using masked photopolymerization controlling and maximising the contact area between the electrode and the gel and leading to large changes in capacitance in response to pressure (Figure 1). Figure 1: Visual representation of the sandwiched (A) micropatterned and (B) flat ionic gel between two copper sheets as the electrodes, (C) schematic representation of the electron double layer (EDL) effect between the micropatterned ionic gel and the electrode, and (D) the corresponding circuit diagram. This project relies on the recent development of facile ‘one-pot’ methods of ionogel synthesis which are compatible with digital light projection-based 3D printing. Using these methods, ionogel sensors with tailored surface patterns can be rapidly fabricated and tested, delivering gains in sensitivity, frequency stability and directionality. Similar acoustic sensors have recently been reported using heat treatment to polymerize the patterned surface within a mould – in one case the mould being formed by a zebra plant leaf’s surface texture. The key to this project is leveraging the power of rapid fabrication to quickly iterate the design parameters of this surface pattern and develop a framework to relate that to the acoustic characteristics of the sensor. Objectives: This pilot project had two aims to establish the feasibility of making 3D-printed ionogel sensors. First to confirm that the compatibility of the documented ‘one-pot’ synthesis methods with vat-based polymerization 3D printing is realisable. The first project stage would attempt to replicate a mould fabricated ionogel structure of simple cones using DLP technology, verifying if this was practical and could generate designs with similar electrical and mechanical responses. We could then compare the sensitivity and frequency response of the device to published results for devices made with more traditional fabrication methods. The second stage is to exploit the 3D printer’s ability to rapidly iterate designs to explore the impact of the surface pattern on the sensor’s characteristics. The goal of this project is to collect sufficient data to form the basis of sensor design models and a subsequent stream of funding and support to drive the technology to a potentially exploitable technology readiness level. Outcomes/Impact*: Verification of 3D printing compatibility of ionogels The ionic gel precursor solution used comprised a matrix of acrylic acid with either 1-Ethyl-3-methylimidazolium chloride ([EMIM][Cl]) or 1-Ethyl-3-methylimidazolium ethyl sulfate ([EMIM][eSO4]) as the ionic liquid in a ratio of 40:60 wt/wt. Polyethylene glycol diacrylate (PEGDA Mn 250) was used as our crosslinker and BAPO or Irgacure 819 as our photoinitiator in ratios of 0.025 mol% and 1 wt% relative to monomer weight respectively. Our initial goal was to verify that these resins could be successfully polymerised and form a stable ionogel, and then to evaluate the ability of the gel to be patterned in the microscale. Three base resin compositions were tested, two using [EMIM][eSO4] as the base ionic liquid at a ratio of 60% wt/wt with variation in photoinitiator from 0.5 – 1% by weight and one resin using [EMIM][Cl] as the base ionic liquid (Figure 2). Figure 2: Schematic illustration showing the one-pot synthesis resin preparation comprising ionic liquid ([EMIM][Cl] or [EMIM][eSO4]), monomer (acrylic acid), crosslinker (PEGDA) and photoinitiator (BAPO or Irgacure 819) and DLP 3D-printing for the fabrication o Three different microcone patterns were generated, each following an even ratio of base diameter to cone height or 0.25, 0.5 and 1 mm respectively. For each microcone patterned ionic gel a corresponding flat ionic gel was printed, quickly generating three sets of data for each of the base resins. The resolution and ionogel structure was evaluated via scanning electron microscopy and ImageJ software to obtain the actual printed dimensions of the cones, while the mechanical behaviour of the material was characterised using a Mach-1 analyser of uniaxial tensile tests of 3D-printed dog-bone shaped specimens. Figure 3 shows the CAD model design for each micropatterned ionic gel (MC1, MC2, and MC3), and SEM images of individual cones. Despite the calculated Jacob’s working curve of the optimal print exposure time being 3.62 s for 50um in the initial material trials, the 7.5 s and 15 s exposure times resulted in better 3D-printing resolution of the microcones with a mean relative deviation of 2.02 ±1.72% and 0.24 ±2.13%, respectively, as opposed to the 22.26 ±2.59% deviation for the 3.62 s print. Thus, for smaller cone heights such as MC2, the 3.62 s exposure time was not taken into consideration. In this case, as shown in Figure 3(B), increasing the exposure time from 7.5 s to 15 s, resulted in a lower cone height, with the former exhibiting a geometrical deviation of 16.88 ±4.26% and 40.03 ±1.52%. By 3D-printing at a layer thickness of 15 µm, the cone height was increased slightly than the 50 µm layer thickness, however this still resulted in geometrical deviations of 46.76 ±6.55% and 56.37 ±2.20%, respectively. The lack of 3D-printing resolution could also be observed from the rounded cone tips as shown by the SEM image. Therefore, in an attempt to improve the 3D-printing of MC3, the part was tilted at a 45° to the platform (or x-axis) to 3D-print at a different orientation. This method has been reported to enhance the print resolution of 3D-printed microneedles resulting in sharper tips. Printing at 45° to the platform at an exposure time of 15 s and layer thickness of 25 µm, resulted in an increase in cone height when compared to printing without a tilting angle (0°), with geometrical deviations of 2.44 ±8.14% and 37.60 ±7.39%, respectively. The SEM image in Figure 3(C) also shows a less rounded cone tip obtained by the different 3D-printing orientation. Mechanical testing of 3D printed samples Previous studies have reported that the combination of IL and acrylic acid results in a highly stretchable ionic gel. Therefore, in this work, the mechanical behaviour of the 3D-printed ionic gels composed of either [EMIM][eSO4] (Resin 2) or [EMIM][Cl] (Resin 3) was evaluated via uniaxial tensile testing. Figure 4 shows the stress-strain curves and the corresponding elastic properties, including the ultimate tensile strength, elongation at break, and elastic modulus, comparing both resin types. From this evaluation, it is evident that the mechanical response depends on the ionic liquid’s anion, since the gels comprising [eSO4]- as an anion exhibited a higher elongation at break yet lower strength and stiffness than [Cl]-. In a similar study investigating [EMIM][Cl] and acrylic acid ionic gels stretchability of up to 2200% strain was reported, which is higher compared to that obtained in the present work. This discrepancy in elastic properties may be attributed to the fabrication process, given that these gels were 3D-printed and not moulded. Moreover, the 3D-printing process requires the incorporation of a photoblocker to obtain good dimensional accuracy, however this has been reported to influence the mechanical behaviour of the 3D-printed gels, specifically it has been observed that the compressive strength decreases with increasing concentration of photoblocker. Figure 3: 3D-printed microcone-patterned ionic gels using Resin 2. CAD design models for three different sized microcone arrays, corresponding SEM images and effect of printing parameters on cone height for (A) microcone pattern 1 (MC1), (B) micrcone pattern 2 (MC2), and (C) microcone pattern 3 (MC3). Columns represent mean ± S.D. (n=3). Red dashed lines show the CAD design cone height. Figure 4: Elastic behaviour of the 60:40 IL:AA resins comprising either [EMIM][ESO4] (Resin 2) or [EMIM][Cl] (Resin 3) as the ionic liquid. Tensile stress-strain curves for (A) Resin 2 and (B) Resin 3 samples. Elastic properties including (C) ultimate tensile strength (UTS), (D) elongation at break (?B), and (E) elastic modulus. Columns represent mean ± S.D. for n=4 (Resin 2) and n=6 (Resin 3). (F) Visual representation of the tensile test performed on Resin 3 ionic gel before stretching, and (G) during stretching prior to sample rupture. Electrical characterisation The electron double layer (EDL) effect, characteristic of ionic gels, was investigated by sandwiching either a flat or micropatterned gel between two copper electrodes as shown in Figure 1. Upon application of an external voltage, counterions accumulate at the electrode-electrolyte interface, contributing to a high specific capacitance. This capacitance highly depends on the electrode-electrolyte contact area. For a patterned electrolyte, such as the microcone patterned ionic gels developed in this work, the contact area is much smaller than for flat ionic gels, contributing to a lower capacitance as can be observed in Figure 6 (top versus bottom panels). However, when the patterned gel is subjected to a mechanical compressive load, this pressure increases the contact area, which in turn increases the capacitance, ultimately contributing to a highly sensitive capacitive pressure sensor. This increase in sensitivity can be explained by the equivalent circuit diagram depicted in Figure 1(D). By considering the EDL capacitance at the bottom and top electrode interface, represented by the capacitors in series: CEDL Bottom and CEDL I, respectively, where i represents the number of cones in the micropattern array, the total capacitance is: ?(1/C_total =1/C_(EDL Bottom) +?_(i=1)^??1/C_(EDL i) ) Given that the ionic gel fully contacts the surface area of the bottom electrode, the capacitance at the bottom is much higher than the capacitance at the top electrode, and therefore the total capacitance and the sensitivity of the sensor are governed by the micropattern. In this work, the frequency-dependent capacitive response to load variations revealed a slight increase in capacitance with increasing loads. The capacitance is also dependent on the thickness of the ionic gel, according to Equation 5 for a parallel plate capacitor, where C is the capacitance, ?o is the permittivity of free space, ?r is the relative permittivity, A is the contact surface area, and d is the distance between electrodes. ?(C=(?_o ?_r A)/d )In this work, for the flat ionic gels, the capacitance behaviour is consistent with Equation 5, where a decrease in thickness of the gel i.e., a decrease in separation between the two electrodes, contributes to an increase in capacitance, as shown in Figure 6 (A) - (C). Figure 6 also shows that the capacitance decreases with increasing frequency irrespective of the ionic gel’s surface pattern, which in combination with the range of capacitance is also consistent with ionic gel behaviour reported in previous results. Figure 5: : Capacitance of (A, B, C) flat and (D, E, F) micro-patterned ionic gels (MC1, MC2, and MC3) sandwiched between copper electrodes as a function of frequency and its response under varying loads (20g, 75g, and 99g). Data points represent one sample for each MC type. *What activities have you undertaken to engage with research users, special interest groups and the general public to inform them about the research? We are compiling the results of this pilot project into a conference submission for IEEE Flexible Electronics and Printable Sensors in July 2024. *Have any new research tools or methods been created or commissioned, if so, provide details: - *What activities have you undertaken to engage with research users, special interest groups and the general public to inform them about the research? *Have any new research datasets, databases and models making, or potential to make, significant difference to your research (or that of others), been created, if so, provide details: - Generation of datasets of mechanical and electrical properties of various combinations of ionogel precursors and micropattern arrays. Conclusion: The pilot project's twin goals of 3D printing a viable ionogel and developing a dataset of rapidly manufactured ionogel properties were met, and gave a clear indication of the next steps for this project. The ionogels were confirmed as compatible with digital light processing, however, the method of manufacture resulted in poorer mechanical properties such as a reduced elongation at break. This is predictable in itself and is part of the trade-off for the flexibility inherent in digital light processing-based manufacturing. The poorer reproducibility of the electron double-layer effect is more concerning and could be attributed to the loss of some resolution of the microcone structure as the dimensions were reduced. Achieving comparable gains in capacitance to devices made by moulding could be possible with dedicated projection optics, further reducing our XY resolution to 1-2 um from the present 40um. In addition, more care will need to be taken over the environmental stability of the ionogel formula, especially those comprising a hydrophilic IL such as [EMIM][Cl] which favours moisture absorption in air. A more hydrophobic IL such as those comprising a BMIM cation tend to demonstrate more stable behaviour in ambient conditions. Both acrylic acid and PEGDA are hydrophilic monomers, and therefore the presence of such components in the ionic gel might have also promoted moisture absorption and hence changes to the sensing performance. Therefore, for future work, it is also beneficial to determine the effect of moisture absorption by measuring changes in gel weight over time. The 3D printed ionogels do have poorer performance than other reported fabrication methods, however, the potential for rapid design steps and development of variation in the structure make this a powerful tool for research and modelling future designs. The next steps in this project will involve generating combined Multiphysics models of the ionic liquid and material strain from the data gathered here and testing the material response to an acoustic signal (rather than direct load as presented here). We hope to compile this information, along with the report above for publication in 2024. Plans for follow-on activities/grants: Beyond the completion of the data gathering for this set of experiments we have a PhD student beginning on 1st October with a remit to investigate patterned and porous gels for flexible piezoelectric transducers, which we hope will build on the research detailed here. Weblink: (to the outcome of the project, the Open Access repository for the data1, or press releases): Open access repository for the data generated here: https://doi.org/10.15129/c0d374ee-4849-4a17-801d-e222f6947f98 List of publications: in peer reviewed or non-peer reviewed literature. If no publications are available, what are the plans to publish? Please follow UKRI guidelines for Open Access https://www.ukri.org/manage-your-award/publishing-your-research-findings/ 1 As a UKRI award holder you must follow their research data policies- https://www.ukri.org/manage-your-award/publishing-your-research-findings/making-your-research-data-open --------------- ------------------------------------------------------------ --------------- ------------------------------------------------------------