Fabrication of Flexible, Fully Organic, Degradable Energy Storage

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Fabrication of flexible, fully organic, degradable energy storage devices using silk proteins Ramendra Pal, Subhas C. Kundu, and Vamsi K Yadavalli ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19309 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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ACS Applied Materials & Interfaces

Fabrication of flexible, fully organic, degradable energy storage devices using silk proteins

Ramendra K. Pal, Subhas C. Kundu, Vamsi K. Yadavalli *

R.K. Pal, Dr. V.K. Yadavalli Department of Chemical and Life Science Engineering Virginia Commonwealth University 601 W Main Street, Richmond VA, USA 23284 E-mail: [email protected]

Dr. S.C. Kundu 3Bs Research Group, Headquarters of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine, University of Minho, Guimaraes, Portugal

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Abstract

Flexible and thin-film devices are of great interest in epidermal and implantable bioelectronics. The integration of energy storage and delivery devices such as supercapacitors (SCs) with properties of flexibility, miniaturization, biocompatibility, and degradability are sought for such systems. Reducing e-waste, and using sustainable materials and processes are additional desirable qualities. Herein, a silk protein-based biocompatible and degradable thin film micro-supercapacitor (µSC) is reported. A protein carrier with the conducting polymer PEDOT:PSS and reduced graphene oxide dopant, is used as a photopatternable biocomposite ink. Active electrodes are fabricated using photolithography under benign conditions and only water as the solvent. These electrodes are printed on flexible protein sheets to form degradable, organic devices with a benign agarose-NaCl gel electrolyte. High capacitance, power density, cycling stability over 500 cycles, and the ability to power a light-emitting diode are shown. The device is flexible, can sustain cyclic mechanical stresses over 450 cycles, as well as retain capacitive properties over several days in liquid. Significantly, the µSCs are cytocompatible and completely degraded over the period of ~1 month. By precise control of the device configuration, these silk protein-based, all-polymer organic devices can be designed to be tunably transient, and provide viable alternatives for powering flexible and implantable bio electronics.

Keywords: supercapacitor; flexible; degradable; silk protein; conducting polymer


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1. Introduction

Rapid progress in the field of (bio) electronics is ushering smart and adaptable systems with diverse applications such as stretchable devices,1 wearables,2 electronic skin,3 epidermal,

4

and implanted devices.5 The need for optimal, autonomous energy storage and supply in such systems has led to developments in batteries, fuel cells, and supercapacitors.6-8 Among these, supercapacitors (SCs) are emerging as an important class of energy storage devices due to fast charge-discharge rates, high power densities, and long cycling.9 In designing such devices for biological working environments, constraints include the need to be mechanically compliant, soft, conformable, and have a small footprint. For instance, micro-SCs (µSCs) can be used in miniaturized systems with low power requirements.10 Transient components with the ability to disintegrate functionally and/or physically at the end of their operational period, while providing intrinsic environmental utility, can potentially resolve risks associated with chronic implants, or the need for additional extractive surgery.11-13 Recent challenges include the desire for ‶green electronics″ sustainable fabrication and lifecycle, with a view to reducing ewaste.14 Energy storage devices with qualities of sustainability, miniaturization, flexibility, biodegradability, and biocompatibility can therefore provide suitable prospects.15

Conventional SCs primarily comprise four components - the active electrode, carrier substrate, gel electrolyte, and charge collectors. To date, various active electrode materials have been identified, including oxides: such as MnO2, and SnO2; carbon based materials: such as nanotubes, pyrolyzed biomass, graphene oxide and reduced graphene oxide; conjugated polymers:

such

as

polypyrrole

(PPy),

polyaniline

(PANI),

and

poly(3,4-

ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS).6, 16-18 The active materials are deposited or printed on inert, flexible carrier substrates including polyimides (PI), polyethylene terepthalate (PET), polycarbonates (PC), or paper.19 Gel electrolytes are used as 3 ACS Paragon Plus Environment


electrode separators and ion conductors. Polymeric matrices such as polyvinyl alcohol (PVA), polyethylene glycol (PEG) and gelatin, and ion conductors such as H2SO4, H3PO4, KCl, KOH, or ionic liquids can be used.20 Electrodes are interfaced with metallic conductors to and for charge transport.16, 21-23 An analysis of these elements shows that material selection for in vivo, subcutaneous, or tissue implantation continues to be a challenge. Design considerations include biocompatibility,

degradability (e.g. bioresorption), and often, sustainable

manufacturing.14 Nondegradable configurations with metal or metal oxide charge collectors printed on plastic substrates, are not “green”, and tend to be unsuitable for “implant-andforget” type of applications, or where relatively short lifespans are required.24

Energy storage devices for bioelectronics applications fabricated from natural polymers or biomolecules can confer biocompatibility and electrochemical stability in physiological media.24-26 These include polysaccharides: chitosan, cellulose, agarose, sodium alginate, pectin; proteins and peptides: gelatin, hemeprotein, silk, DNA; and phenolic polymers: lignin.22,

27

Using directed assembly methods, proteins and bio-based materials (e.g.

nanofibrils) can serve as custom templated building blocks for various applications such as scaffolds, drug delivery systems and biosensors.28, 29 Biopolymers can provide 3D structural support to the electroactive materials,22 dielectric planar support,30 be pyrolyzed to form porous carbon electrodes,31 or form gel electrolytes.22 Integration of the materials of choice with microfabrication tools is needed to reduce the size of conventional SCs which are too large to be used in vivo,19 and integrate them with other electronic components.9 Techniques such as inkjet printing and screen printing have been adopted to form localized microelectrode patterns in miniaturized SCs,. However, these techniques are either limited in design, or rely on physical adhesion of the patterns to the support substrate, whereby the stability of the electrodes in aqueous environments without shorting or under flexure is difficult.9 Unconventional µSCs that can address the above challenges are therefore highly desirable.32 4 ACS Paragon Plus Environment

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Our group has demonstrated an approach to micropattern electroactive, conductive polymers as composites using chemically-modified, photo-crosslinkable silk proteins.33 Here, we report on a biocompatible, degradable, and microfabricated µSC based on protein-based composites that form both the working electrodes and the flexible substrate. The conductive composite bio-ink has a high charge storage capacity, can be micropatterned into circuits using light, and can function without the use of a conductive metal/metal oxide background.34 Interdigitated microelectrodes with high performance and integration with microfabricated devices are produced using a facile, room temperature photolithographic technique using only water as the solvent. Via doping with reduced graphene oxide (rGO), we demonstrate a tunable increase in the capacitive nature of the electrodes.35 For the first time, the reported µSCs is a benign, completely biomaterial based, biocompatible system that can be used in biological microenvironments using an agarose biopolymer and sodium chloride based gel electrolyte.36 The competitive performance characteristics, mechanical flexibility, strength, optically transparency, biocompatibility and biodegradability of the support biopolymer (silk) makes this a versatile choice for transient energy storage.37

2. Results and Discussion

2.1. Micropatterning biofriendly conductive ink and gel electrolyte for supercapacitors The fabrication of the degradable supercapacitor is shown in Scheme 1. Photolithography is used to micropattern high resolution soft electrodes in a scalable and reproducible manner. The water-based electroactive, photopatternable ink using silk proteins and PEDOT:PSS permits the use of traditional photolithographic approaches to fabricate micro and nanoscale circuits on both rigid and flexible substrates. The biochemically modified silk proteins provide stable and biodegradable matrices, as well as support for fabricating functional devices.31 Two µSCs are shown - 1.5 mm x 2 mm with 20 µm electrodes, and 5 mm x 5 mm 5 ACS Paragon Plus Environment


with 250 µm electrodes (Scheme 1). Since sandwich-like configurations tend to introduce challenges for integration with flexible and thin-film based bioelectronic devices, planar interdigitated arrangements were used (Figure S1 in the supporting information shows additional images).17, 38 The designs and complexity of the microarchitectures are only limited by the photomask used, and any type of planar design can be realized. Here, electrodes with line widths down to 5-10 µm are easily patterned via standard photolithographic techniques.

The electrodes are deposited and covalently attached to an underlying silk substrate comprised of a crosslinked photo-fibroin to form a flexible and freestanding device. It must be noted that these devices are completely metal/metal oxide free in comparison to earlier reported degradable silk films.33, 39 SCs with a small footprint (µSCs) or larger scales (inches), can be easily configured for implantable or injectable systems as rolled up or flat devices (Scheme 1). The ink can be easily patterned on substrates functionalized with pendant (meth)acrylate groups to form vinyl bonds. When UV light is irradiated through a photomask on the cast ink, crosslinking reactions occur inside the exposed ink as well as between the underlying substrate and the ink. Owing to covalent bonding between the substrate and the ink, the critical problem of pattern delamination due to mechanical stress can be avoided, resulting in exceptional stability under bending. The entire process is conducted at the benchtop using all-water based processing at room temperature, rendering the fabrication to be sustainable, cost-efficient, and straightforward in comparison to previous reports.

PEDOT:PSS is a promising electrode material for supercapacitor applications due to its electrical and ionic conductivity, electrical and electrochemical stability especially in biological media.40 The capacitive nature of the protein carrier-PEDOT:PSS ink is exploited to fabricate interdigitated electrodes whose functionality is enhanced by doping with reduced graphene oxide (rGO), while preserving the degradable and aqueous nature of the system and 6 ACS Paragon Plus Environment

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process. Nanoscale carbon materials such as graphene have high capacitance with extremely high surface area resulting in better double layer properties, and electrochemical stability. Even though graphene has theoretical electrical double layer capacitance values up to 550 F/g, parallel stacking of sheets tends to limit its capacitance.41 The incorporation of rGO with PEDOT:PSS greatly reduces restacking, while simultaneously improving electrode flexibility.35 Previous reports of conductive polymer-rGO composites showed high specific capacitances ~300 F/g, although non-degradable metallic charge collectors were used.42 Here, the rGO is incorporated in the conductive matrix as a dopant to improve capacitive behavior. Aqueous, room temperature reduction of GO via ascorbic acid is used as a facile and biofriendly process, in contrast to traditionally harsh thermal, chemical, or electrochemical reduction.43

2.2. Electrochemical characterization of the conductive ink To characterize the capacitive properties of this system, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) techniques were utilized. Four different conductive ink compositions were studied based on varying ratios of PEDOT:PSS and rGO (~19:1 and 4:1 at high and low PEDOT:PSS concentrations). Table S1 and Figure S2 in the supporting information discusses the rationale for these parameters, including the characterization of different ink compositions. The frequency response (Nyquist plots) of electrode/electrolyte system as a plot of the imaginary component (Z′′) of the impedance against the real component (Z′) are shown in Figure 1a. The small semicircle in the highfrequency region indicates a low ion resistance, caused by the porous matrix. The matrix porosity was measured from SEM images to be ~25%. In the low-frequency region, the nearly vertical line of the electrode validates the primary contribution of the ideal electrical double layer capacitance (EDLC) which is a characteristic of a supercapacitor material. In the midfrequency region, the sloped curve (Warburg resistance) is closely related to the 7 ACS Paragon Plus Environment


diffusion/transport in the electrolyte ions in the electrodes. The short region observed indicates that the PEDOT:PSS with rGO sheets facilitate the diffusion of the electrolyte ions through a porous matrix. The real part of complex impedance (Z′) obtained by linear interpolation of the low frequency part of the Nyquist curve to Z′′=0 represents the internal or equivalent series resistance (ESR) of the cell, which limits the rate at which the cell can be charged/discharged. Increase of conductive polymer and rGO content in the ink causes lower ESR and therefore, better capacitive performance. Ink with 48% PEDOT:PSS and 8.16% rGO shows a very low ESR of ~130 Ω and was used for subsequent device fabrication. The ESR values for different compositions are reported in Table S2. As the conductive content is increased, the spectra become more vertical at lower frequencies indicative of higher capacitive nature.

The choice of electrolyte is another consideration for such SCs. Typically strong acids, or bases are used as ion conducting materials that tend to be unsuitable for biological applications. Here, agarose - a polysaccharide, and NaCl - a neutral salt, is used to form a benign electrolyte gel.36 Agarose is a hydrophilic polymer obtained from seaweed, widely used for biomedical applications due to its non-immunogenic and anti-fouling properties. It further has the potential to address electrode failure in physiological media due to protein adsorption or biofouling. This electrolyte system was chosen for its superior biocompatibility despite a slight compromise in performance as discussed further below.

Parallel electrodes were patterned on flexible fibroin films using agarose/NaCl as the gel electrolyte. Figure 1b shows cyclic voltammograms at scan rates varying from 5 mV/s to 500 mV/s. The characteristic symmetric and square shape of the voltammogram up to 100 mV/s is indicative of a double layer supercapacitor charge injection mechanism. The electrical doublelayer capacitor (EDLC) dominated mechanism is confirmed by the linear dependence of scan 8 ACS Paragon Plus Environment

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rate on capacitance. At a higher scan rate (500 mV/s) even though the square shape is preserved, the capacitance is not linear with scan rate, indicating that pseudo-capacitance dominates (Figure S6). Comparative CV measurements conducted for different conducting ink compositions, (at a scan rate of 50 mV/s) show that the increase of conductive component in the ink improves capacitance behavior (Figure S3). Theoretically, the capacitive performance can be further increased by reducing the (non-conducting) protein carrier content in composite ink. However, this not only renders the ink unsuitable for patterning via photolithography, but also affects the degradation of the device. For applications not requiring degradation or a miniaturized format, a mixture of PEDOT:PSS and rGO can be considered. The experiments show that the bioink is promising either as capacitive coating, or as a standalone electrode material. The versatility is augmented by the fact that the material can be photolithographically patterned on a wide variety of functionalized substrates in the form of complex circuits.

2.3. Evaluation of supercapacitor performance The capacitive performance was evaluated by galvanostatic charge/discharge experiments.44 Experiments conducted at different ink compositions show similar behavior observed with CV, wherein the increase of PEDOT:PSS and rGO % enhance the capacitive nature. SCs fabricated using 48% PEDOT:PSS and 8.16% rGO provide an impressive specific gravimetric capacitance of 148.3 F/g, which is very competitive in comparison to prior reports.35, 45-46 A contextual comparison of this system with recently published SC works is presented in the supporting information (Table S4). A comparison to other SCs that have utilized silk proteins as elements is presented in Table S5. In terms of areal capacitance, the devices averaged 9.85 mF/cm2 at a current density of 1A/g. The SC uses light-weight and flexible materials with an overall device thickness ~ 50 µm (electrode thickness ~18 µm). This results in the µSCs being extremely light weight (3.95 ± 0.21 mg/cm2). In comparison, comparable 9 ACS Paragon Plus Environment


devices on various substrates (paper, PI, PET etc.) are 2-10x heavier.35, 45-46

The charge-discharge curves for all four compositions at a current density of 1 A/g are shown in Figure 2a. Charge-discharge experiments were done in triplicates and their rate performance are shown in Figure 2b. The linear profile of galvanostatic charge and discharge curves of all compositions and their symmetric triangular shapes are indicative of nearly ideal capacitive characteristics. A potential window of 0-0.5V was used, away from the water electrolysis window (CV data from 0-1 V is presented in the supporting information – Figure S4). It must be noted here that the patterns are made on fibroin films and do not have conventional metallic or metal oxide charge collectors. The material is stable up to 5A/g (Figure 2c). As the current density is increased, the discharge time decreases but no appreciable change in specific capacitance of the system is observed (Table S3). This independence of specific capacitance vs. current density is further characteristic of ideal capacitor behavior. The electroactivity depends on the speed of loading-unloading of charges through its matrix, and the number of active sites offered by the electrode. A small increase in i-R drop from low to high current density is observed. This shows that the internal resistance plays a role on the charge discharge characteristics. Nonetheless, the chargedischarge curves at all current densities remain triangular and symmetric, indicative of stability at different current loading. The ability to power a regular 2V, 20 mA LED was demonstrated (Figure 3a, b). In this case, 4 µSCs were connected in series, showing the potential of these devices to be used for energy storage.

The application of conducting polymers and rGO to form all polymer, metal-free supercapacitors, or with NaCl electrolyte has not been common. A reported capacitance of ~380 F/g in an analogous system, was accomplished using H2SO4 electrolyte, with a carbon cloth charge collector making the system neither biocompatible nor degradable.47 To compare 10 ACS Paragon Plus Environment

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the performance of this system with other configurations, two different experiments were conducted. First, the underlying substrate was changed to rigid ITO/glass. In the second experiment, a conventional PVA-H3PO4 electrolyte was substituted in this flexible system. The specific capacitance increased to ~221 F/g and ~178 F/g for experiments with ITO and PVA-H3PO4 respectively (compare to ~150 F/g obtained with the flexible protein substrate NaCl-agarose system) (Figure S7). These experiments are therefore indicative of the potential of achieving higher capacitive properties with this material with various configurations.

The overall performance of energy storage materials is depicted using a Ragone plot (Figure 2d). The reported electrodes with the highest PEDOT:PSS-rGO content achieve power and energy densities ~3230 W/kg and 5.2 Wh/kg respectively, which are similar or better than prior reports with conductive materials on non-conductive matrices, or using a combination of PEDOT:PSS with rGO.24,

35, 45, 48-50

(A comparative Ragone plot is

presented in the supporting information Figure S8). To test the long-term stability of the SCs, 500 cycles were run in triplicate. This number was selected considering the envisaged applications for this device with a useful lifecycle in the timescale of weeks. A capacity retention of ~ 91% after 500 cycles was observed. In previous work, a retention of electroactivity of 90.5% after 200 electrochemical cycles was shown.34 Importantly, the charge-discharge curves remain symmetric and triangular without an appreciable i-R drop (Figure 3c and d). The % retention shows that most of the electroactivity is retained over the product life cycle. Conductive polymers can be unstable during long-term charge/discharge processes, which has been a major drawback in their use for SC applications.49 In these devices, charge-discharge curves are symmetric over 5 days and the device retains ~99% of its initial capacitance with no perceptible i-R drop (Figure 3e, f). The improvement in electrochemical stability is therefore due to the addition of rGO, which also enhances the mechanical properties of the ink, causing reduction in swelling and shrinking 11 ACS Paragon Plus Environment


during the long-term cycling processes. The overall mechanical stability of the SC is further reflected in the capacitive performance.

2.4. Flexibility and bendability of supercapacitors The nature of applications for flexible and implantable devices involves movement and compliance in space. Therefore, a significant performance criteria is to evaluate device stability under different mechanical deformation states. The SCs show a negligible decrease in capacitance even after a complete 180° bend (Figure 4a). The capacitance retention during cyclic bending was further studied, wherein the device was bent up to 90° and flexed back in a repeated cyclic fashion. Even after 450 cycles of bending, an impressive capacitance retention of 97.5% was observed (Figure 4b). No delamination of the electrodes from the underlying substrate is observed owing to the covalent linkage between the two. Further, the surface morphology of the conducting electrodes was studied using atomic force microscopy (AFM) imaging before and after the cycles of mechanical bending. Even at the nanoscale, no difference in morphology was noticed (no cracks or fractures, with identical surface roughness), indicating that the material is highly stable (Figure S9). These experiments suggest that the device with interdigitated electrodes on the flexible protein sheet has excellent stability under applied mechanical stresses.

2.5. Cytocompatibility and degradability of supercapacitors An important advantage of using natural biopolymers over metals and synthetic polymers is the ability to be bioresorbed or degraded in a physiological environment. The biopolymer nature of the construct coupled with the versatility of silk proteins, permits the biocompatibility and degradability of the entire device in a controllable manner.37 It is important to note that, keeping with the technical definition when referring to a biomaterial, we use the term biodegradation to include processes that result in dematerialization or 12 ACS Paragon Plus Environment

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decomposition, with an ultimate loss of material integrity.52 The modified silk proteins used as the carrier and substrate in this SC were previously shown to be cytocompatible and biodegradable under protease action.33 Previous reports on PEDOT:PSS-rGO also show that they are biocompatible together.53-54 Preliminary cytocompatibility experiments showed that human dermal fibroblasts (HDF) cells attach to the devices and no adverse impacts on their growth or confluence was noted after 4 days in culture (Figure S10). While long term experiments are needed, these initial results show that the fully organic nature of the µSCs are biofriendly and benign to cells.

An enzymatic degradation essay was conducted on the electrodes patterned on flexible fibroin sheets in protease vs. PBS (control) at 37°C to observe the overall device decomposition over time. These samples were removed at specific time intervals (weeks) and imaged to investigate the degradation. Figure 5 shows SEM and optical microscopy images over three weeks. Whereas no appreciable degradation was observed on samples kept in PBS even after three weeks, the samples underwent proteolytic degradation each successive week, with a progressive loss of structure as observed. In ~1 month, the entire device broke down and a fibrous nanoscale morphology was observed (corresponding control samples were still intact). The amount of non-biodegradable component (i.e. the conducting polymer) is very low (~0.06 mg/4.5 mg device). The crosslinking of the proteins as well as the thickness of the electrodes/films can be easily controlled by modulating the pendant reactive moieties. Thus the devices can be precisely engineered to decompose over a specific period. As noted above, this advantage of the device coupled with its flexibility and performance is novel. The performance of the µSCs in aqueous media was separately investigated by studying samples immersed in PBS (n=3 samples). No physical delamination of the functional electrodes from the support film was observed even after 1 month, indicating the mechanical stability of the system in liquid. By control of the thickness of the support matrix and film, the µSCs can be 13 ACS Paragon Plus Environment


fabricated to not only be tunably degradable, but also tunably functional over different periods of time (days to weeks) prior to eventual loss of function.

3. Conclusions In summary, this work demonstrates a green and sustainable fabrication of a flexible microsupercapacitor based on a conducting polymer used with a silk protein carrier and silk substrate to form a fully organic, degradable device. The µSCs show a high capacitance, with a fast, reversible capacitive nature, and deliver power densities comparable to conventional devices without the use of toxic electrolytes and/or non-degradable charge collectors. The benchtop, room temperature and aqueous photolithographic process makes it cost efficient and scalable. The novel approach involving a biofriendly conductive ink for electrodes, and benign gel electrolyte, presents a viable alternative for powering implantable and transient flexible devices. The cytocompatible device is mainly comprised of proteins and polysaccharides (agarose) that can be resorbed inside body, making it suitable for use as a transient bioelectronic system. The device is bendable and durable under cyclic bending cycles which is an essential requirement for implantable bioelectronic devices. The interpenetration of the conducting polymer with an rGO dopant permits a device with high energy density and power density. Optimization of the capacitive performance using different electrolytes and support materials suggest opportunities which are currently being investigated.

4. Experimental Section Synthesis of carrier protein photoresist: The carrier and support were fabricated from silk proteins - fibroin and sericin photoresists (FPP, SPP), prepared following earlier works.55-56 Fibroin was degummed and purified from silk cocoons (B. mori) as discussed elsewhere.57 Sericin was procured in pure form (Wako Chemicals, Richmond, VA). Briefly, the proteins 14 ACS Paragon Plus Environment

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were first dissolved in 1 M LiCl/DMSO solution and reacted with a stoichiometric amount of 2-isocyanatoethyl methacrylate (IEM) at 60 °C and 5 h in a dry nitrogen purged system. The mixture was poured into excess cold ethanol to precipitate out the protein with pendant methacrylate groups. The products were washed in a 50-50 mixture of cold ethanol/acetone, centrifuged, and lyophilized for 48 h to obtain the final product.

Formation of photocrosslinkable conductive ink: The conductive ink to form the electrodes was formed by adding together sericin protein photoresist (SPP) as carrier, PEDOT:PSS as the active material, rGO as dopant, and DMSO as plasticizer. Dry re-dispersible pellets of PEDOT:PSS (Orgacon™, Sigma-Aldrich, St. Louis, MO) were ultrasonicated for 20 min in water, and filtered with 0.25 µm syringe filter to obtain 1.0% w/v solution. 5 % (v/v) DMSO was added to PEDOT:PSS to enhance the conductivity and improve plasticity of conductive ink. In the second step, graphene oxide (GO) was reduced in water at room temperature.43 2 mg/ml graphene oxide (GO) solution (University Wafer, South Boston, MA, USA) was reduced with ascorbic acid (Fisher Scientific, Asheville, NC) (5 mg per 1 mg of GO) under continuous stirring at room temperature for 1 hour. The reduction of GO to rGO was confirmed by UV-Vis absorption spectra which shifted from 230 nm to 262 nm. Freshly reduced rGO was always used to avoid precipitation. SPP and rGO were added to PEDOT:PSS solution to form conductive ink with specific compositions. Darocur 1173 (BASF) (0.1 µl/1mg of SPP) was added as a photoinitiator prior to use.

Fabrication of electrodes on fibroin films: Support films were prepared by casting a 6.15% (w/v) solution of FPP in formic acid (Acros Organics 98%). The substrates were prepared by casting on silicon or glass substrates and crosslinking under 365 nm UV light for 3 s. 2 mg/ml rGO stock solution and 1% (wt./vol) PEDOT:PSS solution having 5% (vol) DMSO were prepared separately. The two solutions were mixed together with the carrier protein and 15 ACS Paragon Plus Environment


photoinitiator to form the conductive ink at different compositions. Conductive ink was drop cast on fibroin film substrate and allowed to air dry in the dark. Following air drying, patterning was performed via contact-photolithography using a dark-field mask and UV exposure for 2.5 s. Micropatterns were developed in water to obtain defined interdigitated electrodes. Electrode patterns were fixed with silver wires and insulated with epoxy (Epotek, Billerica, MA) to ensure that the supercapacitor devices stay metal-free, and avoid false enhancement in capacitive behavior due to contact of electrolyte with the silver.

Electrolyte gel development: A biofriendly electrolyte was adapted for use. NaCl was dissolved in 10 ml of DI water to form a 5M solution. 100 mg agarose (Sigma-Aldrich, St. Louis, MO) was added. The agarose was melted and dissolved in solution by heating in a water bath at 90° C for 2 hours. PVA-H3PO4 was studied as a conventional electrolyte. 0.2 g PVA (9000 MW, 80% hydrolyzed) was mixed with 2 ml of water and heated under continuous stirring until the solution became clear. Then 0.16 g of H3PO4 was added.

Electrochemical experiments: Cyclic voltammetry (CV), galvanostatic charge-discharge (CD) and electrochemical impedance spectroscopy (EIS) were used. Electrodes were formed on ITO/glass substrate and identical electrode areas were defined using PTFE tape. Electrochemical characterizations were conducted using a two-electrode cell configuration. 5 M NaCl-Agarose or 0.8 M H3PO4-PVA gel electrolytes were used with a potential range of 0 to 0.5 V and 0 to 1.0 V for C-D or CV experiments respectively. EIS measurements were done in a three-electrode electrochemical cell with 0.1 M PBS buffer as electrolyte. The EIS data were collected in a frequency range of 0.01 Hz to 104 Hz with a 5 mV AC amplitude. The capacitance was calculated from the galvanostatic C-D curves using equations discussed in the supporting information.44 Microelectrodes were connected to silver wires and sealed using hermetic epoxy to avoid noise at the metal-electrolyte interface. 16 ACS Paragon Plus Environment

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Bio-resorption, stability in aqueous media, and imaging of supercapacitor devices: Degradability of the devices was studied by enzymatic degradation over time. Two sets of devices (4 mg of protein total) were incubated either in 10 ml of protease (1 U/mg of protein) (Protease XIV from S. Griseus, Sigma-Aldrich) or PBS buffer (control) at 37 °C. The enzyme solution was replaced every third day to preserve activity. Every week, one sample from each set was removed, washed with deionized water, and dried for further study. The study was carried for 3 weeks. Films were imaged using scanning electron microscopy (SEM) to observe the changes in the surface morphology. Optical and scanning electron microscopy (SEM) images were taken on a Nikon Eclipse LV-100D and Hitachi FE-SEM SU-70 instruments respectively. The patterns were first sputter coated in 20 Å platinum Denton vacuum V cold sputtering system (Moorestown, NJ). Stability of supercapacitor in liquid environment is an important factor for the device to qualify for implantable applications. Therefore, the stability of the flexible device was studied in 0.1 M PBS buffer medium over 1 week. Three samples suspended in PBS (functioning as liquid electrolyte) were used over this period. Each day, galvanostatic charge-discharge experiments were carried out.

Supporting Information Supporting Information showing additional images and electrochemical characterization, cell compatibility of the SCs is available.

Acknowledgements SEM images were obtained in the Nanomaterials Characterization Center at VCU. The assistance of Dr. Ning Zhang and Chenyang Jiang of the VCU Biomedical Engineering Department

in

cell

culture

experiments

is


gratefully

acknowledged.


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Scheme 1. (a) Fabrication of flexible microsupercapacitors using photolithography. The conductive ink is spin coated on a fibroin substrate and developed using water following UV exposure. SEM images of the electrodes are shown for devices – 50 µm electrodes on a 1.95 mm ⨯ 1.1 mm device; 250 µm electrodes on a 4.75 mm ⨯ 2.9 mm device. The scale bars shown for the SEM images in panels (b) 100 µm and (c) 500 µm. (d) The devices are flexible and freestanding as well as conformable as shown by placing on skin or as a foldable or planar structure.


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Figure 1: Electrochemical characterization of conductive ink (a) electrochemical impedance spectroscopy (EIS) as a function of composition (C1 – 28% PEDOT:PSS + 1.4% rGO; C2 – 28% PEDOT:PSS + 5.4% rGO; C3 – 48% PEDOT:PSS + 2.2% rGO; C4 – 48% PEDOT:PSS + 8.16% rGO), and (b) Effect of scan rate on capacitance behavior of the chosen composition (C4).



Figure 2: Performance evaluation of fabricated microsupercapacitors (a) galvanostatic charge-discharge as a function of composition, (b) Specific capacitance as a function of composition. (c) Charge-discharge as a function of current density for the chosen composition (C4) (d) Ragone plot at two compositions (C3 and C4) of the conductive ink.


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Figure 3: Electrochemical stability of the supercapacitors: (a, b) The panels show that the µSCs can be used to light an LED for a few seconds. In this experiment, 4 µSCs were connected in series to provide 2V. (c) change in charge-discharge characteristics after 500 cycles, and (d) capacitance retention over 500 cycles. (e) change in charge-discharge characteristics over 5 days suspended in PBS, (f) capacitance retention over this time period (98.7 ± 0.3 % for n = 3 devices).



Figure 4: Stability under mechanical stress: (a) under cyclic stress cycles. The different colors represent the number of bending cycles (e.g. black = initial condition, red = 50 cycles, green = 450 cycles) and (b) under steady U-bend (black = flat condition, red = U-bend condition).


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Figure 5: Proteolytic degradation PEDOT:PSS-Silk supercapacitor devices: SEM images of conductive ink-silk electrodes at Day 7, 14 and 21 in (L) PBS buffer (protease –) and (R) enzyme solution (protease +). Proteolytic degradation is clearly evident, whereas the control samples show no significant change in surface morphology. Scale bars on SEM images = 500 nm. The optical images (bottom panels) show visible degradation of the devices after 21 days of degradation. Scale bars on the optical images (bottom panels) = 50 µm.



Table of Contents graphic


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Fabrication of Flexible, Fully Organic, Degradable Energy Storage

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