Soft Electronic Platforms Combining Elastomeric Stretchability and Biodegradability

Research Article

www.advsustainsys.com Soft Electronic Platforms Combining Elastomeric Stretchability and Biodegradability

Martin Held, Alexander Pichler, Joshua Chabeda, Natalie Lam, Philip Hindenberg, Carlos Romero-Nieto, and Gerardo Hernandez-Sosa*

of these devices would, on the one hand, Soft and stretchable electronic devices are expected to offer technological help to reduce the high amount of global advances in the field of robotics, human–machine interfacing, and healthcare. electronic and plastic waste.[8] On the other Employing biodegradable elastomers, hydrogels, and nontoxic conductors hand, it would enable implantable tempo- would add significant value to and minimize the ecological impact of such rary sensors or prosthetics, which would not have to be removed after the therapy, disposable and transient electronic applications. Here, the biodegradable and making a second surgery redundant.[9,10] photo-crosslinkable elastomer poly(glycerol sebacic) acrylate (PGSA) is charac- An increasing number of biodegradable terized for its use in soft and stretchable electronics. Its mechanical proper- electronic elements,[11–13] such as transis- ties are investigated in terms of their chemical composition and compared to tors,[14] capacitors,[15] or light-emitting elec- [16–18] commonly used gelatin hydrogels. Furthermore, these materials are combined trochemical cells (LECs) have been reported in recent years. However, they with interconnects made of liquid Galinstan in order to create functional sub- have been commonly fabricated on rigid or strates with certified biodegradability under ISO standards. The combination foil substrates,[19] which lack stretchability. of these materials produces elastic circuit boards that act as soft platforms for Materials exhibiting evident biodegrada- body-mounted sensors or biodegradable stretchable light-emitting devices. tion such as silk fibroin[20] or gelatin[21,22] These soft platforms reveal linear elongations at a break of 130% to 350% and have been presented as alternatives when similar moduli to nondegradable elastomers and human tissue, without any increasing their elasticity by adding a high content of plasticizer and creating a decrease in conductivity. Advanced applications in biofriendly packaging, soft hydrogel. Furthermore, researchers have robotics, and healthcare will greatly benefit from these biodegradable devices. been looking into synthetic elastomers in order to attain full control of their desired mechanical, chemical, and optical proper- 1. Introduction ties.[23–26] For biodegradation, the elastomer chains need to con- tain connectivities that permit either a hydrolytic or an enzymatic Stretchable electronic devices are currently being considered cleavage.[9,27,28] For example, the degradation of ester groups for numerous applications in bioelectronics as they eliminate in polyesters will be assisted by esterase enzymes in the body, the mismatch between mechanically rigid electronic devices alongside hydrolysis in the aqueous environment.[9,24,27] This is and soft biological tissue.[1,2] These new technologies offer the case for the biodegradable and elastomeric polyesters such exciting possibilities in the field of bionics and soft robotics,[3] as poly(glycerol sebacate) (PGS)[29–32] and poly(glycerol sebacate) for agricultural sensors,[4] smart packaging,[5] or wearable acrylate (PGSA),[33–35] which have been successfully tested as electronic devices in clothing and accessories.[6] Most of the tissue implants in vivo. Such implants require elastic deforma- existing literature reports on this topic present devices sup- tion at high stress, which is typically provided by crosslinks in ported by silicone- or polyurethane-based substrates, while dis- elastomeric materials.[24] Adding double-bond moieties to polyes- regarding their biodegradability.[7] An innocuous degradability ters enables photo-crosslinking, e.g., by adding acrylate [33,35,36] or

Dr. M. Held, J. Chabeda, N. Lam, Dr. G. Hernandez-Sosa Dr. M. Held, J. Chabeda, N. Lam, Dr. G. Hernandez-Sosa Light Technology Institute InnovationLab Karlsruhe Institute of Technology Speyererstr. 4, 69115 Heidelberg, Germany Engesserstr. 13, 76131 Karlsruhe, Germany A. Pichler, P. Hindenberg, Dr. C. Romero-Nieto E-mail: [email protected] Institute of Organic Chemistry The ORCID identification number(s) for the author(s) of this article Heidelberg University can be found under https://doi.org/10.1002/adsu.202100035. Im Neuenheimer Feld 270, 69120 Heidelberg, Germany Dr. C. Romero-Nieto © 2021 The Authors. Advanced Sustainable Systems published by Wiley- Faculty of Pharmacy VCH GmbH. This is an open access article under the terms of the Crea- University of Castilla-La Mancha tive Commons Attribution License, which permits use, distribution and Calle Almansa 14-Edif. Bioincubadora, Albacete 02008, Spain reproduction in any medium, provided the original work is properly cited. DOI: 10.1002/adsu.202100035

In contrast, liquid metals such as Galinstan (eutectic alloy of 68 wt% Ga, 22 wt% In, 10 wt% Sn) combine the high conductivity of metals (3.5 × 104 S cm-1)[57] with an intrinsic ductility due to the liquid state and biochemical inertness.[58–61] In this work, we synthesize and characterize the biodegradable and photo-crosslinkable elastomer PSGA and demonstrate its applicability in soft and stretchable electronics. We investigate its mechanical properties in terms of chemical composition, and compare it to commonly used gelatin hydrogels. Furthermore, we combine these materials with interconnects made of liquid Galinstan in order to create functional substrates. The biodegradation of PGSA is demonstrated via an internationally standardized test. Finally, a biodegradable light-emitting electrochemical cell pixel is mounted on top of these elastic circuit boards in order to attest their functionality as a carrier substrate for electronics. The demonstrated biodegradability and flexibility of our soft electronic platforms open up opportunities to address mechanically challenging applications in the field of smart packaging, agricultural sensors, soft-robotics, or healthcare.

2. Results and Discussion 2.1. Synthesis and Fabrication of Elastic Films

The synthesis of poly(glycerol sebacate) (PGS) consists in a solvent-free polycondensation, in which the natural educts glycerol and sebacic acid react in an environmentally friendly process (Figure 1a, details in Section S1.1, Supporting Informa- tion).[3,24,29,30,62] The polycondensate PGS has a linear structure due to the inert reaction conditions and shows thermoplastic behavior.[63] By thermal curing, its secondary hydroxyl groups may react with an excess of sebacic acid or terminal carboxylic groups to produce crosslinks and restrain its thermoplastic Figure 1. Molecular structure and biodegradability of elastic sub- properties. For this, PGS was kept at 120 °C under pressure for strates. a) Synthesis of poly(glycerol sebacate) (PGS). b) Synthesis of poly(glycerol sebacate) acrylate (PGSA) from PGS. c) Representative at least 3 d, forming an 800 µm thin elastomer film (details in structure of gelatin.[64] d) Biodegradation of PGSA-19 with a cellulose ref- Section S2.1, Supporting Information). Clearly, the large time- erence, tested according to DIN EN ISO 14851 (2004-10). Inset: A patch scale and harsh conditions of this thermal crosslinking process of PGSA wrapped around an Aloe vera leaf. represent a major drawback.[24,31] Note that the crosslinked PGS has been previously applied as a tissue implant and can there- fumarate moieties[37] or using maleic acid as a monomer.[38–42] The fore be regarded as biodegradable.[29,30,32] photocurability of PGSA has been exploited as a glue for heart As an alternative to thermal crosslinking, chemical surgery[43,44] and as bioinspired structural adhesives.[45] Compared crosslinking of PGS by toluene diisocyanate would limit the to natural biopolymers, their synthetic counterparts are, in gen- pot life of the solution and create the risk of a residual toxic eral, immunogenically inert, provide a batch-to-batch uniformity chemical in the elastomer. Conversely, by introducing acrylic and usually undergo hydrolytic degradation, so they show only a groups through the free hydroxy groups of PGS, the resulting minimal site-to-site and patient-to-patient variation.[46] poly(glycerol sebacate) acrylate (PGSA) would offer the pos- A biodegradable elastic substrate requires equally stretchable sibility to be photo-crosslinked faster and at room temperature interconnect electrodes. While gold is intrinsically biocompat- through the double bonds of the acrylic moieties, creating an ible, its evaporated thin films or foils are merely bendable and elastomeric network.[33–35,65] Thus, we conducted an acrylation require prestretched substrates.[47,48] However, prestretching reaction in solution, in which the amount of added acryloyl chlo- and buckling decreased the transmittance of a stretchable LEC ride determined the final degree of acrylation (DA) of PGSA dramatically.[47] As an alternative, gold nanoparticles and nano- (Figure 1b, details in Section S1.2, Supporting Information). wires require annealing and percolation,[49–52] usually achieving The removal of triethylamine chloride, which otherwise formed around half of the conductivity of the evaporated film (1× crystals in the final elastomer film, presented an essential step 105 S cm-1).[43] Additionally, they require strong adhesion to the in the synthesis procedure of elastic PGSA. This step was not elastomer matrix.[53] Other conducting materials like carbon elaborately discussed in previous synthesis protocols[33–35] and is black, graphene, and PEDOT:PSS exhibit lower conductivity therefore detailed in Section S1.3 (Supporting Information). (Graphene: 550 S cm-1,[54] PEDOT:PSS: 0.1–1 S cm-1[50,55,56]) The solution-processability of PGSA opens up various and provide elasticity only for strains of up to a few percent. film fabrication routes, e.g., spin-coating, blade-coating,

Figure 2. Photos of a) PGSA-19 and b) gelatin in linear tensile tests. Stress–strain curves for biodegradable substrate candidates and an Ecoflex reference in a c) destructive and d) cyclic linear tensile test.

microdispensing, making it attractive for industrially relevant sample) to 6.9 and small white flakes of biomass, indicating processes. Thus, we fabricated films with a concentrated and biodegradation. viscous solution of PGSA by drop-casting onto a carrier sub- Recording the oxygen consumption of the degrading sample strate or into a casting mold (details in Sections S2.2 and S2.3, with a respirometer enabled the comparison to the theoret- Supporting Information). A UV photo-crosslinking step with ical oxygen demand, which serves as the figure of merit for the aid of the photoinitiator 2,2-dimethoxy-2-phenylacetophe- biodegradability. A 100% of the theoretical oxygen demand rep- none for the radical polymerization (details in Section S1.4, resents full oxidation of the sample’s carbon. This value may be Supporting Information) successfully produced either thin elas- exceeded by the development of additional CO2 from the primer, tomer films or thicker elastomer films with embedded channels. when the addition of the sample triggers the biodegradation of The lower temperature and shorter timescale of this process are the seeding material (activated sludge), causing a higher oxygen highly advantageous compared to the thermal crosslinking of consumption than the theoretical demand. PGSA-19 and the ref- PGS. Multiple layers with intermediate crosslinking ensure a erence showed a typical exponential decay behavior (Figure 1d). firm structure within the bulk of the film. The cellulose reference reached 133 ± 1% of the theoretical Hydrogels can be used as an alternative concept to elastomers oxygen demand within 68 d. The second rise in degradation for elastic biocompatible substrates,[66,67] despite exhibiting a beyond 21 d and exceeding 100% resulted from the afore-men- drastically different molecular structure and properties com- tioned priming effect. The biodegradation of PGSA-19 reached pared to elastomers. Gelatin forms elastic hydrogel films when 89 ± 10% of the theoretical oxygen demand in the same period. processed from aqueous solution, to which we added glycerol as The plateau beyond 60 d indicates that the sample reached its an environmentally friendly plasticizer (details in Section S2.4, maximum degradation. Consequently, PGSA-19 is certified bio- Supporting Information). Note that the heterogeneous polypep- degradable in aqueous media by reaching 90% biodegradation tide mixture of gelatin exhibits variable molecular weight and in the plateau phase, satisfying the definition of biodegradability composition that heavily depends on its extraction procedure of DIN EN 14995 and DIN EN 13432.[73,74] These results add from collagen. The acidic pretreatment route forms gelatin type to previous data that proved PGSA to be biocompatible and to A that includes roughly 33% glycine, 11% alanine, 11% proline, encourage cellular adhesion and growth in vivo.[33] Likely, PGSA 10% hydroxyproline, 5% arginine, 5% glutamic acid, and 25% will exhibit the same degradation mechanism as PGS, i.e., via remaining amino acids.[68–70] On average, the order of amino a surface erosion mechanism that maintains the mechanical acids follows the pattern Gly-X-Y (Figure 1c).[64,71] On a molecular properties and the integrity of the bulk material.[23] In contrast, level, the intermixed structural polypeptides of the hydrolyzed the bulk hydrolytic degradation of traditional polyesters like collagen provide a high elasticity without chemical crosslinks. poly(glycolic acid) is based on a random chain scission, thus altering the molecular weight and mechanical properties of the bulk.[23,24] The certified biodegradability of PGSA allows a 2.2. Biodegradation of PGSA plethora of applications on living subjects (Figure 1d inset).

As gelatin originates from a natural material and is composed of oligopeptides, it is obviously biodegradable.[66] In contrast to 2.3. Mechanical Properties of Biodegradable Elastic Films gelatin, synthetic elastomers that are suitable for tissue engi- neering, such as PGSA,[33–35] have not been investigated with An application of elastic materials as a substrate for biodegrad- regard to their biodegradability, yet. Thus, the biodegradability able electronics will encompass compressive and tensile strain, of PGSA with a 19% DA (PGSA-19) was investigated with a where the latter truly exposes their advantage over thermo- standardized biodegradation test in aqueous media according plastic foil substrates. Linear tensile tests (Figure 2a,b) revealed to DIN EN ISO 14851 (2004-10),[72] using microcrystalline the mechanical key properties of candidate elastic materials cellulose as a reference. Both test volumes of the reference and (Table S2, Supporting Information). When applied to the body, PGSA-19 revealed a decrease of pH value from 7.0 (blind control the Young’s modulus E of the elastic substrate should match the

E of the respective body tissue in order to minimize the need Cyclic linear tensile tests confirmed the results of the destruc- for geometric compensation in case of a difference in stiffness. tive tensile tests (Figure 2d). The small hysteresis emphasized As gelatin originates in the collagen of connective tissue, it the reproducibility of the discussed mechanical properties under offers very similar mechanical properties to the tissues of the periodic load and the absence of a dominant viscoelastic compo- human body. The low Young’s modulus E = 48 kPa of this gel- nent in the Young’s moduli. In order to add a visual perspective atin-plasticizer hydrogel matches the modulus range of muscle to the stress–strain data, Figure S6 (Supporting Information) tissue (10–500 kPa).[24,75–77] Additionally, gelatin exhibited a high shows photos of stretching each of the elastic materials by elongation at break εB = 155% (Figure 2c). However, as gelatin hand. Combinations of two materials could further enhance the is a natural product with nonuniform chemical composition, its mechanical properties,[78] as the bonding strength of PGSA–gel- mechanical properties are subject to batch-to-batch variations. atin exceeded the one of gelatin–gelatin.[79] Thus, εB may be as low as 120% (Figure S5b, Supporting Infor- As a result of this extensive comparison of biodegradable mation). The mechanical properties of gelatin could be further and nondegradable elastic material candidates, the mechanical tuned by mixing gelatin products of different gel strengths. properties of gelatin and PGSA-19 in combination with the Unlike gelatin, PGSA offers the possibility to tailor its ability of solution processing qualify them to be further investi- mechanical properties by adjusting the DA. Therefore, the gated for biodegradable stretchable platforms. properties of the target tissue can be imitated with PGSA by controlling an additional parameter besides changing the molecular weight or adding additives.[33–35] Our experiments 2.4. Biocompatible Stretchable Interconnects revealed that PGSA with a 28% DA (PGSA-28) exhibited the expected rubber-elasticity of an elastomer but with a much In order to equip the PGSA-19 and gelatin substrates with bio- higher E = 592 kPa than gelatin. With a similar ultimate tensile degradable electronic components in a printed-circuit-board- [80–82] strength σUTS, this led to a low εB of only 20%. By reducing the like manner, a biocompatible stretchable conducting DA to 19%, the reduced number of crosslinks decreased the E material is required for the interconnects. Galinstan was to 143 kPa, still in the range of muscle tissue, and increased chosen because of its biocompatible constituent elements and εB significantly to 130%. PGSA-19 exhibited more rubber- constant conductivity under deformation, resulting in infinite elastic behavior than PGSA-28. Reducing the DA further to 0% mechanical compliance.[83,84] (PGS) retained E, but decreased the εB to 103% due to a lack of Several patterning concepts for liquid metals in elastomers crosslinks. Thus for an optimized εB with ductile deformation, have been reported previously, including lithography pro- one must find an optimum DA between 0% and 28%. cesses, injection into predefined features (e.g., microchannels), However, a sheet of prematurely crosslinked PGSA subtractive methods, i.e., selective removal of the metal, and (PGSA-19*) emphasized the full potential of PGSA’s mechan- additive methods where the metal is deposed only in desired ical properties. The obtained Young’s modulus of 80 kPa, high regions, e.g., via inkjet printing.[60,61,85] However, none of these ultimate tensile strength of 265 kPa and a high elongation at turned out to be compatible with PGSA-19 and gelatin. Depos- break of 350% (Figure S5c, Supporting Information) are com- iting gelatin or PGSA-19 solution into a laser-cut cast mold petitive to soft silicone rubbers, such as Ecoflex with a low enabled drop-casting the liquid metal into these predefined E = 34 kPa and very high εB = 522%. Increasing the molecular channels (details in Section S2, Supporting Information). This weight of PGSA would certainly increase the ultimate tensile step could be readily upscaled with a microdispenser or a modi- strength further. fied inkjet printhead.[60,61,86,87] Galinstan dewets from the sur- Silicones, such as Ecoflex or poly(dimethylsiloxane) (PDMS), face of gelatin and PGSA-19, which predicts a stable laminar are commonly used as stretchable substrates due to their ability flow in the channel. Subsequently, the Galinstan trace was of liquid processing. However, they are not biodegradable and covered with a second layer of elastomer (PGSA solution or a thus inferior to PGSA and gelatin for ecofriendly or health- solid gelatin layer), embedding the Galinstan interconnects care applications. The stiffness of PDMS (E = 1124 kPa) is in in the substrate material. This fabrication process enables the the range of skin (0.7–16 MPa)[24] and significantly larger than patterning of biodegradable soft platforms, i.e., elastic circuit that of Ecoflex, PGSA, or gelatin. The same disadvantages apply boards, on an industrial scale with arbitrary complexity. to vulcanized natural rubber, which also cannot be cast after vulcanization. Biodegradable alternatives to gelatin and PGSA are com- 2.5. Soft Platforms under Strain mercially available latex milk and “Terratek Flex”, which is a thermoplastic elastomer with a 35% biobased content and Four different patterns of interconnect traces were tested in a certified compostability. Both exhibit a drastically higher linear tensile test: A straight line and three serpentines with a Young’s modulus in the MPa range, much higher than that sinusoidal, semicircle, and horseshoe shape (Figure 3a,b). Even of typical muscle tissue. Conclusively, these two materials are though serpentine geometries are well known to be beneficial too stiff for skin-mounted devices. Terratek Flex even revealed for solid and especially printed interconnect traces,[88–92] the a plastic deformation above 26% tensile strain, despite being liquid metal equivalent has not yet been compared to linear marketed as a biodegradable rubber replacement. Likewise, interconnects. poly(ethyleneoxide) with a trimethylolpropane ethoxylate plas- Samples of PGSA-19 and gelatin, each with embedded Galin- ticizer did not show elastic properties (details in Section S2.5, stan interconnects, were stretched repeatedly with an increasing Supporting Information). elongation while the resistance of a serpentine geometry and

Figure 3. Embedded serpentine Galinstan interconnects in a) gelatin and b) PGSA-19 for c) cyclic tensile tests. Stress and relative resistance of the interconnect versus strain for d) PGSA-19 and e) gelatin.

the stress were simultaneously measured (Figure 3c). The ini- with the measurement clamps due to the increased pressure tial absolute resistance ranged around 2 Ω for each trace and in the embedded channel because of the Poisson effect. Corre- remained constant for every cycle. Hence, the strain-dependent spondingly, a fatigue test, i.e., repetitive but constant strain, of resistance was normalized by the initial resistance for every surface-mounted Galinstan serpentines on PGSA-19* revealed cycle to account for the different lengths and thicknesses of a reduction of the resistance by around 6% within 500 strain the serpentines. For PGSA-19, none of the geometries exhib- cycles (Figure S7, Supporting Information). ited a rise in resistance neither within a strain cycle nor after Overall, both biodegradable elastic materials were success- various strain cycles, highlighting the high strain compatibility fully equipped with stretchable and biocompatible intercon- of the liquid metal electrode material (Figure 3d). A serpentine nects of arbitrary shape. Each of the four trace geometries geometry of the interconnect did not result in a lower resist- exhibited full compliance for a cyclic tensile strain of up to ance or more strain-resistant behavior compared to the straight 100%. Thus, these ecofriendly stretchable platforms are well trace. Similar results were obtained for the gelatin sample suited for reliable biodevices that experience high strain. (Figure 3e). A slightly decreasing resistance at higher strain In order to demonstrate the application of the embedded Gal- can be attributed to a better interfacial contact of the Galinstan instan interconnects in a biodegradable matrix, we embedded a

Figure 4. a) Application of a stretchable gelatin–Galinstan soft platform on a human elbow. b) The resistance was recorded throughout the bending motion of the elbow. c) Repeating the experiment seven times. straight trace in a gelatin substrate and attached it to a human setup of the experiment. Interfacing with hard wire electronics elbow (Figure 4a). The elbow constitutes a part of the human could be more reliable when employing nanowire-elastomer body where the skin gets stretched with a very high magni- blends,[53] nanoparticle-elastomer blends[86,93,94] or HCl vapor tude and frequency. As the ends of the specimen are fixed, it treatment.[95] The overall reproducible and constant change experiences both bending and tensile stress, where the Galin- in resistance of these stretchable and biodegradable intercon- stan trace is roughly located in the neutral axis. The resistance nects renders them suitable as a soft platform for applications was measured by piercing wires into the terminal reservoirs on human patients or farm animals, exploiting the biodegrada- of the interconnect, exhibiting an initial value of 0.6 Ω. When bility in case of accidental detachment. bending the elbow, the recorded resistance plateaued at 0.7 Ω at a 90° bending angle, corresponding to a strain of approxi- mately 30% (Figure 4b). When straightening the elbow, the 2.6. Soft and Biodegradable Circuit Boards resistance returned to its initial value. This reversible change [81] of R/R0 = 16% exceeded the relative resistance variation of A further step toward biodegradable circuitry is mounting the straight trace in the linear tensile test (R/R0 = 8% for 30% electronic devices onto the soft platforms that are made of strain in Figure 3e) due to a combination of the tensile stress embedded Galinstan in biodegradable elastomer matrices. induced Poisson effect and the bending stress induced com- First, two embedded interconnects in the PGSA-19 or gelatin pression of the channel. Still, this absolute change in resistance substrate linked a light-emitting diode (LED) to a power supply of 0.1 Ω is very small and constant over seven bending cycles (Figure 5a). (Figure 4c, videos in the Supporting Information). The very While many previous reports focus on testing such an short resistance spikes are attributed to the volatile interfacial arrangement merely under linear strain,[94,96] stretching the resistance of the moving wire in the Galinstan reservoir, which elastomer film over an object emulates the isotropic bending is not considered a part of the device but was necessary for the stress of a body-mounted device, as it was produced on the

Figure 5. Application of the stretchable biodegradable platforms with electronic devices. a) Schematic connection of an LED to the biodegradable soft electronic platform. b) Photograph of isotropic stretching of the PGSA-19 elastic substrate. c) Side view and d) top view of a mounted LED under an isotropic bending strain of 21%. e) Current and luminance of the LED at an increasing isotropic bending strain.

Figure 6. Fully stretchable and biodegradable light-emitting device. a) Schematic device stack and b) photograph of the biodegradable light-emitting electrochemical cell (LEC) pixel. c) LEC pixel in operando. d) Schematic setup of the LEC pixel on the soft electronic platform. e) Isotropic stretching of the elastic substrate with the mounted LEC (indicated by the red arrow) at a strain of 21%. f) Current density and normalized luminance of the LEC at an increasing isotropic bending strain.

elbow (Figure 5b). Increasingly larger half-spheres in the setup partial biodegradation in aqueous media of 62% theoretical produced an increasing strain of up to 21% (Figure 5c). Indeed, oxygen demand in 82 d (Figure S8a, Supporting Information), this value falls much below the maximum linear strain of the corresponding to a 46% relative degradation compared to the tensile tests, but the isotropic nature of the puncturing load did cellulose standard (Figure S8b, Supporting Information details not permit a Poisson-effect-like compensation of the stretched in Section S5). A portion of the incomplete degradation of this material as it would be the case at linear stress. device can be attributed to the biologically inert PEDOT:PSS. Under mechanical load, the embedded Galinstan inter- Metals, such as gold, are not considered for the theoretical connects remained visually undistorted, even at a high strain oxygen demand. However, certified tests in a compost environ- (Figure 5d), confirming the durability of the PGSA-19 elas- ment have revealed full biodegradability for a device made of tomer and gelatin hydrogel as presented in the linear tensile the same cellulose acetate, a gelatin-based electrolyte, printed and elbow stretching tests. Thus, the Galinstan interconnects gold ink, and PEDOT:PSS.[97] Hence, the cellulose acetate likely in both elastomer substrate materials supplied the LED with a constitutes the degradation bottleneck in water and such elec- constant current up to a 21% strain (Figure 5e). Consequently, tronic devices require compost conditions to degrade properly. the luminance remained constant as well, as the variations can The bendable LEC pixel was attached to the stretchable be assigned to a slightly varying orientation of the LED with substrate (Figure S9a–c, Supporting Information). As the size respect to the photosensor. of the contact spot (2 × 2 mm) was relatively small compared to the substrate, the presence of the pixel did not interfere with the mechanical properties of the elastic substrate. The Galin- 2.7. Fully Biodegradable Light-Emitting Device under Isotropic stan directly contacted the gold and PEDOT:PSS electrodes Bending Strain of the LEC pixel and linked it to the external power supply (Figure 6d). Printing the LEC layers directly onto an elastic In order to take full advantage of the biodegradability of the substrate resulted in excessive swelling and nonfunctional PGSA-19 and gelatin substrates and the biocompatibility of the devices (Figure S9d, Supporting Information). Galinstan interconnects, the LED was replaced with a printed Again, isotropic stretching by semispherical objects up to and partially biodegradable light-emitting electrochemical cell a strain of 21% (Figure S9e, Supporting Information) emu- (LEC) pixel (Figure 6a).[16–18] The LEC comprised a printed gold lated the strains in wearable applications (Figure 6e). Upon cathode, a printed PEDOT:PSS anode, and a blade-coated active increased bending, the current density of the pixels remained layer, i.e., a mix of a biodegradable solid polymer electrolyte and independent of the isotropic strain (Figure 6f). The decrease a yellow polymer emitter. A biodegradable thermoplastic foil in current density for the gelatin substrate beyond 14% can be served as a substrate for the pixel (Figure 6b).[19] attributed to a gradual loss of contact with the power supply. The entire LEC stack was tested for biodegradability under The luminance of the pixel’s yellow emission (Figure 6c,f) was the same conditions as the PGSA-19 substrate and revealed equally independent of the applied strain and followed the

Adv. Sustainable Syst. 2021, 2100035 2100035 (7 of 10) © 2021 The Authors. Advanced Sustainable Systems published by Wiley-VCH GmbH www.advancedsciencenews.com www.advsustainsys.com current density fluctuations. These variations were a property Biodegradation Tests: The certified Fraunhofer UMSICHT Institute of the pixel itself rather than a result of the bending (Figure conducted the biodegradability test according to DIN EN ISO 14851 S9f,g, Supporting Information). (2004-10) “Determination of the ultimate aerobic biodegradability of plastic materials in an aqueous medium—Method by measuring the In essence, the combination of a bendable pixel, liquid metal oxygen demand in a closed respirometer.” The testing volume of 250 mL, interconnects and a stretchable substrate material provided filled with an optimized mineral media and with the test and reference fully biodegradable light-emitting devices for isotropic bending substance at a concentration of 400 mg L-1, was seeded with 140 mg L-1 strains of up to 21%. Further small biodegradable devices could activated sludge from a sewage plant and kept at an incubation be attached to the soft platforms by state-of-the-art printed circuit temperature of 20 ± 1 °C. Microcrystalline cellulose (20 µm, Sigma board placing technology. This concept decouples the device Aldrich, product number 310697, total organic carbon 44%, theoretical CO amount 118.5 mg/100 mg) served as the reference substance. properties from the mechanical properties of the rubber-elastic 2 The blind control was prepared with the mineral media and inoculum substrate. Hence, the failure of the circuit is limited by the perfor- (seeding material, 0.28% dry mass, 20% of dry mass are organic) mance of the interconnects rather than the devices themselves. without any reference or test substance. The test substances were small chunks of crosslinked PGSA (19% DA), flakes of dried PEDOT:PSS (Clevios VPAI 4083, Heraeus), and shreds of a printed LEC stack on cellulose acetate (printed PEDOT:PSS FHC Solar, printed ZnOx, super 3. Conclusion yellow, P(CL-TMC), TBABOB, PEDOT:PSS FHC Solar). Three parallel runs were conducted for the reference, blind, and test samples. The We fabricated soft electronics platforms on the basis of the bio- validity criteria of a maximum oxygen demand of the seeding material degradable synthetic elastomers in combination with the liquid of 2 mg per mg dry mass (0.21 mg mg-1 dry mass after 82 d), as well as metal alloy Galinstan. The mechanical properties of the syn- a biodegradation degree of the reference substance of >60% (60% after thetic polymer PGSA were adjusted by the degree of acrylation 19 d) were fulfilled. The theoretical oxygen demand of the reference and compared to state-of-the-art biodegradable and nonbiode- and test substance was calculated from a chemical elemental analysis gradable rubber-elastic materials. The PGSA elastomer with a (elemental analyzer, “Elementar”) of the dry mass according to appendix A of DIN EN ISO 14851. 19% degree of acrylation was selected as the best performing Tensile Tests and Interconnect Resistance Measurement: An Alluris FMT- biodegradable elastic substrate for optoelectronics, on which 310BU tensile tests machine was employed, where the force was measured light-emitting devices were mounted. The certified biodegra- either with an FMT-310FUC5 500N (precision of 0.1 N) or an FMT- dability of PSGA and the evident ecofriendliness of gelatin 318FUB5 50N (precision of 0.005 N) unit at a speed of 10 mm s-1. The renders them more suitable than the commonly used PDMS Young’s modulus was calculated from the stress–strain slope of the initial for applications requiring bio- and ecofriendliness. 10% strain, with an error of about 2%. The Young’s modulus, ultimate tensile strength, and elongation at break exhibit a variation of up to 20% Embedding Galinstan conductive traces into PGSA and gel- between samples. The fabrication of the plain thin films for all materials atin created elastic electronic platforms that function for linear and embedded interconnect films for PGSA-19 and gelatin, including strains of up to 90% and were successfully applied to the body. the elbow-mounted interconnect, is detailed in Section S2 (Supporting These embedded interconnects supplied a standard LED and a Information). The thickness of the elastomer films was measured with biodegradable LEC pixel with power, revealing a stable perfor- a digital micrometer (Shut geometrical Metrology, 908.750), using mance for isotropic bending strains of up to 21%. Ultimately, the two Teflon sheets to avoid the torsion of the film by the micrometer. maximum strain of these stretchable devices is limited by the The resistance of the interconnects was measured by a Keithley 2612B SourceMeter continuously at 50 Hz through a custom sample clamp. The elastic substrate material, since the functionality of the Galinstan Galinstan (Ga:In:Sn = 62:22:16 by weight, eutectic, ≥99.99% purity) was electrode materials and the mounted pixel is strain independent. acquired from VWR or metallpulver24.de. The experiments with elbow- Fabricating biodegradable and fully stretchable pixels will remain mounted devices were performed in compliance with the KIT’s policy on challenging as long as emitter polymers and PEDOT:PSS elec- ethics, the informed consent of the human subject was obtained. trodes do not offer rubber-elasticity, form cracks after a few duty Mounted LEDs on Biodegradable Soft Platforms and Isotropic Bending cycles and limit the maximum linear strain to 30%.[53,98] Test: The yellow LEDs were purchased from Everlight and stuck directly into the outlets of the embedded Galinstan interconnects in the PGSA-19 These fully stretchable biodegradable devices emphasize the and gelatin substrates. The luminance–current–voltage characteristics possibility to transfer biodegradable technology to applications were recorded with a Botest Source Meter Unit (Botest LIV LED SMU, where high elasticity is required. The employment of biode- Asys Group). In order to apply an isotropic bending strain onto the gradable and body-compatible materials highlights the ecof- suspended elastic film, differently sized half-spheres were placed riendly potential of stretchable electronic devices and points underneath the film, stretching the suspended film in an isotropic out design routes beyond silicone and polyurethane elastomers. manner. The bending strain was calculated from the curvature and peak of the half-spheres. In contrast to the dynamic linear tensile tests, the Applications in agriculture, healthcare, packaging, and dispos- isotropic bending test was static. able electronic consumer goods would greatly benefit from this Light-Emitting Electrochemical Cell Fabrication: The LEC was fabricated unique combination of rubber-elasticity, biodegradability, and by printing and spincoating onto a 500 µm thick sheet of cellulose conductivity. acetate (Rachow Kunststoff-Folien), which was flattened by a silicon wafer in the nanoimprinter (NIL CNI V1.0) at 120 °C at 5.5 bar for 10 min, as previously published,[19] and treated with a 5 min oxygen 4. Experimental Section plasma. The bottom electrode was inkjet printed (Fujifilm Dimatix DMP 2831) with gold ink (DryCure AU-J without binder, C-Ink Japan) Synthesis and Origin of Elastic Materials: The synthesis of PGS and sintered in the nanoimprinter at 120 °C from 30 min. The active and PGSA is described in great detail in Section S1 of the Supporting layer combined the polymer emitter super yellow (PDY-132, Merck), the Information. The source of gelatin, Ecoflex, PDMS, natural rubber, latex, ion-conducting polymer poly(caprolactone-co-trimethylene carbonate) Terratek Flex, and PEO:TMPE is detailed in Section S2 (Supporting (80:20, synthesized) and the salt tetrabutylammonium bis(oxalato) Information). borate (synthesized). Each component was dissolved at 10 g L-1 in

anisole and mixed at a volume ratio of 1:0.1:0.075. The mixed solution [4] A. Luvisi, Agron. Sustainable Dev. 2016, 36, 189. was spincoated at 2000 rpm, 1000 rpm s-1 for 60 s and dried at room [5] K. B. Biji, C. N. Ravishankar, C. O. Mohan, T. K. Srinivasa Gopal, temperature in vacuum for 5 h. The top electrode was printed with the J. Food Sci. Technol. 2015, 52, 6125. same inkjet printer from a PEDOT:PSS dispersion (Clevios F HC Solar, [6] A. Neudeck, Y. Zimmermann, U. Möhring, J. Solid State Electro- Hereaus Germany) with 0.25 wt% zonyl surfactant and dried at room chem. 2015, 19, 3. temperature in vacuum. The LEC samples were cut into individual pixels [7] T. R. Ray, J. Choi, A. J. Bandodkar, S. Krishnan, P. Gutruf, L. Tian, of 10.5 × 7 mm size with an active area of 4 × 6 mm. R. Ghaffari, J. A. Rogers, Chem. Rev. 2019, 119, 5461. Mounted LEC Pixels on Biodegradable Soft Platforms: The LEC pixels [8] M. Heacock, C. B. Kelly, K. A. Asante, L. S. Birnbaum, Å. L. Bergman, were attached to the PGSA-19 or gelatin substrate (25 25 mm) with × M.-N. Bruné, I. Buka, D. O. Carpenter, A. Chen, X. Huo, M. Kamel, embedded stretchable Galinstan interconnects by a biodegradable P. J. Landrigan, F. Magalini, F. Diaz-Barriga, M. Neira, M. Omar, adhesive film (NatureFlex Silver-30 with BioTak S100). A small hole in A. Pascale, M. Ruchirawat, L. Sly, P. D. Sly, M. van den Berg, each contact pad of the LEC pixel acted as a via to connect the Galinstan outlet from below the pixel with the electrode on the planar pixel’s W. A. Suk, Environ. Health Perspect. 2016, 124, 550. surface. This via hole is essential to keep the Galinstan bead attached to [9] L. S. Nair, C. T. Laurencin, Prog. Polym. Sci. 2007, 32, 762. the pixel’s underside when the pixel’s edge gets lifted up from the elastic [10] A. Sadeghi-Varkani, Z. Emam-Djomeh, G. Askari, Int. J. Biol. Mac- substrate upon increasing isotropic strain. 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