Soft Electronic Platforms Combining Elastomeric Stretchability and Biodegradability
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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 Adv. Sustainable Syst. 2021, 2100035 2100035 (1 of 10) © 2021 The Authors. Advanced Sustainable Systems published by Wiley-VCH GmbH www.advancedsciencenews.com www.advsustainsys.com In contrast, liquid metals such as Galinstan (eutectic alloy of 68 wt% Ga, 22 wt% In, 10 wt% Sn) combine the high conduc- tivity 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 biodegrad- able 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 biodegra- dation of PGSA is demonstrated via an internationally standard- ized 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 elec- tronics. The demonstrated biodegradability and flexibility of our soft electronic platforms open up opportunities to address mechanically challenging applications in the field of smart pack- aging, 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