Biodegradable Polymeric Materials in Degradable Electronic Devices

Abstract

Biodegradable electronics have great potential to reduce the environmental footprint of devices and enable advanced health monitoring and therapeutic technologies. Complex biodegradable electronics require biodegradable substrates, insulators, conductors, and semiconductors, all of which comprise the fundamental building blocks of devices. This review will survey recent trends in the strategies used to fabricate biodegradable forms of each of these components. Polymers that can disintegrate without full chemical breakdown (type I), as well as those that can be recycled into monomeric and oligomeric building blocks (type II), will be discussed. Type I degradation is typically achieved with engineering and material science based strategies, whereas type II degradation often requires deliberate synthetic approaches. Notably, unconventional degradable linkages capable of maintaining long-range conjugation have been relatively unexplored, yet may enable fully biodegradable conductors and semiconductors with uncompromised electrical properties. While substantial progress has been made in developing degradable device components, the electrical and mechanical properties of these materials must be improved before fully degradable complex electronics can be realized.

Figure 1

(A) Biodegradable electronics have numerous promising applications within the body and the environment. A typical electronic device, like the one depicted on this leaf, is built up from four main classes of materials: semiconductors (blue), conductors (silver), dielectrics (orange), and substrates (light green). (B) In general, biodegradable materials with desired electronic properties consist of an active material (dark blue) dispersed within a biodegradable matrix (light blue). For example, dielectric materials may use high dielectric constant fillers as active materials, whereas semiconductors and conductors use conjugated polymers as active materials that provide electronic conduction pathways within the matrix. Regardless of their electronic properties, the biodegradable materials discussed in this review can be classified into one of two categories. Type I materials are disintegrable, though only the matrix, which holds together nondegradable active materials, can be fully broken down into small molecule building blocks. On the other hand, both the matrix and active materials comprising type II materials can be fully broken down into monomers or oligomers. Because type II materials are fully biodegradable, they can potentially also be recycled.

Figure 2

Chemical structures of moieties susceptible to hydrolysis (A) and oxidation (B) are shown. Hydrolyzable bonds and sites of oxidative attack are highlighted in red and marked with an asterisk. (C) Ester hydrolysis may occur chemically (acid or base) or enzymatically. The mechanism for acid-catalyzed hydrolysis is shown, where the ester bond is cleaved to yield a carboxylic acid and alcohol byproducts. (D) PVA degradation is triggered by oxidation of 1,3-diols in the backbone, which can be catalyzed by either secondary alcohol oxidase (SAO) or periplasmic PVA dehydrogenase (PDH). Iterative oxidation and further degradation by aldolase and β-diketone hydrolyase along the PVA backbone lead to simple byproducts, such as acetic acid. Image adapted with permission from refs (10) and (11). Copyright 2008 Woodhead Publishing and 2014 Wiley.

Figure 3

(A) Scheme for general transfer process of devices fabricated on temporary silicon substrates to a final biodegradable substrate. The transfer method enables a broad choice of substrate materials, though may suffer from limited scalability. (B) Photographs displaying the dissolution of a biodegradable device on a PVA substrate in water over time. Scale bar is 5 mm. (C, D) Chemical structures of some common biodegradable substrate materials. Cellulose (C) is a naturally derived material frequently used as a substrate due to its flexibility and thermal stability. To create ultrathin cellulose films, it can be processed with sacrificial trimethylsilyl groups that can be removed via hydrolysis to yield films as thin as 800 nm. Poly(octamethylene maleate (anhydride) citrate) (POMaC) (D) is a synthetic biodegradable elastomer that can be used as a stretchable substrate. Images adapted with permission from refs (, , , and 23). Copyright 2014 Wiley, 2014 AIP, 2017 National Academy of Sciences, and 2010 Royal Society of Chemistry.

Figure 4

(A, B) The dielectric constant of degradable composites can be increased by incorporating high-κ additives such as Al₂O₃ (A) and carbon nanotubes (CNTs) (B). (C) DNA can be solution-processed into thin films by complexing it with cationic surfactants like hexadecyltrimethylammonium chloride (CTMA). (D) Structures of DNA nucleobases used as thin film dielectrics in OFETs. (E) Pyramidal microstructures enhance the sensitivity of capacitive pressure sensors made with the elastomeric dielectric PGS. Pressure sensor arrays fabricated from these devices are capable of detecting the presence of a 5 mg grain of rice. Images adapted with permission from refs (, , , , and 37). Copyright 2017 Springer, 2016 Royal Society of Chemistry, 2010 Wiley, 2010 Springer, and 2015 Wiley.

Figure 5

(A) SEM image of electrospun PLGA fibers coated with PPy, (B) which show enhanced growth and stimulation of PC12 cells as compared to noncoated ones. (C) SEM image of a lyophilized biodegradable hydrogel made from gelatin grafted with polyaniline. (D) Hyaluronic acid doped PEDOT (PEDOT-HA) increases the degradation rate of PEDOT-HA/PLLA composites by introducing additional hydrophilic domains. (E) PPy is doped by an oxidizing agent like FeCl₃. (F, G) Dopant-free conductive polyurethane (DCPU) comprising aniline oligomers are self-doped by incorporating dimethylolpropionic acid in the backbone. The resulting biodegradable elastomers are also highly stretchable. Images adapted with permission from refs (, , , , and 64). Copyright 2017 Elsevier, 2015 Springer, 2009 Elsevier, 2014 Royal Society of Chemistry, and 2016 authors of ref (64).

Figure 6

(A) Highly flexible devices made with imine-degradable semiconductor PDPP-DP on ultrathin cellulose substrates can be transferred to target surfaces after dissolution of an underlying sacrificial dextran layer. (B) PDPP-DP contains imine bonds that can hydrolyze under acidic conditions to corresponding aldehydes and amines. (C) Chemical structure of indigo, along with a photo of degradable indigo-based transistors on shellac. (D, E) The comproportionation reaction between melanin and water has been proposed to explain the doping effect of water on melanin (D), as evidenced by the observed increase in melanin conductivity with hydration (E). Images adapted with permission from refs (, , and 77). Copyright 2017 National Academy of Sciences, 2012 Wiley, and 2012 National Academy of Sciences.