Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 1-290, Cambridge, MA

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This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi:

10.1002/adma.201904720.

Conductive Silk-based Composites Using Biobased Carbon Materials

Diego López Barreiroâ, Zaira Martín Moldesâ,b, Jingjie Yeoâ,b,c,d, Sabrina Shenâ, Morgan J. Hawker^b, Francisco J. Martin-Martinezâ, David L. Kaplan^b, Markus J. Buehlerâ*

aLaboratory for Atomistic and Molecular Mechanics (LAMM), Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 1-290, Cambridge, MA 02139, USA.

bDepartment of Biomedical Engineering, Tufts University, 4 Colby Street, Medford, MA 02155, USA.

cInstitute of High Performance Computing, A*STAR, Singapore, 138632.

dSibley School of Mechanical and Aerospace Engineering, Cornell University, NY 14850, USA

*corresponding author, mbuehler@MIT.EDU and +16174522750

Abstract

There is great interest in developing conductive biomaterials for the manufacturing of sensors or flexible electronics with applications in health-care, tracking human motion, or in-situ strain measurements. These biomaterials aim to overcome the mismatch in mechanical properties at the interface between typical rigid semiconductor sensors and soft, often uneven biological surfaces or tissues for in vivo and ex vivo

applications. In this paper we demonstrate the use of biobased carbons to fabricate conductive, highly stretchable, flexible and biocompatible silk-based composite biomaterials. Biobased carbons are

synthesized via hydrothermal processing, an aqueous thermochemical method that converts biomass into a carbonaceous material that can be applied upon activation as conductive filler in composite biomaterials.

We combine experimental synthesis and full-atomistic molecular dynamics modeling to synthesize and characterize these conductive composite biomaterials, made entirely from renewable sources and with promising applications in fields like biomedicine, energy, and electronics.

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Submitted to: Advanced Materials (adma.201904720)

Keywords: Biomaterial; Composite; Silk; Bioinspired; Biomass; Biocarbon; Nanomaterial

Main Text

Silk fibroin proteins obtained from Bombyx mori cocoons display outstanding mechanical properties that stem from a delicate hierarchical structure spanning different length scales^[1]. As shown in earlier work, the biocompatibility and biodegradability of silk fibroin proteins provides utility for the manufacturing of materials with applications in biomedicine, flexible electronics or water purification^[2,3]. Furthermore, adding plasticizers, such as glycerol^[4] or CaCl₂^[5], increases the match of mechanical properties of silk fibroin with biological tissues and enables the development of silk-based, soft, conformable biomaterials.

Generating conductive silk-based biomaterials has the potential to enable the development of sensors and flexible electronics with applications in health-care monitoring^[6], in-situ human motion tracking^[7], and strain measurements^[8], among others. Such conductive biomaterials would overcome the mismatch in mechanical properties at the interface between typical rigid semiconductor sensors and soft, uneven biological surfaces or tissues^[9]. However, turning silk fibroin into a conductive biomaterial is not straight- forward, and it requires the use of conductive fillers such as intrinsically conductive polymers^[10], carbon nanotubes (CNT)^[11], or graphene^[8]. In many cases, the hydrophobic nature of carbon-based fillers makes them immiscible with aqueous silk fibroin solutions, and usually requires the aid of complex organic solvent mixtures to disperse them in protein aqueous suspensions ^[8,12]. An alternative is to use carbon nanomaterials with polar functionalities to facilitate dispersion in aqueous silk fibroin solutions, such as graphene oxide. In earlier work, silkworms^[13] and spiders^[14] were also directly fed with graphene or CNTs to obtain conductive silk composite fibers. However, the content of carbon nanomaterials in the resulting fibers was insufficient for conductivity, requiring additional carbonization steps^[13].

An unexplored conductive filler material for silk-based biomaterials is biobased carbon, which can be obtained by physical activation of the hydrochars derived from hydrothermal processing (HTP) of biomass.

HTP is an aqueous thermochemical process that turns biomass into a solid carbon-rich material termed

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hydrochar^[15]. This hydrochar can be turned by physical activation into conductive biobased carbons, which display interesting properties that arise from their hierarchical porous nanostructure and their chemical functionalities^[16]. These properties make them promising materials for electrocatalysis in energy storage systems^[17], or carbon capture and sequestration^[18]. Their nanostructure and chemical functionalities can be tuned by varying HTP parameters, such as biomass feedstock, temperature, biomass:water ratio or reaction time, among other variables^[19]. One key property of biobased carbons is that they are readily doped with polar functionalities -mainly oxygen and nitrogen- that facilitate their dispersion in polar matrices like silk fibroin suspensions.

In this paper we report the manufacture of environmentally matched and conductive silk fibroin-biobased carbon biomaterial composites that combine the excellent mechanical properties of silk fibroin with the conductivity of biobased carbon fillers. These materials are compared against silk fibroin-graphene materials produced using a commercial 3D printing graphene ink. We also explored the effects of different silk fibroin:biobased carbon ratios (from 0.1 to 20.0 wt% loading of biobased carbon) on the electrical, mechanical and biological properties of thin films. These films were cast by dissolving silk fibroin and biobased carbon in formic acid at a 1:10 ratio (mass:volume). CaCl2 was added as a plasticizing agent to increase the compliance of silk fibroin. The experiments were complemented by atomistic molecular dynamics simulations to obtain fundamental understanding on how different silk fibroin domains

(amorphous, crystalline and N-terminus) interact with biobased carbons. Different doping elements, namely oxygen and nitrogen, were considered to represent the composition and chemical functionalities of biobased carbon as revealed by XPS^[17] (Figure 1). Details are available in the Experimental Section of the

Supporting Information.

For the production of the biobased carbons, two biomass sources (chitin and wood) were subjected to HTP (200°C, 6 h). Thereafter, the resulting hydrochar was physically activated (850°C, 2 h, N₂ atmosphere) to obtain biobased carbons. Both feedstocks are carbohydrate-rich, and therefore prone to high yields of hydrochar through repolymerization reactions under HTP conditions^[15]. The effects of HTP parameters on the yields, and the chemical and textural properties of wood- and chitin-derived carbons were discussed in detail elsewhere^[17]. In brief, both biobased carbons were mainly composed of aromatic carbon. XPS

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analyses revealed the presence of nitrogen (4.9 wt%) and oxygen (5.2 wt%) in the carbon framework of chitin-derived carbons due to the presence of an acetyl amine group in the chitin structure, whereas wood- derived carbons appeared only doped with oxygen (6.7 wt%). In terms of elemental composition, biobased carbons were comparable to reduced graphene oxide^[20].

Biobased carbons possess enhanced wettability when compared to conventional carbon nanomaterials due to the nitrogen and oxygen functionalities within the carbon framework^[17], a relevant feature for the present work. This wettability facilitates mixing between silk fibroin and biobased carbons. With our HTP-based approach, biobased carbons obtained after physical activation readily dissolve in formic acid, together with silk fibroin and CaCl₂, for casting silk fibroin-biobased carbon thin films. These films are flexible

(Supplementary Movie 1) and stretchable at room temperature and relative humidity of 50-55%. When applying the same procedure for the synthesis of silk fibroin-CNT films, the CNT hydrophobicity prevents uniform dispersal in the silk fibroin solution (Figure S1). Similarly, an early report on silk fibroin-graphene films using the same synthetic route as the one in the present work^[8] required several additional chemicals (polylactide-co-glycolide, dichloromethane, ethylene glycol butyl ether, and dibutyl phthalate) to prepare a graphene ink that mixed with silk fibroin. In contrast, biobased carbons are easily incorporated into silk fibroin solutions without the need for exogenous chemicals.

The mesostructure of the surface and cross-section (thickness of ca. 150 µm) of silk fibroin films with varied concentrations of biobased carbon or graphene was characterized with scanning electronic microscopy (SEM) (Figure 2a-f). The films are continuous and homogeneous matrices at low biobased carbon concentrations (Figure 2c,e), as well as in control films without biobased carbon incorporated (Figure 2a). Notably, high biobased carbons contents (20.0 wt%) cause a noticeable accumulation of the biobased carbon phase on the bottom layer of the films due to settling during the casting and drying process (Figure 2d,f). In contrast, the formation of a distinct carbon phase is not observed in silk fibroin-graphene films (20 wt%) (Figure 2b), indicating that the aforementioned organic additives in the commercial graphene ink contribute to the homogeneous dispersal in the silk matrix.

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In addition to investigating film topographies via SEM, in order to evaluate electrical and mechanical testing of silk fibroin-biobased carbon films, dog-bone-shaped gauges were laser-cut from the films and tested at 25 °C with relative humidity of 55%. Increasing concentrations of biobased carbon lead to a significant reduction in the resistivity of the gauges (Figure 2g). This reduction is greater for chitin-derived carbon than for wood-derived carbon, which may be attributed to the doping effect of nitrogen

functionalities in chitin-derived carbon^[21]. The gauges from chitin show resistivities ranging from 62.9 Ω·m (carbon content: 0.1 wt%) to 1.9 Ω·m (carbon content: 20.0 wt%). For wood, the resistivity varies from 45.0 Ω·m (carbon content: 0.1 wt%) to 11.2 Ω·m (carbon content: 20.0 wt%). Notably, silk fibroin-only films show inherent conductivity (resistivity = 72.4 Ω·m) at a relative humidity of 55 %, which disappears after drying the films at 60°C for 2 h. Since silk fibroin is an insulating material, this finding suggests that the presence of CaCl2 in films at relative humidity of 55% provides some ionic conductivity^[6] as CaCl2 can coordinate with up to 8 molecules of H2O^[22]. However, the ionic conductivity devoted to CaCl2 is clearly enhanced by the electronic conductivity attributed to biobased carbon, being this effect observed even at low biobased carbon concentrations.

The gauges display a synchronous strain-dependent resistance that responds in real time to the application of strain (Supplementary Movie 2). The effects of biobased carbon content are only noticeable during the low strain regime (<25%), which covers the range of physiologically relevant strains (up to 25-30%)^[23,24], such as breathing, pulse, or skin deformation^[7]. At high strains, the resistance becomes similar for all the gauges, regardless of the carbon concentration. These results indicate that above a certain strain, the carbon network surpasses its percolation threshold and no longer contributes to the conductivity of the gauge. Still, the presence of CaCl₂ allows the gauges to show a strain-dependent resistance at very high strains (>300%).

This makes them suitable for tracking both small and large strain deformations. In most cases, the value of the resistance exceeds the detection limit of the multimeter (55 MΩ) before the gauge breaks. It should be noted that the resistance at strain zero of the gauges could not be entirely recovered after large

deformations, due to the relaxation of the silk fibroin chains after tensile stress. This makes the present gauges applicable as disposable eco-friendly strain sensors.

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The resistivity of silk fibroin-biobased carbon gauges is lower than that from silk fibroin-graphene films produced by the same experimental procedure and with the same load of conductive filler (1.9 Ω·m for gauges with 20 wt% chitin-derived carbon, 13.6 Ω·m for gauges with 20.0 wt% graphene^[8]). Silk fibroin- graphene films synthesized in this study confirm these results. This is intriguing, because the conductivity of biobased carbon is lower than that of graphene. The reason is probably linked to the formation of a biobased carbon-rich layer on one side of the films, as revealed by SEM, due to the aforementioned settling effects upon casting. This localized carbon layer could help in reaching the percolation threshold for a conductive network at lower concentrations of biobased carbon than of graphene, since graphene appears to be more homogeneously dispersed within the silk fibroin matrix. Moreover, we cannot rule out that some of the reagents used to synthesize the graphene ink (including polylactide-co-glycolide, dichloromethane, ethylene glycol butyl ether, and dibutyl phthalate) are not entirely volatilized and act as insulators.

In terms of mechanical properties, gauges with low concentrations of carbon (both chitin- and wood- derived) show much higher failure strain (up to 1030 ± 129%) than pure silk fibroin gauges (679 ± 144%) (Table 1). A similar effect was previously reported for silk fibroin fibers with graphene oxide^[4], or bioplastics with graphene^[25]. Further increasing the biobased carbon content in the composites causes a decline in the failure strain, but even at carbon contents of 20.0 wt%, the failure strain achieves a

remarkable 239 ± 31% for chitin-derived carbon and 386 ± 84% for wood-derived carbon. These strains are more than two orders of magnitude larger than typical commercial strain gauges^[26]. Furthermore, the gauge failure always occurs at strains significantly larger than those typically found under physiological

conditions (25-30%)^[23,24]. This impressive behavior demonstrates the potential of these biomaterial composites as sensors in biological tissues, with potential applications in vivo (e.g., coating of metal-based neural probes) and ex vivo (e.g., human motion tracking). The gauge factor (GF) provides additional information pertaining to mechanical properties, as it describes the variation of resistance with strain. The GF values up to 175% strain are remarkably high for the gauges with biobased carbon content of 20.0 wt%

(296 for chitin-derived carbon, 32 for wood-derived carbon) (Figure S2). These values are much higher than those of conventional metal gauges (2.0) in the same strain range^[27], and confirm the sensitivity of silk fibroin-biobased carbon composites for sensing both subtle and large deformations. Furthermore, at a

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conductive filler concentration of 10.0 wt%, the mechanical properties of silk fibroin-biobased carbon gauges are similar to those from previously reported silk fibroin-graphene films^[8]. However, at 20.0 wt%

filler concentration, the peak stress and the toughness of silk fibroin-graphene increases by an order of magnitude, whereas the variation of these two properties for silk fibroin-biobased carbon is much more modest. The poorer mechanical strength of biobased carbons compared to graphene (the toughest material known to mankind) is the most likely reason for the lower stress sustained by silk fibroin-biobased carbon composites at 20.0 wt% carbon content.

Besides good mechanical properties, low cytotoxicity and good mammalian cell adhesion are also important features for biomedical materials. Thus, cytocompatibility was evaluated by seeding L929 fibroblasts on the composite films to ensure that the amount of graphene or biobased carbon in the films was within a safe range for the cells. Cytotoxicity was assessed by live/dead staining. After 3 days post-seeding, the number of cells that adhere on the 0.5 wt% silk fibroin-biobased carbon films is similar to the number of cells adhered to silk fibroin films, but lower than that of the tissue culture plastic (TCP) control sample (Figure S3). It has been extensively reported that cell adhesion on silk films and gels is lower than on TCP, likely due to the hydrophobic nature of silk^[28–30]. The cells adhered to the film surfaces exhibit a morphology similar to those on TCP, indicating that biobased carbons do not significantly affect cell functions.

Interestingly, the number of cells observed for the 0.1 wt% silk fibroin-graphene films is approximately half of that for silk fibroin and 0.5 wt% silk fibroin-biobased carbons (Figure S3), hinting at cytotoxicity by the graphene ink. A decrease in the number of cells adhered is observed at increasing carbon concentrations, especially 10.0 and 20.0 wt%, compared to TCP (Figure 2h-l), but the cells retain the same morphology observed on the TCP. Notably, the reduction in cell adherence in silk fibroin-graphene films is more drastic:

a concentration of 1.0 wt% shows an adherence comparable to that of 20.0 wt% of biobased carbon.

Moreover, a concentration of 10.0 wt% of graphene is toxic enough to prevent adherence of cells. This toxicity is likely caused by the additives used to manufacture the graphene ink, which aid in the dispersion of graphene in silk fibroin suspensions, and support the aforementioned hypothesis that these additives do not entirely volatilize after casting the films. These results highlight the cytocompatibility of silk fibroin- biobased carbon biomaterials to synthesize flexible conductive composite biomaterials.

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Biobased carbon interactions with various silk fibroin domains (amorphous domain, β-sheet crystals and N- terminus), which are thought to play a central role in composite formulations, were assessed using

molecular dynamics simulations (Figure 1b). Computational models representing biobased carbons were built using a single aromatic carbon monolayer. The degree of functionalization of the surface was adjusted to match the content in carbon, nitrogen and oxygen functionalities as determined by XPS analyses^[17]. The chitin-derived carbon model was functionalized with nitrogen and oxygen groups, whereas wood-derived carbon was only functionalized with oxygen. These biobased carbon structures were parameterized using the Force Field Toolkit^[31] for CHARMM General Force Field^[31]. The interactions of the β-sheet domain with graphene were also assessed (Figure 3a), since this domain is critical for the mechanical strength of silk fibroin. The results indicate that polar groups in both chitin- and wood-derived carbons help retain the native secondary structure of silk fibroin (Figure 3b,c). The amount of β sheets and turns in the crystalline domain of silk fibroin are reduced by 4.5% when chitin-derived carbon is used, and by 10.5% when wood- derived carbon is used. A much larger reduction (26.8%) occurs when the crystalline domain of silk fibroin interacts with graphene. This agrees with previously reported molecular dynamics simulations of silk fibroin-graphene composites^[32], and shows that the strong hydrophobic-hydrophobic interactions between silk fibroin and graphene lead to major alterations in the secondary structure of silk fibroin. Our simulation results suggest that the electrostatic interactions between silk fibroin and biobased carbon counterbalance the strong van der Waals interactions between silk fibroin and the aromatic carbon network (Table S1). The polar atoms in biobased carbons stabilize the native silk fibroin secondary structure (as shown in Figure 3d-f), thereby allowing the silk fibroin-biobased carbon films to have similar mechanical strength as pristine silk fibroin films. The number of hydrogen bonds as well as the percentage of ordered and random domains are similar for chitin- and wood-derived carbons (Table S1), and do not vary dramatically with respect to the native state of the silk fibroin domains. This explains why the experimental peak stress of silk fibroin-biobased carbon gauges are kept within the same order of magnitude as pristine silk fibroin for the range of carbon concentrations tested (0.28 MPa for silk fibroin and 0.61 MPa for 10.0 wt% of wood- derived carbon), much more modest than the variations reported when using graphene^[8].

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The strain gauges made with low concentrations of biobased carbon (below 1.0 wt%) have increased toughness and failure strain, compared to pristine silk fibroin films. The enhanced mechanical properties are attributed to the higher degree of entanglement within the composite induced by the presence of carbon nanomaterials^[25]. Molecular dynamics simulations suggest that biobased carbons could act as connecting nodes between silk fibroin nanofibrils through electrostatic interactions without modifying the secondary structure of silk fibroin. This would enhance the load transfer across the silk fibroin-biobased carbon structure, acting in a similar way to “sacrificial physical crosslinks” that dissipate energy upon tensile stress, endowing the gauges with enhanced toughness and larger failure strains. As the carbon

concentrations increases (10.0-20.0 wt%), the composite becomes less extensible, due to excessive electrostatic interactions that restrict its ability to stretch, causing an earlier failure. Our findings establish biobased carbon as a promising conductive filler for the preparation of silk-based conductive composites without causing major alterations in the secondary structure or the mechanical properties of pristine silk fibroin. This highlights the potential of composite silk fibroin-biobased carbon biomaterials for the development of flexible conductive biomaterials that are compliant with biological interfaces.

In summary, the work reported in this paper demonstrates for the first time the use of biobased carbons to fabricate conductive, highly-stretchable, flexible and biocompatible silk-based biomaterials with the following key properties:

 Biobased carbons are prepared via HTP, an entirely aqueous process, followed by physical activation. This approach is environmentally friendly and less expensive than conventional routes to produce advanced carbon nanomaterials like graphene or CNTs.

 The resistivity of silk fibroin-biobased carbon thin films is an order of magnitude lower than that of previously reported silk fibroin-graphene biomaterials.

 These biomaterials overcome the mismatch in mechanical properties between typical rigid semiconductor sensors and biological surfaces or tissues.

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 Silk fibroin-biobased carbon biomaterials exhibit good cytocompatibility, probably because they are synthesized without the need of toxic organic solvents.

 Molecular dynamics simulations reveal that biobased carbons provide conductivity to silk fibroin without causing major rearrangements in its native secondary structure or hydrogen bonding patterns, due to enhanced electrostatic interactions with biobased carbons.

The results have impact for the development of conductive, highly stretchable, flexible and biocompatible biomaterials entirely made from renewable resources that could help reducing electronic waste. The amenability of Bombyx mori silkworm silk for large-scale production and the wide availability of biomass resources to synthesize biobased carbons are promising avenues for the scaled-up manufacturing of the materials presented here. Looking ahead, this work encourages the study of biomass resources with different biochemical compositions, and varying HTP conditions (e.g., temperature, biomass:water ratio, reaction time) to manufacture biobased carbons with a wide range of morphologies and properties for biomaterials synthesis. The experimental space yet to be explored for combining biobased carbons and biopolymers like silk is vast and holds great promise for the development of sustainable biomaterials for applications in fields like biomedicine (e.g., coating of neural probes), energy (e.g., flexible battery electrodes), environment (e.g., water filtration membranes) or electronics (e.g., flexible wearable sensors).

ACKNOWLEDGMENTS

DLB, ZMM, JY, FJMM, DLK and MJB acknowledge support from the National Institutes of Health (U01 EB014976). MJB, DLB, FJMM acknowledge support from ONR (N00014-16-1-233). JY acknowledges support from Singapore’s Agency for Science, Technology and Research (A1786a0031). SS acknowledges support from the MRSEC Program of the NSF (grant number DMR-14-19807). MJH acknowledges support from NIH NIGMS K12GM074869. DLK acknowledges support from the AFOSR (FA9550-17-1-0333).

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Figure 1 – (a) Silk fibroin-biobased carbon thin films for the preparation of flexible electronics. (b)

Atomistic-level all-atom molecular dynamics simulations to study the interaction of silk fibroin with carbon surfaces. Different domains of silk fibroin protein (amorphous, beta-sheet crystals and N-terminus) are placed atop of aromatic carbon surfaces with different level of doping elements (nitrogen and oxygen) to study how this affects the interaction of the carbon surface with silk fibroin and its secondary structure.

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Figure 2 – Cross-sectional SEM images of silk fibroin films: (a) silk fibroin only; (b) with 20.0 wt%

graphene; (c) with 0.5 wt% of chitin-derived carbon; (d) with 20.0 wt% of chitin-derived carbon; (e) with 0.5 wt% of wood-derived carbon; (f) with 20.0 wt% of wood-derived carbon. (g) Resistivity of gauges with different biobased carbon concentrations. Fluorescence microscopic image of the films seeded with L929 fibroblasts after culturing for 3 days and analyzed using live/dead assay staining. Images show live cells in green and dead cells in red from (h) TCP (control), (i) silk fibroin films, (j) 10.0 wt% silk fibroin+chitin- derived carbon films, (k) 10.0 wt% silk fibroin+wood-derived carbon films, and (l) 10.0 wt% silk fibroin- graphene films. The figure shows representative images; all samples were analyzed in triplicate.

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Figure 3 – (a) Change in the secondary structure of the β-sheet crystalline after 100 ns of stochastic

dynamics with graphene, chitin-derived and wood-derived carbon; (b) change in the secondary structure of the silk fibroin domains after 100 ns of stochastic dynamics with (b) chitin-derived carbon and (c) wood-derived carbon; snapshots of the protein in its native form and after 100 ns of molecular simulations with chitin-derived and wood-derived carbon: (d) amorphous domain, (e) β-sheet

crystalline domain and (f) N-terminus domain.

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Table 1 – Mechanical properties of silk fibroin-biobased carbon gauges

Biobased carbon content (wt%)

Failure strain (%)

Peak stress (MPa)

Toughness (MJ/m³)

Silk fibroin - 679 ± 144 0.28 ± 0.06 1.44 ± 0.32

Silk fibroin+Chitin-derived carbon

0.1 1030 ± 129 0.21 ± 0.01 1.58 ± 0.11 0.5 635 ± 101 0.53 ± 0.10 2.48 ± 0.67 1.0 851 ± 75 0.54 ± 0.14 2.90 ± 0.23 5.0 585 ± 83 0.50 ± 0.09 2.16 ± 0.57 10.0 497 ± 115 0.57 ± 0.16 1.97 ± 0.57 20.0 239 ± 31 0.60 ± 0.17 1.18 ± 0.40 Silk fibroin+Wood-derived

carbon

0.1 820 ± 128 0.44 ± 0.07 2.35 ± 0.50 0.5 873 ± 50 0.50 ± 0.07 2.81 ± 0.50 1.0 670 ± 146 0.47 ± 0.04 2.19 ± 0.15 5.0 415 ± 72 0.61 ± 0.10 1.86 ± 0.68 10.0 458 ± 19 0.61 ± 0.01 2.25 ± 0.21 20.0 386 ± 84 0.44 ± 0.08 1.31 ± 0.22

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Table of Contents

This experimental-computational work demonstrates the use of biobased carbons to fabricate conductive, highly stretchable, and biocompatible silk-based composite biomaterials. Biomass is converted into a carbonaceous material via hydrothermal processing, and subsequently applied upon activation as

conductive filler in silk thin films. These conductive composite biomaterials, made entirely from renewable sources, have promising applications in fields like biomedicine, energy, and electronics.