In this work, we report a method to produce a wood-derived, fully bio-based, and environmentally friendly flexible electronic circuit. The main difference between our circuit and other cellulosic circuits resides in the fact that substrate and conductive elements are both obtained via wood nanotechnologies. The substrate, a flexible and transparent wood film (TWF), maintains the cell walls and CNFs original orientations. The structure of the cellulose fibers is well preserved, and their crystallinity is increased due to the mild chemical extraction process. The highly aligned and crystalline CNFs confer to the TWF higher mechanical properties compared to previous studies using similar top-down approach. The film properties position it as a suitable candidate for electronic applications. Lignin, an abundant byproduct from wood processing with high carbon content, was electrospun and then carbonized into conductive carbon fibers. Taking advantage of the strong adhesive properties of amyloid fibrils, a fully bio-based and renewable amyloid/lignin-derived carbon fibers (LCF) conductive ink was formulated and printed on the TWF substrate to produce an electronic circuit. The LCF ink shows excellent printability, adhesion, and flexibility on the TWF substrate. The synergistic combination of TWF and LCF ink enables the production of fully bio-based, flexible electronic circuits with stable electro-mechanic performance. We envision that this concept can be industrially relevant, as its environmentally friendly production could be scaled-up. As a proof-of-concept, the wood-based, flexible, and printed electronic circuit is demonstrated as a durable strain sensor. This fully bio-based, wood-derived flexible electronic device possesses an extensive potential for a wide range of applications including flexible optoelectronics, smart packaging devices and sensors.

Wood can also be deconstructed into cellulose nanofbrils (CNFs), which have a low thermal expansion coefficient and favorable optical properties. (18) Those nanofibrils can be chemically funcationalized and/or utilizeda bottom-up strategy to form CNF nanopaper. Colloidal suspensions of CNFs are dehydrated to produce randomly self-assembled (isotropic), polymer-free, two-dimensional (2D) films with high strength, flexibility, and optical transparency. (19,20) Thus, CNF nanopaper is used as a substrate for several innovative electronics, optoelectronics, and photonics applications (., sensors, energy storage, and organic light-emitting diode display). (21−29) The main challenges limiting the implementation of CNF nanopaper as a substrate for electronic devices at industrial scale lie in the time and energy-consuming manufacturing with multiple steps, such as nanocellulose extraction, dewatering, and formation of high-quality films. Although the fiber size and structure of the cellulose 2D film are tunable, the mechanical performance of the material is mainly governed by the fiber orientation. Therefore, nanocellulose materials require to be highly aligned to improve their mechanical properties. Such unidirectionally aligned nanocellulose materials have been prepared using wet spinning, hydrodynamic, mechanical stretching, and electric and magnetic field methods. (30−34) However, scaling up the production of highly oriented cellulosic materiala bottom-up strategy is even more challenging.

A top-down strategy conserves the anisotropic structure of wood to produce a mechanically robust substrate ideally suited for electronics. For instance, Zhu. created a film with aligned cellulose/hemicellulose by delignifying and compressing wood. (13,14) A flexible and transparent cellulosic film with relatively high mechanical performance was produced (fracture strength of 350 MPa). However, the strength of this material is suboptimal due to the harshness of the chemical treatment process, leading to a partial degradation of the cellulose fibers. A stronger material (specific strength of 451 MPa cm) was later developed by partially extracting lignin and hemicellulose from bulk wood followed by densificationhot compression. (15) However, this material is neither transparent nor flexible and thereby not ideally suited as a substrate for high-value electronic applications. In other studies, anisotropic conductive wood films were achieved by impregnation of epoxy resin into the delignified wood followed by coating with silver nanowire conductive layer. (16,17) Yet, due to shrinkage during the polymerization process, defects or cracks appeared at the interface between cell walls and polymers, leading to reduction of the mechanical properties of the material as well as mechanical brittleness in fiber direction. Moreover, these electronics are neither fully bio-based nor biodegradable. Therefore, the combination of multiple properties such as high transparency, flexibility, conductivity and mechanical robustness into an environment friendly electronics device is still highly desirable and challenging.

Manufacturing fully bio-based, eco-friendly, and sustainable electronics has been attempted for production of high-performance devices. (1−3) Wood is a renewable, biodegradable, environmental friendly, and natural material. Wood nanotechnologies are not only associated with extraction and use of nanocellulose or lignin but also with tailoring and functionalizing the hierarchical nanostructure of bulk wood for functional materials. (4,5) Applying these technologies allow wood or cellulose-based porous materials to be used as templates with favorable mechanical and electrical performances for smart ion and compressible sensors. (6,7) More specifically, wood-based thin films have become highly attractive as substrates for flexible electronics and optoelectronics due to their high mechanical properties, flexibility and optical characteristics. (8−12)

Results and Discussion ARTICLE SECTIONS Jump To

via hydrogen bonds resulting in a coherent material with a homogeneous refractive index ( Wood is a highly porous and hierarchically structured material. Secondary cell walls are typically organized in three layers ( Figure S1 ), where the S2 layer has the lowest microfibril angle orientation and makes up the greatest proportion of the wall thickness. Due to these characteristics, the S2 layer is the main contributor to the wood axial mechanical properties. (35) A top-down strategy was designed to maintain the hierarchical structure and anisotropic nature of wood to create a strong substrate. A two-step approach was carried out to manufacture a flexible electronic circuit ( Figure 1 ). The initial step consists of removing most of the lignin and half of the hemicellulose from the original balsa wood, followed by compression and dehydration to produce the TWF substrate ( Figure 1 a, center). This process preserves the CNFs orientation generating a highly anisotropic material. Moreover, the collapsed cell walls are boundhydrogen bonds resulting in a coherent material with a homogeneous refractive index ( Figure 1 a, center). (36) The TWF is thereby optically transparent, flexible, and strong ( Figure 1 b). These characteristics are ideal for a substrate to be used in flexible electronic applications. During the second step, circuits and patterns are printed with the LCF conductive ink on the substrate using either a conventional inkjet printer or a rod coater ( Figure 1 a, right).

Figure 1 Figure 1. Processing of TWF for flexible electronics application. (a) Illustration of the process. Lignin and half the of hemicellulose are removed from the wood tissue. The treated wood is then pressed and dried under ambient conditions. The collapsed cell walls are bound together via hydrogen bonding. The hierarchical structure of TWF consists of cellulose microfiber bundles, nanofibrils, and cellulose chains, which include crystalline and amorphous regions. The bio-based LCF ink is printed in a tree-shaped circuit on the TWF substrate. (b) Photograph of original wood, treated wood, and TWF.

–3) is a mesoporous material mainly composed of tubular fibers (73.5 vol %) aligned parallel to the tree growth direction.2 g–1, a density of 0.085 g cm–3, and a porosity of 94.4% (2 g–1) and an increase in density to 1.2 g cm–3 ( Balsa wood (0.17 g cm) is a mesoporous material mainly composed of tubular fibers (73.5 vol %) aligned parallel to the tree growth direction. (37) The fiber lumen has a polygonal cross-section with a diameter between 20 and 45 μm and a cell wall thickness around 1.8 μm ( Figure 2 a, left). Balsa wood structural components are cellulose 50%, hemicellulose 25%, and lignin 25% ( Table 1 ), excluding ash (2%) and extractives (1–3%). Lignin is a phenolic biopolymer, distributed throughout the cell wall and highly accumulated in the compound middle lamella and in the cell wall corners ( Figure 2 a, right). (38) Lignin and extractives contain chromophores that are responsible for the light-yellow color of natural wood under visible light ( Figure 1 b). The chemical treatment preserves the honeycomb structure of balsa, but its color changes to white ( Figures 1 b, 2 b, and S2 ). After treatment, the intensity of the peak (540–580 nm) attributed to lignin autofluorescence decreases by an order of magnitude ( Figure 2 c). The lignin content decreases from 24.5 wt % to 1.6 wt % of the original wood total weight, corresponding to 93.5% reduction ( Table 1 ). The chemical treatment is associated with the removal of half the hemicellulose (53.5%) and 18.2% of the cellulose content. The original wood has a cellulose content of 50 wt %, with a crystallinity index of 70.1% ( Table 1 ). After chemical treatment, the treated wood has its cellulose content raised to 75 wt % and cellulose crystallinity increased to 77.3%. The chemical treatment mostly removes the cellulose amorphous phase (60.4%). By applying the crystallinity index to the cellulose content, the relative crystalline cellulose content is calculated to be 35 and 58 wt % for the original and treated balsa wood, respectively. Altogether, this shows that the treated wood is mainly constituted of crystalline cellulose. The treated wood is also characterized by a surface area of 49.7 m, a density of 0.085 g cm, and a porosity of 94.4% ( Table 1 and Figures S3–4 ). The removal of 45.4% of the wood mass leads to the complete disappearance of the cell wall corners, and the appearance of microspaces in the compound middle lamella and nanopores within the S2 layer ( Figures 2 c,d and S3 ). The treated wood is then compressed in ambient conditions until dry (see the Experimental Section ). The compression transforms the white and opaque treated wood into a thin and dense TWF ( Figure 2 d). Compared to the original wood, the TWF shows a reduction of ∼95% in thickness and 55% in surface area (0.5 m) and an increase in density to 1.2 g cm 1 Tables and S1 ).

Figure 2 Figure 2. Structural characterization of original wood, treated wood and TWF. (a) Confocal scanning light microscopy images of original balsa wood at low (×200) and high (×5000) magnifications. (b) Confocal scanning light microscopy images of treated wood at low (×200) and high (×5000) magnification. The image intensity has been increased by 80%. (c) Fluorescent emission spectrum of original and treated wood. (d) Photographs of TWF sample, the cross-section (red rectangle), and surface (green rectangle) are further described. (e) SEM images of TWF cross-section. The collapsed cell walls form a laminated layers structure material. The yellow arrows are pointing toward CNFs cross sections. (f) Schematic of a single densified cell wall. The cell wall is hierarchically structured by microfiber bundles, nanofibrils, and cellulose molecular chains. The X-ray synchrotron beam is applied perpendicular to the cell wall surface. (g) SEM images of the TWF surface. A single fiber contour is marked by a yellow dashed line. Cellulose microfiber bundles and nanofibrils (blue arrow) are highly aligned along the fiber growth direction. (h) X-ray synchrotron WAXS scattering diffractogram of TWF substrate. (i) Radial integration of the diffractogram shown in (h). (j) Azimuthal integration of the (200) scattering plane from the diffractogram shown in (h).

Table 1. Chemical Compositions and Mechanical Properties of Original Wood, Treated Wood, and TWF materials lignin (wt %) hemicelluloses (wt %) cellulose (wt %) mass loss (wt %) cellulose crystallinity (%) density (g cm–3) Ea (GPa) specific Ea (GPa cm3 g–1) σa (MPa) specific σa (MPa cm3 g–1) original wood 24.5 ± 0.8 24.9 ± 0.9 50.6 ± 2.1 N/A 70.1 ± 2.3 0.17 ± 0.01 0.24 ± 0.5 13.9 ± 3.0 17.6 ± 2.7 103.5 ± 15.7 treated wood 1.6 ± 0.3 11.6 ± 0.5 41.4 ± 1.6 45.4 ± 0.9 77.3 ± 4.6 0.085 ± 0.01 1.0 ± 0.2 11.8 ± 2.5 7.5 ± 0.9 88.3 ± 10.6 TWF 1.6 ± 0.3 11.6 ± 0.5 41.4 ± 1.6 45.4 ± 0.9 77.3 ± 4.6 1.2 ± 0.1 49.9 ± 10.8 41.6 ± 9 469.9 ± 114.3 391.6 ± 95.2

–1 corresponds to the crystal (200) reflection of cellulose I ( Scanning electron microscopy (SEM) images taken from TWF cross-section reveal that the collapsed cell walls are creating a laminated layers structure reminiscent of nacre layered structure ( Figure 2 e). The microvoids and nanopores in middle lamella, cell wall corners, and secondary walls have disappeared ( Figures 2 e and S3c ). (39) The process maintains the original alignment of fibers, microbundles, and CNFs ( Figures 2 f,g and S5 ). The CNFs alignment is confirmed by using 2D wide-angle X-ray diffraction (WAXS) ( Figure 2 f). Peeling off the TWF surface exposes “brush-like” CNFs ( Figure 2 g), revealing their orientation along the tree growth axis ( Figure S6 ). For the TWF sample, the reflections of cellulose crystal planes (200) and (110) have obvious confinement of their arc patterns, indicating a preferential alignment of CNF in the tree growth direction ( Figure 2 h). The azimuthal variation of the scattering peak at 15.7 ± 0.1 nmcorresponds to the crystal (200) reflection of cellulose I ( Figures 2 i,j). The (200) crystal plane is aligned with the CNF, and its scattering pattern is used to quantify the orientation of cellulose crystals ( Figure 2 i). The sharp peak observed corresponds to a high degree of CNF alignment (orientation index = 0.89) ( Figure 2 j). In contrast to the TWF, the scattering diffractogram of CNF nanopaper (random dispersion) shows typical ring patterns of cellulose I crystal (200) and (110) ( Figures S7a,b ). (27) The broad azimuthal peak of CNF nanopaper indicates an isotropic distribution of the CNFs (orientation index = 0.66) ( Figure S7c ). (30)

3 g–1 and 41.6 ± 9 GPa cm3 g–1, respectively. Those values are comparable to previously published on materials made from aligned cellulose macrofibers and 2D films (3 g–1), and the specific strength value is comparable to most steels and alloys ( A general challenge in material design is to produce materials with high specific strength and modulus. (15) The structure of TWF offers a set of mechanical and optical characteristics ( Figure 3 ). The compression process eliminates the porosity and transforms the treated wood into a densified material that is mechanically stronger than the original wood in both longitudinal and transverse directions ( Table 1 Figures S4 and S8–11 ). In the fiber direction, the TWF demonstrates a specific tensile strength and Young’s modulus of 391.6 ± 95.2 MPa cmand 41.6 ± 9 GPa cm, respectively. Those values are comparable to previously published on materials made from aligned cellulose macrofibers and 2D films ( Table S2 ) as well as 10–20 times higher than copy paper and polyethylene terephthalate (PET) ( Figure 3 a). The high mechanical properties of TWF are attributed to its highly oriented CNFs (orientation index = 0.89), (14) high cellulose content (75 wt %) and crystallinity (77.3%) ( Table 1 ). (37) When TWF is stretched in fiber direction, the glucose rings and glucosidic and hydrogen bonds of the cellulose microfibrils act in synergy, resulting in high mechanical properties. (40) Covalent bonds contribute to 75% and hydrogen bonds to 20% of the tensile property of cellulose crystals. (41) Due to the presence of numerous hydrophilic hydroxyl groups, the TWF is sensitive to moisture, and its mechanical properties decreased with increasing relative humidity ( Figures S12–13 ). However, even after being exposed for 48 h at 95% relative humidity, the TWF ultimate strength is still close to 240 MPa (specific strength of 200 MPa cm), and the specific strength value is comparable to most steels and alloys ( Table S3 ).

Figure 3 Figure 3. Mechanical and optical properties of TWF. (a) Typical stress–strain curves of TWF, PET, CNF nanopaper, and copy paper. The black arrow shows that TWF was tested longitudinally (along the fibers growth direction). (b) The Ashby chart of specific Young’s modulus versus specific strength for various materials. Details of the mechanical properties are included in Supplementary Tables S2–3. The TWF position is indicated with a red star. (c) The flexibility and shape recovery of TWF rolled on cylindrical objects. TWF rolled on a 0.15 mm radius wire (insert). (d) Photographs of the TWF excellent flexibility demonstrated by the possibility to make a knot, roll, twists, and bends. (e) Photograph showing the optical transparency of the TWF. The black dashed line area refers to the TWF film. The text is clearly visible when the TWF is placed on a printed paper. (f) Total transmittance and optical haze of a 55 ± 20 μm thick TWF. Anisotropic light scattering pattern (insert).

versus specific strength). This highlights its potential to replace petroleum-based polymers or engineered materials for some applications, such as packaging and automotive. In addition, The TWF has high mechanical properties to weight characteristics ( Figure 3 b). Interestingly, its specific strength and modulus are also greater than most natural fibers, (42) polymers and plastics (PET, nylon-6, PVC, ABS, epoxy, and HDPE) (43) and metal or engineered alloys (stainless steel, Fe–Al steel, gray cast iron, Ti–Al alloy, Mg–Al alloy, and Ni–Fe alloy) (44,45) (summarized in Figure 3 b and Table S3 ). The TWF occupies a preeminent position in the Ashby chart (specific modulusspecific strength). This highlights its potential to replace petroleum-based polymers or engineered materials for some applications, such as packaging and automotive. In addition, Figure 3 c,d presents the qualitative and quantitative flexibility of the TWF in both directions. The TWF can be rolled on a 0.15 mm radius metal wire ( Figure 3 c insert). When the TWF is longitudinally or transversally wrapped around a 5 mm radius cylindrical object, a shape recovery of approximately 95% is observed ( Figure 3 c). The TWF also has an excellent flexibility, as it can withstand various types of deformations such as knotting, rolling, twisting, and bending without sustaining any obvious cracks or damage ( Figure 3 d).

The treated wood mainly contains cellulose and hemicellulose, which have a similar refractive index (≈1.53). (46) The densification step removes the air-containing micro- and nanopores from the material (air refractive index = 1). Therefore, the TWF has a nearly homogeneous refractive index making it optically transparent. The TWF has optical clarity, as when it is placed in direct contact above a printed text, the text is perfectly readable ( Figure 3 e). However, clarity is distance dependent, and when the distance between the TWF and the specimen is above 10–15 mm, the text becomes fuzzy ( Figure S14 ). At a wavelength of 550 nm, the total transmittance and optical haze of the TWF (55 ± 20 μm thick) are 80% and 85%, respectively ( Figure 3 f). The transmittance of the film decreases proportionally to the increase of thickness ( Figure S15 ). This is attributed to the attenuation and scattering of light through the material ( Figure 3 f insert).

Substrates for printed and flexible electronics require superior mechanical, thermal, and optical properties as well as smooth surface profile. (47) For this application, the TWF is an attractive candidate, as it combines strong mechanical performance, high flexibility, and transmittance and a relatively low surface roughness ( Figures S16–17 ). To create environmentally friendly printed electronics, a fully bio-based conductive ink was developed ( Figures S18–21 ). The ink composition includes LCF as a conductive component and amyloid fibrils for dispersion and adhesion. Amyloid fibrils have intrinsic thermodynamic metastability under harsh conditions, such as high temperature and extreme pH. (48) In addition, strong adhesion of amyloid fibrils to both hydrophobic and hydrophilic surfaces has been reported. (49) The amphiphilic properties of the β-lactoglobulin amyloid fibrils are utilized as a stabilizer in the LCF ink system. Once the ink is dry, the amyloid fibrils also enhance LCF–LCF and LCF–substrate interactions without any prior treatment of LCFs or substrate. (49) The addition of amyloid fibrils therefore improves the stability of the LCF ink and the conductive circuits printed with it. This highly patternable and fully bio-based ink was printed on the TWF substrate, producing a totally bio-based circuit suitable for flexible electronic applications, such as strain and wearable sensors ( Figure 4 ).

Figure 4 Figure 4. Electrical performance, flexibility, and structural surface characterization of the TWF flexible electronics. (a) Photograph of flexible TWF electronic circuit. (b) Cross-sectional SEM image of the wood-based flexible electronics. (c) SEM image of LCF ink dispersed on surface of the flexible TWF. (d) SEM image of the TWF surface on the edge of a printed circuit, with an ink-coated and uncoated area. (e) and (f) Photographs of a printed flexible circuit connected to a 9 V battery powering a LED with a bent (e) and folded (f) electronic circuit. (g) Sheet resistance of the flexible film electronics versus folding–unfolding cycles. The resistance was tested for each printed circuit line which were folded between 90° and 180°. (h) Normalized relative resistance variation as a function of number of peel off repeats. (i) Normalized relative resistance variation as a function of applied strain. The strain loading was performed parallel to the fiber growth direction (black arrows). (j) Normalized relative resistance variation as a function of time during 90° folding. The conductive pattern was printed along the fiber direction.

A tree-shaped pattern was printed on the TWF substrate using a conventional printer ( Figure 4 a). The LCF ink is homogeneously coated on the TWF surface ( Figures 4 b,c and S20–21 ). The carbonized nanofibers are randomly oriented and overlapped to form an electrically conductive, interconnected network enabling long distance electron transport with a minimal reduction of electrical properties. Even when the printed circuit is bent or folded, it could still activate a blue light-emitting diode (LED), suggesting favorable flexibility and conductivity of the bio-based electronics circuit ( Figure 4 e,f and inserts).

The TWF/LCF circuit was evaluated for flexible electronics applications by performing cyclic folding/unfolding tests ( Figure 4 g,h). After 500 folding/unfolding cycles, only negligible changes in resistance were measured ( Figure 4 g). Around 80% increase in resistance was detected after 1,000 cycles. After 10,000 cycles, no further degradation was found. This reliable electrical performance, even after numerous folding/unfolding cycles, indicates that the bio-based printed circuit could be developed for flexible electronics purposes. A peel-off test was carried out to assess the adhesion between the printed LCF layer and the TWF substrate ( Figure 4 h). The result shows an increase (12%) of relative resistance after 10 peel-off repeats, which suggests a strong interaction between the conductive ink and the substrate. The strength of the interaction might be attributed to a combination of hydrogen-bonding and van der Waals forces present between the cellulosic substrate and the LCF/amyloid fibrils in the ink.

The electro-mechanical performance of the printed flexible circuit was assessed and demonstrated as a strain sensor ( Figure 4 i,j). With the strain increasing from 0% to 1%, the randomly oriented protein fibrils/LCF fibers network was rearranged. This phenomenon, coupled with the strong adhesion between the ink and the cellulose, might explain the dramatic increase of relative resistance change (∼15%) of the LCFs circuit on CNF nanopaper and copy paper ( Figure 4 i). Interestingly, under 1% strain, the relative resistance change of LCF on both TWF and PET only increased around 5%. At 2% deformation strain, the TWF/LCF circuit relative resistance change rose to 40%. The ink relative resistance change increased up to 100% when the PET substrate underwent 7% strain deformation. This result could be explained by the poor adhesion between the LCF ink and the PET substrate. When the plastic PET is stretched, the high flexibility of PET and the less stretchable ink causes detachment from the substrate. This could be cccommodated by cracking, sliding and susequent separating or disconnecting between PET and LCF ink deformations, which ultimately cause the change of the resistance. These data suggest that TWF/LFC sensor has better electro-mechanical performance compared to other substrates due to its absence of strain deformation at low loading and a strong adhesion with the ink.

A strain sensor for bending test using printed LCF circuit on TWF substrate was developed as a proof-of-concept ( Figures 4 j and S22 ). The sensor was attached on the joint between the second and the third index phalanges. The finger was repeatedly bent at 90° and straightened. The normalized relative resistance changed with an amplitude of 30% and varied in synchrony with the finger movement ( Figure 4 j). Similarly, an 8% variation in relative resistance change was observed in the longitudinal direction ( Figure S22 ). Flexible electronics are often processed through thermal treatment during manufacture and are used in circumstances where the temperature fluctuates. Consequently, substrates need a high thermal stability to withstand thermal variations. Compared to PET and copy paper substrates, TWF and CNF nanopaper showed better thermal stability for electrical performance ( Figure S23 ). This is mainly due to the low thermal expansion coefficient of nanocellulose. (50) Finally, a recycling process for the eco-friendly electronics is proposed. The circuit can be dispersed in water by simple mechanical stirring ( Figure S24 ). This step would allow the fibers to be reused in a traditional paper making process, for example, paper mill recycling, shredding, and pulping.