Cellulose‐Derived Wearable Carbon Nanoflake Sensors Customized by Semiconductor Laser Photochemistry

Multi‐functional wearable electrical materials have been regarded as one of the most pivotal cornerstones for the booming internet of things (IoTs), biomimetic robotics/science, and sensory e‐skins. Nevertheless, customizable, high‐throughput, batch‐fabricated, function‐integrated wearable electronics remain technologically challenging to traditional material engineering. Hereby, a cellulose‐converted active amorphous carbon nanomaterial is developed via a transfer‐free, precursor‐free rapid laser synthesis method incorporating deformation‐tolerant waste papers. The lattice fringe spacing of laser‐synthesized carbon nanoflake is ≈0.305 nm topologically distinct from graphene or carbon dots. The nanostructured three‐dimensional (3D) carbon network exhibits desirable mechanical flexibility, high hygroscopicity/electrical conductivity, large ion storing capacity for Zn2+ or Na+, high sensitivity to pressure, and a natural microwave absorbing ratio (> 37 dB at the terahertz range). Abundant percolation pathways inside cellulose/carbon composite networks offered fast electrolyte diffusion and carrier mobility. A series of low‐cost highly‐deformable interdigitated supercapacitors, tactile sensors, electrical circuits, and functional coatings are experimentally fabricated and identified, enabling waste paper as a function‐magnified meta platform for e‐skins, wearable energy devices, or IoTs interfaces.


Introduction
The green, facile, digitalized laser synthesis, [1] a forerunner method for low-dimensional high-performance materials, is silently developing our high-tech products in multi fields ever since. [2][3][4][5] Photons from laser resonant cavities hit the precursors, causing spatially-confined chemical reactions driven by light energy different from hydrothermal/electrical chemistry or chemical vapor deposition. To date, multitudinous semiconductors, electrical circuits, sensors, robotics, and interfaces have sprung up in the last decade using ultrafast laser ablation, two-photon polymerization, [1] lifting off, [3] or photochemical reduction on hard and brittle wafers. [4] Unfortunately, despite the merits of volume reduction/function controllability, laser-based synthesis remains stifled for wearable applications. In the world of electronics, most electric products rely on bio-incompatible metal alloys or oxides, require tedious processing (annealing, etching, photolithography, or so on), and are constrained to rigid substrates. Currently, with technological advancement, there emerges an irresistible trend toward wearable electronics [5] in unconventional mechanic forms (such as sensory e-skins, [6] soft human-machine interfaces, [7] intelligent perception, [8] internet of things (IoT), [9,10] smart clothes, soft energy devices, [11][12][13] and even bendable displays for wearable device applications [14,15] ), which challenges laser-based synthesis. Since wearability becomes a critical priority for fabricating e-skins/IoT interfaces, thereby, inventing an appropriate laser direct synthesis method on deformable substrates is highly valued for broadband applications and innovative development.
In global human societies, tons and tons of waste papers [16] turn into useless garbage after usage every year, but few works have explored effective methods of recycling these useless papers. The main ingredient of paper, plant cellulose, a bendable fibrous renewable organic material consisting of -1,4-linked anhydrous-D-glucose units, [17] has nearly inexhaustible sources reproduced from bacteria, algae, invertebrates, and marine animals, which is a desirable raw material for green synthesizing carbon. [18] Inside waste papers, cellulose blends with lignin, lubricant, and lime powders into a deformable matrix, promising favorable degradability, bio-compatibility, and bendability eliminating mechanical The as-prepared carbon-based mandarin words (meaning laser direct writing on-paper electronics), the author's WeChat QR code, the bendable spider pattern, and different peripheral circuits all get conductivity improvement by adding the ion/PEDOT solution. b) illustration of the micro/nano-stacked carbon particles and flakes on the bumpy paper surface. c) 3D cellulose/carbon composite framework inside the laser-processed papers. mismatch issues of hard metal/wafers. [19,20] Hygroscopic cellulosic network offers high solvent or electrolyte retention than solid electrodes due to its abundant migration channels inside the micro/nano network. Interestingly, previous research has confirmed that cellulose fibers could be phase-transformed into amorphous carbon nano-materials [21] by directly absorbing sufficient thermal energy. Hereby, in our scenario, combining the low-cost, degradable cellulose-based papers with a noncontact laser synthesis will eliminate the tedious preparation of precursor materials. The sophisticated chemical processes were simplified, and laser synthesis maximizes the digitalized advantages [22,23] of computer-assisted fabrication, [1][2][3][4]22,23] garnering considerable interest in developing wearable electronics.
To begin with, we employ a high-throughput semiconductor laser direct writing (SLDW, Figure 1) incorporating cellulose to conformally construct three-dimensional (3D)-ordered amorphous carbon micro/nanostructures in the principle of photochemistry, the focused laser beam triggers a series of dehydration, nucleation, and carbonization, forming high-purity stacked carbon nanoflakes (Figure 1b; Figure S1, Supporting Information) on the bumpy paper surface at an instant. Facility requirements are lowered because there are no extra precursors or pre-preparation involved. Intuitive evidence was found in optical observation ( Figure S2, Supporting Information), spectroscopic characterizations, and performance identification. The synthesized stacking amorphous carbon allotropes contain carbon particles and carbon flakes (Figure 1c) topologically distinct from graphene, [22] in a user-designed layout for customization. We easily enrich the charge carrier by using aqueous con-ductive hydrogels or ions to immerse the carbon stakes (Figure 1; Figure S3, Supporting Information) in water-proof tape encapsulation ( Figure S4, Supporting Information). Because wearable tactile sensing, [9] microwave shielding, [24][25][26] energy storing/conversion, [27][28][29] and IoTs interface [11] have become the critical priority in wearing techniques, [12] several wearable devices are fundamentally designed, fabricated, and comprehensively identified. Results elucidate that amorphous carbon possesses a disruptive potential in wearability, function integration, cost reduction, controllable performance, and easy tailoring to fuel the innovative development of wearable electric or energy devices.

Results and Discussion
This precursor-free transfer-free reproducible laser synthesis depending on selective laser conditions, the influence of material properties, film thickness, and laser-matter interaction was thoroughly analyzed here. Cellulose molecules absorbed laser energy and conversed into massive amorphous carbon nanomaterials (Figure 2) loading to the concave paper substrate, and this carbon nanoflake/cellulose chain composite material maintained a bendable 3D matrix (Figure 1c), forming a dynamic conductive micro/nanostructure as evidenced by the LCM technique (Figure 2b; Figure S4, Supporting Information, where the 3D carbon micro/nanostructure featured engraving profiles and cellulose fibers entangled on the paper). The surface resistance reaches 5.45 × 10 −6 Ω sq −1 not inferior to the reported semiconductors ( Figure S5, Supporting Information). Molecular interaction Figure 2. a) Chemical transferring mechanism of laser synthesis. b) 3D micrograph of the amorphous carbon on cellulose matrix. c) Raman spectra of waste paper and amorphous carbon layers at gradient optical power (1, 2, 3, and 5 W), presenting the gradually-emerged D-peak (1320 cm at 5 W) and G-peak (1570 cm at 5 W). d) SEM image of a carbonaceous layer in the grooves, and the zoomed-in figure. e) XRD diffraction curves of papers and amorphous carbon. f) A batch of FTIR spectra on three different positions of the paper sensor, position1 (P1) was pure paper surface, P2 was the area covered by PEDOT hydrogel on paper, and P3 was the carbon nanoflake engraved area. P3 curve overwhelmed cellulose and PEDOT peaks with typical carbon features at ≈1132 cm −1 , P2 FTIR curves showed character of PEDOT, while P1 curve matched well the current analysis of paper.
between hydrophilic cellulose and sodium/zinc/lithium ion or poly 3,4-ethylene dioxythiophene (PEDOT) [22,30] solutions promoted at least two-order magnitude electrical conductivity by electron/ion/hole carriers. In Raman spectra, the representative D, G, and G' peaks of sp 2 carbon materials gradually become observable using incremental laser power (Figure 2c). The initial bulging envelope (at low optical power) of the Raman curve gradually separated into two sub-peaks at ≈1370 and 1530 cm −1 at 5 W. As confirmed, the optical intensity directly determined the carbonization degree during amorphous carbon transformation to some extent, based on this, more interface-correlated performance or material optimization strategies could be further investigated.
Scanning electron microscope (SEM, Figure 2d) images exhibited the porous carbon nanoflakes stacking on and interconnecting with the tangled cellulose fibers. The zoomed-in view showed the nanoflakes loaded to deformable paper for an ag-gregated carbon-cellulose composite structure, possessing short interflake distances for convenient electron jumping or ion migration, abundant redox active sites, and oxygen-rich defects. The flexible morphology would reinforce electrical contact and ion transferring/absorbing capacity, laying the foundation for multifunctional applications. Energy dispersive X-ray spectrometry (EDS) mapping ( Figures S6 and S7, Supporting Information) detected the existence of trace elements (Ca, Fe, and Ni,) as fillers in the paper, and SLDW changed carbon element content significantly from 47.79 to 70.96 wt.%. In X-ray diffraction (XRD), the diffraction peaks of cellulose/lignin disappeared in the black curve of Figure 2e, replaced by a red flat curve without graphene peaks, implying the generated nanoflakes were amorphous carbon of incomplete crystalline degree. [29] Another batch of Fourier infrared spectroscopy further confirmed the composition change before and after the fabrication, where spectra illustrated observable characters of PEDOT and carbon mate- From the beginning, interactions between carbon-to-PEDOT ( -to-noncovalent effect), and PSS-to-cellulose gradually generated on flat shape, resulting in structural balance. For wearing situations (Figure 3b), carbon paper was possibly reconfigured into wave or torsion shapes under tensile deformation, and amorphous carbon (Figure 3c,d) was re-distributed at 3D space by outside applied impact at a high breaking strength, without crack propagation. The interconnection of carbon/PEDOT:PSS/cellulose maintained the structural integrity of paper for particular electronics. In mechanism, shortened channels of aggregation state accelerated the ion immigration [31][32][33] between flake surfaces (Figure 3c), and electrons jumped conveniently at the nanoscale from one nanoflake to another. Otherwise, electron jumping happened at a smaller probability due to the long distance, ion migration became relatively inactive. Therefore, a small amplitude shape-changing of paper modulated carrier transferring locally.
The photochemistry-generated carbonaceous 3D micro/nanostructures on pliable paper substrates featured abundant active edges/defects due to the re-stacked carbon nanoflakes. The mechanism underlying electric performance can be expounded below: 1) High-content carbon elements provided high intrinsic conductivity, serving as active materials of electric/electrochemical performance. 3D configuration of the composite networks reduced the ionic diffusion pathway, resulting in a high resistance variation and high power density for electronics. 2) Electron/carrier jumping happened easily between the nanoscale interspace of re-stacked carbon flakes of the deformation-modulated paper. Fewer carriers migrated in the tensile areas, but more carriers migrated in compressed areas through closer interconnection. [32,34,35] 3) Sodium/zinc ions or PEDOT long-chain molecules self-interacted with cellulose/carbon further facilitated [36] electrolytes diffusion and offered a mixed conduction mechanism. [37] Thus, through modulation of the interspacing of nanoflakes, this carbon paper guaranteed the realization of next-generation flexible electronics by ensuring available electrical contact in dynamic conditions, where the electric/electrochemical performance depended on re-stacking morphologies, and material composition.
Since SLDW optically stacked carbon nanoflakes into 3D porous patterns/structures (Figures S1 and S4a, Supporting Information), the microwave penetrability declined a lot by the carbon-based structures for intrinsic electromagnetic shielding properties. [38] The time-domain Terahertz (THz, Figure 3d) [39] spectral peaks shifted synchronously because of loading carbon . a) single-unit ACIS on deformable paper, and four in-parallel ACISs demonstrating a boosted energy storage/conversion ability. b) numerical simulation of interdigital electrodes showed strong electric fields and potential distributions implying typical capacitive behavior. c) integrated electrodes were tested by GVD methods, where integration density was responsible for the increased output potential. d) the deformable ACIS was encapsulated in a Zinc ion solution bag for CV tests to define the positive effect. e) the capacitance retention of flat parallelling electrodes tested in 1000 circles, f) the fluctuated capacitance due to shape-deformation in the continuous test. and PEDOT in the range from 21 to 32 GHz, and spectral intensity decreased from 0.150, 067, 0.041, to less than 0.006, meaning that re-stacking porous morphology blocked and dissipated THz wave off at simple layout design. Since the THz wave played an increasingly important role in consumer electronics, 5/6G communication, and military defense, this carbon paper unexpectedly offered huge potential for the above industries. Nearly perfect THz wave shielding/absorbing became achievable by multi-layer laminating the carbon papers, which eliminated electromagnetic interruption for protecting communication devices or being used to turn off 5/6G signals. The evidentlydecreased intensities (> 37 dB in logarithmic form) were accompanied by the increased electrical conductivities (from the insulating state, 0 Sm −1 , to the maximum 8200 Sm −1 ), similar to the graphene. [38,39,41] After more rational designs, the carbon flake papers could be sensory components such as THz absorbers or frequency-filtering metasurfaces.
In the last decade, tremendous work had been devoted to developing wearable capacitive sensors or energy storage devices, however, few substantial strides in deformable occasions were reported. Here, carbon nanoflakes provided a large surface/volume ratio with metallic conductivity, theoretically offering a high volumetric capacitance and conductivity. Compared to current 2D materials (Mxene or graphene), carbon nanoflakes deserved attention because of their facile photochemical synthesis process, interfacial properties, simplified encapsulation, and huge com-mercial potential from almost zero cost. At the nanoscale, carbonincorporating fibers formed a conductor-to-insulator capacitor morphology. The vast design volume of SLDW promised a nearly endless diversity of patterns (seen in Figure S9, Supporting Information), geometries, and integration density using carbon as electrodes. Further, to ameliorate the capacitive or energy-storing potential of nanoflake sensors, we scanned various interdigitated electrodes in designed shapes, numbers, spacing, and integration density. Taking advantage of hygroscopic and flexible cellulose matrix, the stacked nanoflakes were bestowed with stable ion storage in repeated deformation not possessed by those solid or stiff components done by the slurry casting method.
As verification, we identified the mainstream chip-scale amorphous carbon interdigital supercapacitors (ACIS) for enhanced electrochemical energy storage and conversion. Figure 4a displayed the hysteresis cyclic voltammetry (CV) curves obtained from a single capacitor unit and the in-parallel arrangement of four units connected in series. It can be seen that the output current multiplied that of the single unit, and the working voltage decreased to 20% at the initial 3 V by dropping ion/hydrogel solution, indicating that the geometrical assembly and additive ions or PEDOT conductive hydrogel maximized the use of carbon flakes and outputted a more powerful current. The areal capacitance calculated on the electrochemical tests and 3D microscope reached 7.3 mF cm −2 by pure carbon, and a high 55.1 mF cm −2 for the 4-unit parallel arranged ACIS with ions. The high ionic areal capacitive behavior outperformed the current 2D plane metal capacitors, which stemmed from the high volumetric capacitance of nanoflakes at an aggregated stacking state. Furthermore, the bent and flattened shapes affecting capacitive behavior were compared under the same testing parameters in the ionic state, results displayed a slight shape variation in CV curves much better than purecarbon electrodes, meaning that the interconnection through soft cellulose effectively mitigated the mechanical mismatch issues for stable ion migration even at large-angle twisting deformation.
To qualitatively analyze the electric field and electric potential energy of the amorphous carbon-constructed electrodes, we modeled a monolayer ACIS resembling the geometric parameters of the practical sample (Figure 4b) numerically. Electric/geometric properties of the calculated ACIS model (the thickness equaled 10 m) followed the instrument-measured parameters and were injected with sinusoidal alternative voltages at a low frequency of 2 kHz and amplitude of 3 V at two end ports. The finite element calculation plotted a dense field distribution mostly surrounding the interdigitated electrodes, while the two end ports had relatively low electric field variation. Moreover, this ultrathin 3D carbon layer generated high opposite electric potential energy (detailed information seen in Figure S10, Supporting Information) not inferior to the conventional double-layer or Triangular metal capacitors, implying high charge storage. The visualized 2D simulation concluded the energy-storing/supplying efficacy of electrodes as-prepared by the SLDW methods.
Subsequently, the carbon/PEDOT worked as basic electrodes, and Zinc/Sodium ions worked as active energy-storing materials, to reinforce the capacitive behaviors. The ions were self-enriched around the stacked carbon nanomaterials for at least one-order conduction improvement or moved by the applied voltages, forming an evidently-enhanced charge and discharge process. The typical laser direct writing framework [40] promised vast patterns, segmented designs, or parallel integration of ionic carbon electrodes, leading to controllable working voltages/currents and energy density. As seen in Figure 4c, a simple interdigitated electrode, a bracket-grouped array of electrodes at mediate integration density, and a circle grouping lots of interdigital electrodes at high density were properly designed and fabricated. The resultant galvanostatic charge-to-discharge (GCD) curves showcased soaring output potentials, confirming that the integration noticeably affected power densities. More geometric modulation of the ACISs to be arbitrarily grouped and integrated would enable ondemand control of the voltage/current of wearable electronics in actual requirements.
A group of comparison experiments (Figure 4d) reflected ion activity, where the in-ion and in-air conditions shifted the I-V curves of the same ACIS substantially. In air-conditioning, the charge migrated under the action of the electric field force, the stored charge depended on the separated carbon electrodes, and the fast surface redox reactions of PEDOT, featuring a pseudo capacitance just better than traditional metal electrodes con limited at the smooth surface. The ionic solution provided another charge-storing strategy by the fast ion migration behaviors under applied voltages for higher output current at a lower voltage threshold. As the above prediction, the bracket-connected array of electrodes provided a higher current peak than the ACIS in Figure 4a. In materials, the high aspect-ratio network provided abundant channels to facilitate ion diffusion, and the re-stacked carbon offered a high volume for embedding ions, concluding a real supercapacitor application on the inexpensive waste papers. In the capacitance test, the repeated capacitance remained over 75% of initial storage but fluctuated dramatically if electrodes were deformed (Figure 4e,f), implying a practical capacitive sensing ability. The scalable wearable carbon electrodes anchoring on bendable papers could be seamlessly compatible with other conductive coating/inks, peripheral circuits, or electronic skin, therefore, by changing the layout for customizing electrochemical properties, SLDW promised massive possibilities in fields of energy storage, circuit protection, electric coupling/decoupling, and suppression of electromagnetic interference.
Besides the wearable energy storage devices, these non-noble metal, resistance-tunable carbon stacking layers revealed another tempting application of the human-machine interfaces for tactile sensing motion parameters. As known, epidermal electronics had revolutionized the way of physical/biological signals detection (Figure 5a-f). Through the integrations with biomicrofluidics and wireless communication, wearable tactile sensors allowed physicians to remotely monitor patient conditions. In this case, even slight extension or compression of paper led to significant shifting of the carrier density attributed by the large surface/volume-ratio carbon nanoflakes/PEDOT/ion matrix. Followingly, we fabricated and tested a batch of low-cost on-paper e-skins glued to the human body. These carbon/PEDOT/ion sensors, as SLDW-patterned as serpentine lines (Figure 5b) or discs (Figure 5d), or an array as a sensing platform, were readily integrated with peripheral circuits or instruments. Predictably, the tailorable sensory papers can be further modified for systematic or distributed monitor applications.
As known, the human body's prominentia laryngea changed its shape in speeching to reflect the tone and speed of speech, the elbow joint changed shape in the bending action, the finger curls to tap table surface, and the pelma changed shapes in walking due to force-induced local tensile/compressive configuration. Human motion-induced small-amplitude shape deformation denoted the health situations or interactive commands crucially coral to IoTs interfaces, thereby, the wearable carbon nanoflakes promised natural tactile sensing merit hard to metal materials due to their tunable electric properties at soft mechanics. These SLDW-prepared tactile sensors were encapsulated and applied to detect different-position motions as evidence. The real time electric signal was real time interrogated using a 4-wire resistance analyzer in V/I mode ( Figure S11, Supporting Information) to record the muscle contraction and relaxation induced by human motions. The response latency of a single finger-taping motion was less than 0.1 s in Figure 5c, satisfying most of the actual requirements. 10 4 Cycles deformability tests showed no mechanical mismatch, breaking up, or crack happening to the as-prepared paper sensors, which implied a long-termed using possibility. Worth mentioning, that the slaty sweat, tears, and tissue fluid also affected the resistance of sensors, implying another bio-sensing perspective.
Furthermore, we employed SLDW to scan amorphous carbon patterns on a 4 × 4 array to upgrade the material's position detection as a sensing platform. One bottle (weight of ≈27 g) and a tape roll (11 g) were placed on the platform respectively, leading Representative human motion-induced resistance variation curves (speaking "ouch", bending the elbow to 90°, finger tapping tables, and walking) provided information for further biological or medical diagnosis, where the magnified inset figure of rising edge lasted less than 100 ms. e) Photograph of a position sensing via a 4 × 4 array at carbon-based pixels. The below two figures exhibited the resistance variation 3D mapping of the deformable carbon paper platform placed by bottle/tape. f) The varied resistance of coughing detected from the human throat. g) The retained relative resistance of a frequency-shifted damping vibration, and the resistance variation recorded when force increased in periodic compressing paper sensors.
to local deformation. The resistance variation of each pixel was recorded and plotted in a 3D mapping graph. Based on the linear deformation-to-resistance relationship, the obtained mapping spatially described the weight distribution of the tested objects (Figure 5e). In sensitivity calibration, we confirmed that the ion solution quadrupled the resistance sensitivity at a range of 2-1200 mN with remarkable reproducibility (> 10 5 cycles). Then, more performance tests were conducted, Figure 5f reflected the possibility of medical monitoring by detecting the cough using as-prepared sensors. The frequency-changed and amplitudechanged deformation were real time recorded to indicate the quasi-linear force-to-resistance relation (Figure 5g). Since the ion-active carbon paper became massively reproducible with desirable force sensitivity, there lay a foundation for comprehensively sensing the movement of prosthetics, humanoid robots, and human wearable devices. Anyway, the waste papers now become a tremendous asset utilized in wearable technologies using our proposed methods.
In a nutshell, the scalable mask-free semiconductor laser synthesis successfully induced cellulose carbonization into wearable amorphous carbon nanoflakes topologically distinct from graphene, the stacked 3D carbon/cellulose composite layer exhibited deformable mechanics, ultrahigh electrical conduction (up to 10 6 S m −1 ), microwave absorbing ability, wearable ionic energy storage (tunable power with areal capacitance over 50 mF cm −2 ), recyclability, and an ultralow cost as the most competitive footstone for the wearable IoTs interfaces or e-skins. Large quantities of amorphous carbon flakes provided abundant diffusion/migration channels and huge space for storing ions, leading to excellent electrochemical characteristics at excellent mechanical strength. Importantly, the free-standing carbon layers demonstrated a diversity of shape-designable electrodes at specific interspacing/geometries/integration density, with a higher fabrication efficiency than pyrolysis or hydrothermal carbonization needing high temperature or gas protection. As found, the cellulose-derived amorphous carbon flakes offered reliable guidance for innovatively developing cost-effective, function-integrated wearable electrical products including efficient energy storage devices, circuits, motion sensing systems, and microwave shields.

Experimental Section
Conductive Hydrogel/Ion Solution Preparation: The conductive hydrogel [3] was diluted from commercial Clevios PEDOT: PSS solution from 1.4 to 0.8 wt.%, which was commercially available from Aldrich. Na + ion and Zn 2+ solutions were prepared by diluting sodium chloride crystal (NaCl, Sigma-Aldrich) and zinc sulfate hydrate (ZnS0 4 •6H 2 O, Sigma-Aldrich) in water. No precursor material or pre-processing on papers was necessary.
Mask-Free Semiconductive Laser Direct Writing: Waste papers were picked from waste package boxes, and were tailored at suitable sizes (from 20 × 20 × 0.2 to 300 × 300 × 2 mm 3 ) as the laser-writing object. A continuous blue-color (405 nm) semiconductor laser diode (power at 0.1-5 W) wrote out the black amorphous carbon (1-5 m thick) for carriertransporting on the upper surface of the paper.
More customized peripheral circuits, electrodes, and specific coating patterns could be digitalized in AutoCAD and inputted into driven software for path planning, the laser system (K5-BT, DAJA) adopted transversal and longitudinal precise guide rails for the semi-conductive laser moving in pre-designed paths (at stepsize of 0.05 mm). Laser focus height and optical power (1-5 W) were tunable via a rotatable focusing lens on the laser head. The altitude of laser focus was settled down at the paper-to-air interface during scanning.
Encapsulation of Wearable Amorphous Carbon Electronics: The asfabricated wearable carbonaceous layer was injected with PEDOT:PSS hydrogel or ion solution via a medical 10 mL syringe ( Figure S1, Supporting Information), and was placed in a dark room for 4 h. Hydrogels interacted with the carbon/cellulose to form a protective layer, ions permeated into the composite matrix. Copper foils were tailored and laminated on carbon patterns to link with external circuits/instruments. The on-paper electronics could be further film-sealed by covering a polyethylene or polyvinyl chloride film for further protection.
3D Micrograph by Micro/Nanoscale Characterization: The composite cellulose/carbon matrix and nanostructured materials were observed by the environmental scanning electron microscopes (SEM, GeminiSEM 460, ZEISS) with Energy dispersive spectroscopy (EDS) for element analysis. 3D in-depth photographs were taken by a non-contact 3D laser conformal microscopy (3DLCM, VHX-7000, Keyence, resolution of 1 nm in the longitudinal direction, the example seen in Figure S2, Supporting Information), and a transmission electron microscope with high-resolution sC-MOS camera (TEM, JEM-1400Flash, JEOL, resolution < 0.4 nm).
Spectroscopic Observations: The wearable carbon papers were fabricated at selected optical parameters for comparison. This photontriggered photolysis process was spectroscopically investigated via Raman (InVia, Renishaw, laser wavelength at 532 nm, power at 5 mW, ranging from 0 to 3000 nm). The as-prepared paper samples were furtherly tested by a THz spectrometer (TAS7500, Advantest) from 1 to 50 GHz at a step size of 0.2 GHz for transmission time-domain terahertz spectra, the waveabsorbing band covered from 22 to 32 GHz. Crystalline features of powderstate carbon nanoflakes were checked via XRD techniques (D8, Bruker, with 30 kV X-ray source, 2°ranging from 10°to 45°). Fourier Transform Infrared Spectra (FT-IR) at wavenumbers of 650 to 1800 cm −1 to deduct using Nicolet™ Nexus 670 model FT-IR spectrometer from Thermo Sci-entific™.
Electrical and Electrochemical Properties Measurement: A 4-wire highresolution resistance analyzer (using 2400 source meter, Keithley) real time recorded the dynamic resistance of carbon papers at sub-millisecond time intervals and 0.01 Ω resolution to calculate electrical conduction. The clips bite the copper foils of papers, and the varied equivalent resistance induced by applied force was recorded. A Labview-based analysis software removed the baseline noise to show the fluctuation of resistance. Sensing sensitivity was calibrated using a multi-purpose electronic testing machine (DDL100, Doli) with 1 mN force resolution. All sensing wearable papers only involved non-invasive tests, as-fabricated on common papers with skin grafting, without affecting human health/ethics negatively, which worked within the safe voltage range (< 3.2 V) throughout this study.
The integrated interdigital capacitors were investigated via repeated cyclic voltammetry and galvanostatic charge-discharge measurement by a semiconductor parameter analyzer (4200A-SCS, Keithley, 1 kHz-10 MHz frequency range, current sensitivity at pA level), and a CHI660E electrochemical station (Sensitivity at A level, the initial voltage was 0 V, high voltage was 3 V, and scan rate was 0.5 V s −1 , sample interval was 0.001 V) respectively.
Data Analysis: Before TEM tests, the laser-synthesized bare amorphous carbon nanoflakes were peeled off from the paper, mixed in deionized water, and dropped on a copper mesh. The raw TEM images were loaded into Digital Micrograph software in dm3 format, first fast Fourier transforming (FFT) calculation were used to convert TEM images in a square area into frequency-domain data, then, the mask tool of Digital Micrograph software was used to pick the periodic Bragg point of the frequency-domain data, a filter at 5 pixels was used to remove background noise. Subsequently, inverse FFT was used to recover clear TEM images showing typical diffraction fringes, and the specific linewidth was measured according to the system scale bar.
3D geometries of cellulose-carried carbon/PEDOT were given by profiling tool of Keyence analysis software to obtain the height, thickness, and linewidth at a resolution of less than 3 nm. The footprint areas of the interdigital electrodes and sensors were calculated from the length, height, or radius. The size of nanoparticles or nanoflakes was summarized from the Nano-measure software by picking at least thirty samples with average calculation. The repeated measured resistance variation was recorded via Labview software of Keithley 2400 source meter repeatedly. The longtermed capacitance values were recorded by CHI660E software at least 1 × 10 3 times.
Numerical Simulation of Interdigital Capacitors: Numerical simulation of electric field and electric potential energy distributing around the interdigital electrodes was conducted on a commercial finite element software COMSOL 6.0 version. The geometries and electric properties rigidly followed the practical measured results. Inputting signals were a lowfrequency (2KHz frequency, and 3 V amplitude for Figure 4b) alternating voltage at electrostatic field mode, the generated electric fields, and energy potential around interdigital electrodes were illustrated at sub-millimeter mesh accuracy.
Informed Consent for Wearing Sensor Experiment: The tested person voluntarily participated in the sensor tests as single human subject. The sensor tests were conducted only after the tested person knew the specific details and approved. The tested person proved the used methods were not harmful to human health, the scientific research means were noninvasive experiments, and the relevant technologies were not harmful to human health.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.