Direct Patterning of Conductive Structures on Hydrogels by Laser‐Based Graphitization for Supercapacitor Fabrication

Hydrogels have emerged as promising supporting materials for wearable or implantable electronic devices, owing to their high biocompatibility. Among the many techniques for patterning conductive structures on supporting materials, laser‐based graphitization allows the simultaneous synthesis and patterning of conductive structures. Here, it has been demonstrated for the first time, the direct patterning of conductive structures on hydrogels by laser‐based graphitization, and the application of this method to the fabrication of hydrogel‐based supercapacitors. Conductive graphitic carbon is formed on the hydrogel along the trajectory of laser scanning. One‐step fabrication of supercapacitors is achieved by laser irradiation of hydrogels containing both lignin and NaCl electrolyte. Notably, the areal capacitance of the fabricated supercapacitor is retained after drying and swelling, indicating its reusability. The method realizes rapid fabrication of hydrogel‐based supercapacitors that can be stored in a dry state, providing a facile method for storing and delivering energy‐storage devices.


Direct Patterning of Conductive Structures on Hydrogels by Laser-Based Graphitization for Supercapacitor Fabrication
Rikuto Miyakoshi, Shuichiro Hayashi, and Mitsuhiro Terakawa* DOI: 10.1002/aelm.202201277 ties allow them to be dried by exposure to the ambient atmosphere and swelled reversibly by immersion in water. Several researchers have reported the reusability of hydrogel-based electronic devices after drying and swelling. [4] For instance, Liu et al. reported hydrogel-based strain sensors that were reusable after drying and swelling. [4a] Preserving hydrogel-based electronic devices in a dry state can suppress the changes in device performance due to the oxidation of conductive structures as well as the spontaneous leakage of liquids, providing a low-cost and facile method for storing and delivering these devices.
Because most hydrogel-based electronic devices are fabricated by combining hydrogels and conductive structures, their fabrication requires the patterning of conductive structures on the hydrogels. Previously, conductive structures were patterned on hydrogels using transfer printing [5] and laser-based ablation. [6] Transfer printing can be divided into three steps: preparation of conductive structures and a mask or mold, patterning of conductive structures on the donor material, and transfer of conductive structures from the donor material to the hydrogel for patterning. Rahimi et al. reported the patterning of conductive structures on hydrogels by laser-based ablation as a technique that realized spatially selective processing. [6a] The patterning procedure involved preparing the hydrogel and a conductive Zn film separately, placing the Zn film on the hydrogel, and laser irradiation to selectively remove the Zn film, followed by the removal of the excess film via peeling.
Organic materials can be directly modified into conductive graphitic carbon by laser irradiation, which is generally referred to as laser-based graphitization. By utilizing this technique, conductive structures can be directly patterned along the trajectory of laser scanning. Laser-based graphitization has been used to pattern conductive structures on supporting materials for the fabrication of electronic devices. Several studies have been conducted on the fabrication of energy-storage devices such as lithium-oxygen batteries [7] and electric double-layer capacitors (i.e., supercapacitors), [8] by laser-based graphitization. To date, various materials including polyimide, [9] paper, [10] and coconut [11] have been used as precursor materials for laserbased graphitization. However, to the best of our knowledge, the direct patterning of conductive structures on hydrogels by laser-based graphitization has yet to be demonstrated. Establishing a novel method for patterning conductive structures on Hydrogels have emerged as promising supporting materials for wearable or implantable electronic devices, owing to their high biocompatibility. Among the many techniques for patterning conductive structures on supporting materials, laser-based graphitization allows the simultaneous synthesis and patterning of conductive structures. Here, it has been demonstrated for the first time, the direct patterning of conductive structures on hydrogels by laserbased graphitization, and the application of this method to the fabrication of hydrogel-based supercapacitors. Conductive graphitic carbon is formed on the hydrogel along the trajectory of laser scanning. One-step fabrication of supercapacitors is achieved by laser irradiation of hydrogels containing both lignin and NaCl electrolyte. Notably, the areal capacitance of the fabricated supercapacitor is retained after drying and swelling, indicating its reusability. The method realizes rapid fabrication of hydrogel-based supercapacitors that can be stored in a dry state, providing a facile method for storing and delivering energy-storage devices.

Introduction
The application of biocompatible materials as supporting materials in wearable and/or implantable electronic devices is being researched extensively. Hydrogels are highly biocompatible materials with Young's moduli that are similar to that of biological tissues. [1] Owing to their unique properties, hydrogels have been applied as supporting materials in electronic devices that are used in contact with biological tissues, such as smart contact lenses [2] and muscle-stimulation devices. [3] In addition to their high biocompatibility, their water-retention proper-www.advelectronicmat.de hydrogels along the trajectory of laser scanning will contribute to the facile fabrication of hydrogel-based electronic devices.
In this study, we demonstrated the direct patterning of conductive structures on a hydrogel by laser-based graphitization, for the first time, and applied the patterned structures to the electrodes of supercapacitors, which can be stored in a dry state. Lignin, a natural wood-derived polymer, was added to the hydrogel to facilitate graphitization. To discuss the effect of lignin on the formation of conductive graphitic carbon structures, the relationship between the crystallite size of graphitic carbon and the concentration of lignin added to the hydrogel was discussed via Raman spectroscopy. Electrochemical analysis revealed that the conductive graphitic carbon structures formed by laser irradiation on hydrogels could be used as the electrodes of supercapacitors. Furthermore, based on the capacitance retention of the supercapacitors after drying and swelling, we demonstrate that the supercapacitors can be reused even after storage in a dry state.

Results and Discussion
3 mm line structures were directly fabricated by a single scanning of femtosecond laser pulses on agarose hydrogels containing lignin with different concentrations. The optical absorbance of the agarose hydrogel containing 0.001 wt.% lignin was ≈1.9 times higher than that of the agarose hydrogel without lignin, at a wavelength of 522 nm ( Figure S1, Supporting Information). The laser-irradiated hydrogels were analyzed after immersion in deionized water, unless otherwise mentioned. Figure 1a shows the digital microscope images of the hydrogels after laser irradiation. No visible modification was observed for the hydrogels without lignin. In contrast, the hydrogels containing 5-25 wt% lignin were visibly modified into black-colored structures. At 25 wt% lignin, the cross-section of the structure showed a nearly semicircular shape ( Figure S2, Supporting Information). High surface roughness was confirmed at the hydrogel containing lignin based on the scanning electron microscope (SEM) image ( Figure S3, Supporting Information). Figure 1b shows the Raman spectra obtained at a distance of 15 µm from the center of the fabricated structures in a direction perpendicular to the laser scanning direction. Peaks were confirmed at ≈1350 and ≈1600 cm −1 from the Raman spectra at 5-25 wt% lignin. These peaks correspond to the D and G bands, respectively, indicating the formation of graphitic carbon. [12] On the other hand, no peak was confirmed for the Raman spectrum obtained from the hydrogel containing 25 wt% of lignin before laser irradiation ( Figure S4, Supporting Information). The peak intensity ratio of the D and G bands (I D /I G ) is known to be negatively correlated with the crystallite size. [13] Note that a lower I D /I G indicates a larger crystallite size of graphitic carbon. I D /I G decreased with an increase in the concentration of lignin from 5 to 25 wt%. This result suggests that the crystallite size of graphitic carbon increased with the increase in concentration of lignin. The full width at half maximum of the D band increased with an increase in the concentration of lignin, which suggests an increase in the number of defects of graphitic carbon. [14] The transmission electron microscope image of the structure fabricated at 25 wt% lignin ( Figure S5, Supporting Information) shows the lattice spacing of ≈0.32 nm. This lattice spacing corresponds to the (002) plane of graphitic carbon. [9] Figure 1c shows the conductances calculated from the resistances measured via the four-probe method. The resistances were measured at the point where the Raman spectra were obtained. The electrical conductivity was confirmed for the structures fabricated at 5-25 wt% lignin. The conductance increased with the increase in concentration of lignin. To the best of our knowledge, the results shown in this study present the first demonstration of The formation of graphitic carbon on agarose hydrogel by laser irradiation can be explained in terms of the optical property and chemical structure of the lignin added to the hydrogel. The irradiation of femtosecond laser pulses with a high repetition rate (63 MHz) can induce photodegradation and thermal degradation of the precursor material owing to the high peak intensity and heat accumulation, respectively. In addition, lignin has been considered as an effective-precursor material for the laser-based graphitization because they are composed of aromatic carbon (i.e., sp 2 carbon). [8d,15] For the laser-based graphitization of wood, which is a lignin-containing material, it is speculated that the sp 2 carbon cluster formed by the degradation of wood acts as the nucleus, and that it grows by recombining with carbon radicals to form graphitic carbon, as reported previously. [15] Similar to the case of wood, the sp 2 carbon cluster formed by the degradation of lignin is assumed to act as a nucleus of graphitic carbon and grow by recombining with the carbon radicals formed by the degradation of lignin and agarose.
The concentration of lignin added to the hydrogel affected the crystallite size, conductivity, and dimensions of the fabricated structure. The increase in the concentration of aromatic carbon and that in the absorbed optical energy, with the increase in the concentration of lignin, probably led to an increase in the crystallite size of graphitic carbon. Because the thermal stability of graphitic carbon increases with the increase in its crystallite size, [16] the thermal ablation of graphitic carbon may be suppressed at a high concentration of lignin. This suppression of thermal ablation probably resulted in the black-colored structure remaining on the hydrogel near the focal spot, at high concentrations of lignin. The electrical conductivity of the fabricated structure is attributable to the formation of graphitic carbon. A previous report showed that the crystallite size of the graphitic carbon formed by the irradiation of femtosecond laser pulses on cellulose nanofibers was correlated with the electrical conductance. [17] Similarly, the increase in conductance with an increase in the concentration of lignin is attributable to the increase in the crystallite size of graphitic carbon. The results of hydrogels prepared with 25 wt% lignin, which is the condition under which the fabricated structure exhibited the highest conductance, are shown below. Figure 2a shows digital microscope images of the structures fabricated on the hydrogels with different laser scanning speeds and laser powers. No visible modification was observed in the hydrogels irradiated with a laser power of 25 mW at a laser scanning speed of 125 µm s −1 . Black-colored structures were discontinuously formed in the hydrogels irradiated with a laser power of 25 mW at lower laser scanning speeds (i.e., 75 and 100 µm s −1 ). At a consistent laser power (25 mW) and even lower laser scanning speed (50 µm s −1 ), the continuous formation of black-colored structures was confirmed. It is known that the laser spot size and the number of pulse overlap largely affect the formation of graphitic carbon. [11] The increase in energy with the decrease in laser scanning speed probably induces an increase in the maximum reached temperature, which presumably resulted in the formation of black-colored structures with a laser power of 25 mW and low laser scanning speeds (i.e., 50, 75, and 100 µm s −1 ). Figure 2b shows the conductance of the fabricated structures shown in Figure 2a. The SEM images of the structures fabricated with a laser power of 100 mW at a laser scanning speeds of 50, 75, and 100 µm s −1 indicated the increase in surface roughness with the decrease in laser scanning speed ( Figure S6, Supporting Information). The results shown below were obtained from the structures fabricated with a laser power of 50 mW at a laser scanning speed of 75 µm s −1 , which is the condition wherein the structure exhibits the highest conductance. Figure 3 shows the SEM images obtained from 3 mm line structures fabricated on hydrogels containing both lignin and NaCl electrolyte. The addition of NaCl before laser irradiation may contribute to the one-step fabrication of the supercapacitor and the formation of highly porous structures that increase the areal capacitance. As shown in Figure 3, the number of pores increased with an increase in the concentration of NaCl for www.advelectronicmat.de the structures fabricated with 0, 4, and 8 wt% NaCl. Compared to the structure fabricated with 8 wt% NaCl, pores with larger dia meters (≈1 µm) were observed in the structure fabricated at 12 wt% NaCl. The formation of highly porous structures has been reported in the cases of heating organic polymers containing NaCl crystals, in a furnace. [18] Similarly, it is presumed that the presence of recrystallized NaCl formed after the evaporation of water led to the formation of highly porous structures, in the present study. Note that the diameter of the pores increased with an increase in the concentration of NaCl, similar to the case of heating phenolic resin containing NaCl crystals in a furnace. [18a] Interestingly, high conductance was calculated from the structures fabricated on the hydrogel containing NaCl ( Figure S7, Supporting Information), which is desirable for the electrodes of the supercapacitor.
A pair of conductive graphitic carbon structures was patterned with a gap of 1000 µm, to fabricate the electrodes of a supercapacitor. Agarose hydrogels containing both lignin and NaCl (8 wt%) were irradiated with laser pulses, as illustrated in Figure 4a. A photograph of the hydrogel after laser

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irradiation is shown in Figure S8, Supporting Information. The electrodes patterned on the hydrogel remained on its surface after immersion in deionized water and NaCl aqueous solution, whereas lignin was extracted from the hydrogel, as shown in Figure 4b. Figure 4c shows the cyclic voltammetry (CV) curves of the supercapacitors, with and without immersion in deionized water and NaCl aqueous solution. In both conditions, a pseudorectangular CV curve was confirmed, which is the typical CV curve of an electric double-layer capacitor. [19] The nearly triangular shape of the galvanostatic chargedischarge (GCD) curve ( Figure S9, Supporting Information) also indicates the typical behavior of electric double-layer capacitor. From the GCD curve at 20 µA cm −2 the energy density and the power density were calculated to be 4.5 nWh and 10.0 mW cm −2 . Based on the CV curves, the areal capacitance of the supercapacitor with immersion was found to be 97% of that of the supercapacitor without immersion. It is worth mentioning that the ionization degree of lignin is lower than that of NaCl, which exhibits an ionization degree of ∼1 when dissolved in deionized water. Therefore, the number of ions decreased negligibly with the extraction of lignin, which possibly resulted in a minor decrease in the areal capacitance. The results obtained from supercapacitors immersed in deionized water followed by immersion in 8 wt% NaCl aqueous solution are shown below. Figure 4d,e shows the CV curves and derived areal capacitances, respectively, of the supercapacitor, at different voltage scan rates. The shape of the CV curve shifts to a more rectangular shape with a decrease in voltage scan rate, which is consistent with the previous reports of carbon-based supercapacitors. [20] The areal capacitance decreased with an increase in the voltage scan rate, which is also consistent with the typical results of supercapacitors. [21] It is known that the ion diffusion cannot follow the electric-field change at high voltage scan rates, which results in a low areal capacitance. [22] The results shown below are from the analyses of the supercapacitors at a voltage scan rate of 1500 mV s −1 , to investigate the electrochemical performance at a high voltage scan rate. The capacitance retentions were calculated based on the CV curves obtained for 1000 consecutive cycles from the supercapacitor (Figure 4f). 91% of the areal capacitance was retained for the supercapacitor after 1000 consecutive cycles. Comparable retention of areal capacitance (88%) was calculated from the GCD curve obtained for 1000 consecutive cycles at a current density of 20 µA cm −2 ( Figure S10, Supporting Information). The retention of capacitance is also comparable to that of the supercapacitor fabricated via laser-based graphitization of lignin/poly(ethylene oxide) film [23] and that fabricated via placing the hydrogel on electrodes patterned by laser-based graphitization of polyimide. [24] The fabricated supercapacitor retained the areal capacitance for 82% after the bending at the radius of 9 mm, based on the CV curves obtained at different bending radii ( Figure S11, Supporting Information), suggesting the potential for the application of wearable devices. Figure 5a,b shows the CV curves and derived areal capacitances, respectively, at different concentrations of NaCl during laser irradiation. The areal capacitance increased with the increase in the concentration of NaCl for the supercapacitors at 0, 4, and 8 wt% NaCl. The areal capacitance of the supercapacitor at 8 wt% NaCl was higher than that of the supercapacitor at 12 wt% NaCl. In the case of an electric doublelayer capacitor, the number of ions adsorbed on the electrode increases with the increase in the surface area of the electrode, resulting in an increase in the areal capacitance. [8a] Considering the effect of NaCl on the surface morphology of the electrode (Figure 3b,c), it can be inferred that the areal capacitance changes with the concentration of NaCl, owing to the change in the surface area of the electrode. The equivalent series resistance (ESR) of the supercapacitors fabricated at 0 and 8 wt% of NaCl were 435 and 131 Ohm, which were calculated from the high-frequency region of the Nyquist plots shown in Figure S12, Supporting Information. The difference in ESR is consistent with the relationship between the conductance of the structure and the concentrations of NaCl ( Figure S7, Supporting Information).
To investigate the reusability of the fabricated supercapacitors after drying and swelling, they were dried in ambient atmosphere and then swollen by immersion in 8 wt% NaCl aqueous solution (Figure 6a). The photographs of the supercapacitors at each step are shown in Figure 6a. The electrode www.advelectronicmat.de pattern did not change significantly with drying or swelling. Figure 6b shows the CV curves of the supercapacitors with and without drying and swelling. From the CV curves, it can be found that the areal capacitance of the supercapacitor subjected to drying and swelling was 79% of that of the supercapacitor without immersion. Despite the moderate decrease in the areal capacitance, our results demonstrate that the fabricated supercapacitors can be used after drying and swelling. Figure S13, Supporting Information, shows a digital microscope image of the electrode of the supercapacitor after drying. Although the electrode became curved during drying, no cracks were observed. Because the hydrogels retain water, the volume of the hydrogel decreases with drying. This volume reduction probably induced stress, which deformed the electrode; however, the stress was not sufficient to induce cracking. Therefore, the decrease in the effective surface area was probably moderate, which presumably resulted in the retention of the areal capacitance (i.e., 79%) with drying and swelling. The dried hydrogelbased supercapacitor is small and lightweight, allowing for facile storage and delivery (Figure 6c). Optimization of the chemical structure of the hydrogel to suppress shrinking when exposed to the ambient atmosphere, as well as the shape of the electrode may further improve the capacitance retention after drying and swelling.

Conclusion
We demonstrated the direct patterning of conductive structures on hydrogels by laser-based graphitization and applied the patterned structures to the electrodes of supercapacitors. The formation of conductive graphitic carbon was confirmed in the laser-irradiated hydrogel containing lignin, indicating the facilitation of graphitization by lignin. As our technique allows for the patterning of electrodes via simple laser irradiation, the one-step fabrication of supercapacitors was demonstrated by laser irradiation of a hydrogel containing both lignin and NaCl as an electrolyte. Interestingly, the porosity of the fabricated structure increased with the increase in the concentration of NaCl in the hydrogel. Furthermore, the areal capacitance of the supercapacitor was retained after drying and swelling. Our method realizes rapid fabrication of hydrogel-based supercapacitors that can be stored in a dry state, providing a facile method for storing and delivering energy-storage devices.

Experimental Section
Fabrication of Structures: Commercially-available agarose (4.0%, w v −1 , Sigma-Aldrich Co., LLC, USA), lignin (Tokyo Chemical Industry Co., Ltd., Japan), and NaCl (Sigma-Aldrich Co., LLC, USA) were added www.advelectronicmat.de to deionized water. Agarose hydrogel shows sol-gel transition by physical crosslinking, and the detailed properties have been reported elsewhere. [25] The prepared solution was heated at 80 °C for 30 min and poured into a mold (0.2 cm × 0.8 cm × 1.0 cm), followed by cooling at room temperature for 30 min for gelation. All structures in this work were fabricated by irradiating laser pulses with a central wavelength of 522 nm (second harmonic wave of a 1045 nm femtosecond laser) and pulse width of 192 fs at a repetition rate of 63 MHz using a femtosecond laser system (High Q-2, Spectra-Physics Inc., USA). The laser pulses were focused on the surface of the hydrogels using an objective lens (MPLN20x, Olympus Corp., Japan) with a numerical aperture of 0.4. The laser spot diameter is calculated to be ≈1.6 µm. The laser pulses were scanned using an automatic three-axis (xyz) translation stage.
Characterization: The absorption spectra of the hydrogels were obtained using a spectrometer (UV-3600, Shimadzu Corp., Japan). Resistances were measured via a four-probe method (probe distance:100 µm) with a digital source meter (2401, Keithley Instruments Inc., USA), as illustrated in Figure S14, Supporting Information. The dimensions and surface morphologies of the fabricated structures were observed using a digital microscope (MS-100, Asahikogaku Co., Ltd., Japan) and SEM (SU-70, Hitachi High-Tech Corp., Japan), respectively. The chemical analyses of the fabricated structures were conducted using a Raman spectrometer (InVia Raman Microscope, Renishaw Inc., UK). An excitation wavelength of 532 nm was used for the Raman analyses, and the spectra were recorded for a wave number of 1000-3000 cm −1 .
Electrochemical Analyses: A potentiostat (Gamry 1010E Interface, Gamry Instruments Inc., USA) was used to characterize the fabricated supercapacitor. The areal capacitance (C C ) was calculated based on the capacitance derived from the CV curve using Equation (1): where v is the voltage scan rate, V f and V i are the potential limits of the CV curves, S is the two-dimensional surface area of the electrode, and I 1 (V) is the voltammetric current. [9] The areal capacitance (C G ) was calculated based on the capacitance derived from the GCD curve using Equation (2): where I 2 is the discharge current, dV/dt is the slope of GCD curve. The energy density (E) and the power density (P) was calculated based on the capacitance derived from the GCD curve using Equations (3) and (4), respectively: where ΔV is the discharge potential range and Δt is the discharge time.
Nyquist plot was obtained in the frequency range of 10 mHz to 100 kHz with potential amplitude of 10 mV. Bending Test: The fabricated supercapacitor was placed onto the curved platform with different radii. The CV curve was obtained before and after bending in a flat state.
Drying-Swelling Test: The supercapacitor fabricated on the hydrogel containing both NaCl (8 wt%) and lignin (25 wt%) was dried in ambient atmosphere (48 h) and swelled by immersion in 8 wt% NaCl aqueous solution (48 hours) prior to the electrochemical analysis, for the dryingswelling test.

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