Hao Wang, Zifeng Zhao, Panpan Liu, Xiaogang Guo

Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing 10 0 081, China

Keywords: Laser-induced porous graphene Polyimide (PI)/PDMS composite kirigami-inspired strain sensor

A B S T R A C T The laser-induced porous graphene (LIG) prepared in a straightforward fabrication method is presented,and its applications in stretchable strain sensors to detect the applied strain are also explored. The LIG formed on the polyimide/polydimethylsiloxane (PI/PDMS) composite exhibits a naturally high stretchability (over 30%), bypassing the transfer printing process compared to the one prepared by laser scribing on PI films. The PI/PDMS composite with LIG shows tunable mechanical and electronic performances with different PI particle concentrations in PDMS. The good cyclic stability and almost linear response of the prepared LIG’s resistance with respect to tensile strain provide its access to wearable electronics. To improve the PDMS/PI composite stretchability, we designed and optimized a kirigami-inspired strain sensor with LIG on the top surface, dramatically increasing the maximum strain value that in linear response to applied strain from 3% to 79%.

Recently, the stretchable strain sensors with a significant gauge factor, a wide tunable strain range are of increasing demand due to their applications in a wide range of electronics, such as power generators [ 1 , 2 ], supercapacitors [ 3 , 4 ], optoelectronic devices [5] ,sensors [6–10] , and actuators [ 11 , 12 ]. In addition, graphene-based nanomaterials have demonstrated their advantages in serving as composite fillers [ 13 , 14 ], functional materials in wearable electronics [ 15 –17 ], and electrodes of batteries [ 18 –20 ] owing to their unique physical, chemical, mechanical and electric properties.Though the advances devoted to exploring the synthesis methods of graphene materials, it is still challenging to fabricate precisely patterned graphene films that have shown their potential in wearable electronics and energy storage devices. As a straightforward method, the laser-induced fabrication of 3D porous graphene on a polymer layer (i.e., polyimide (PI)) paves the way toward the controlled formations of graphene-based stretchable strain sensor[21] . In this method, the high temperature due to the laser scribing breaks the C-O, C = 0, and N-C bonds, resulting in the recombination ofCandNatoms. Furthermore, the rapid liberation of carbonaceous and nitric gases gives rise to the formation of 3D porous structures. Though this strategy is a one-step and chemical-free synthesis method for the porous graphene, some post processes are required to fabricate a strain sensor with a high stretchability,such as the process of transferring the prepared porous graphene from the rigid PI film to the top of soft and stretchable substrate[ 22 , 23 ]. This transfer process’s efficiency imposes certain limitations to the fabrication of strain sensors with a large area and complex patterns. This paper develop a straightforward fabrication method for stretchable strain sensors with large area and complex patterns. Here, the graphene is formed on the PI/PDMS (polydimethylsiloxane) composite through the laser scribing according to the pre-designed CAD files. The commercial PI film is an ideal precursor material for making porous graphene by providing the carbon source. At the same time, PDMS serves as a stretchable substrate for the design of strain sensors considering its soft property.After the formation of graphene by laser scribing, the stretchable sensor is completed at once, where the post process of transferring the graphene to the top of other stretchable substrates from rigid PI film is unnecessary. Additionally, the PI/PDMS composite shows tunable mechanical and electronic performances with different PI particle concentrations in PDMS. The cyclic stability of the prepared strain sensor was experimentally verified under the strain range of 3%. The results demonstrate its potential in wearable electronics. Finally, a kirigami-inspired strain sensor with preciously patterned porous graphene layer on the top surface was designed and optimized and dramatically increases the maximum accessible strain value in linear response to applied strain from 3%to 79%.

Figure 1 a illustrates the fabrication process of laser-induced porous graphene (LIG) and experimental images of the prepared specimen with different PI particle (500 mesh, Dupont) concentrations in PDMS (i.e., 1:4, 1:3, and 1:2 from left to right). This fabrication process began with PI/PDMS composite preparation by mixing the PI particles and PDMS with a specific mass ratio and stirring for 20 min with a constant speed. After pouring into the Al mold, the PI/PDMS composite was cured at 70 °C for one hour. The stretchable precursor for making the porous graphene using laser scribing was fabricated after removing it from the mold, with a dimension size of 75 mm ×20 mm ×1.5 mm. The laser scribing onto the top surface of PI/PDMS composite photothermally converted the sp3-carbon atoms in PI particles to sp2-carbon atoms,and formed the 3D porous graphene, resulting in a high electrical conductivity along the trace of laser induction. After painting the conductive silver to the two sides of regions of LIG as the electrodes and soldering Cu wires on it, the stretchable sensors with 3D porous graphene were completed, as shown in Fig. 1 b.

Fig. 1. Fabrication of the stretchable strain sensors based on LIG and its specimen. (a) Schematic illustration of the fabrication process of stretchable sensor based on porous graphene; (b) Experimental images of LIG sensor with different PI particle concentrations in PDMS (i.e., 1:4, 1:3, and 1:2 from left to right).

Fig. 2. Mechanical and electric testing instruments, and the illustration of loading processing stretching mode.

Since the concentration of PI particles (φPl) in PDMS has a significant effect on the stretchable sensor’s mechanical and electric performances, the modulus and the variation of resistance of preprepared sensors concerning tension strain were experimentally explored in a loading machine (as shown in Fig. 2 ). Here, the electric performances were recorded by a digit multimeter (KEYSIGHT,34465A). The patches in Al were adhered on the top and bottom surfaces of the end of the stretchable sensor, as shown in Fig. 1 b.

Figure 3 presents the mechanical properties of the specimen with different PI particle concentrations in PDMS, along with their modulus. The modulus of pure PDMS is about 0.67 MPa, which is a relatively small value. As the PI particle concentrations increases, the modulus of the specimen increases nonlinearly. Significantly, the modulus of PDMS with 33% PI particles can reach about 1.12 MPa, nearly two times higher than that of the pure PDMS. The tunable range of the modulus of PI/PDMS composite(from 1.038 MPa to 1.118 MPa) and considerable promising strain(over 30%) prove an excellent potential for its application in wearable electronics to fit various target objects.

Figure 4 gives the electric performances of stretchable sensor based on laser-induced porous graphene versus tensile strain with different PI particle concentrations (i.e., 20%, 25% and 33% from left to right). As the gradual formation of crack in LIG, the resistance of the specimen increases nonlinearly as the increase of tensile strain. The normalized resistances of the specimen even achieve 50 0 0% forφPI= 20% at 8% strain, 3700% forφPI= 25% at 8.5%strain and 920% forφPI= 33% at 10% strain respectively. Additionally, the gauge factor (GF), a parameter to evaluate the sensitivity of sensor for the applied strain, is calculated usingGF=(ΔR/R0)/ε,whereΔR,R0andεdenote the resistance change, original resistance and applied strain, respectively. As illustrated in Fig. 4 c, the prepared strain sensor exhibits three gauge factors as the tension proceeds. For the specimen ofφPI= 33% , theGFis relatively small,only about 40 in a low strain range (i.e.,ε<3% ). Due to the gradual formation of crack perpendicular to the loading direction, theGFincreases to 100 for 3%<ε<7% , and then to 181 forε>7% .This intrinsic advantage promises its applications in the fields that require a widely tunableGF. In spite of the nonlinear dependence of resistance ratio on the applied strain, a linear increase of the resistance ratio with a scope of 3% can also be observed at the beginning of stretching (as shown in the inset images of Fig. 4 ),which is a crucial essential during the design of flexible electronics. The other two specimens (i.e.,φPI= 20%,φPI= 25% ) show the same dependence but different values, as shown in Fig. 4 a and b.

Figure 5 shows the electric performances of the prepared strain sensors with different PI particle concentrations during mechanical cycles, where the maximum tensile strain was about 3%. As evidenced by the experiments shown in Fig. 5 d, the normalized resistance first decreases nonlinearly, and then a stable electric performance can be observed after 10 mechanical cycles. These observations demonstrate the good cyclic ability of the stretchable strain sensors based on porous graphene. Additionally, we will explore the cyclic stability of the strain sensor under extremely-high-cycle loading in the future.

Fig. 4. Normalized resistance versus tensile strain of stretchable sensors. (a–c) The resistance ratio versus strain of the specimen with different PI particle concentrations;(d)The trends of gauge factor.

To address the demand of the wearable or flexible electronics on the large accessible mechanical deformation, a kirigamiinspired stretchable sensor using LIG as the functional material is also design, as shown in Fig. 6 a. Here, the concentration of PI particles in PDMS was selected as 33%. By considering the linear response of the laser-induced graphene material along with the tensile strain and using 3% as the crucial strain for this functional material, we can estimate the value of maximum tensile strain of the kirigami-inspired stretchable sensor in PI/PDMS composite (as shown in Fig. 6 b and 6 c). Figure 6 d presents the dependence of maximum accessible tensile strain on the film thickness(tkiri). Because of the emerging restriction effect of the film thickness on the out-of-plane deformation, the maximum strain decreases nonlinearly from 79% when the thickness is 0.1 mm to 8.5%when the thickness is 0.5 mm. Despite this, the FEA result also demonstrates the capability of this kirigami-inspired strain sensor in serving as the detecting component in wearable electronics devices.

Fig. 5. The cyclicability of strain sensor under several loading cycles. (a–c) The resistance ratio versus time of the specimen with different PI particle concentrations; (d) The resistance ratio versus the cycle number.

Fig. 6. Kirigami-inspired strain sensor in PI/PDMS with a large accessible mechanical deformation. (a) Schematic illustration of the kirigami-inspired strain sensor using LIG as the functional material ( L 1 = 15 mm, L 2 = 13 mm, W 1 = 4.5 mm, W 2 = 1.5 mm); (b, c) The FEA result of the kirigami-inspired strain sensor under a uniaxial tension; (d)Maximum strain of the kirigami-inspired LIG stretchable strain sensor with different film thicknesses.

In this paper, we developed a straightforward fabrication method for the stretchable LIG strain sensors. The LIG prepared on the PI/PDMS composite by the laser scribing exhibits a naturally high stretchability (over 30%). To address the demand of the stretchable strain sensor for the linear response in a broad applied strain range, we explored and optimized the kirigamiinspired strain sensor, dramatically increasing the accessible maximum strain from 3% to 79%. The combined capabilities in a broad tunable elastic modulus range, good stability, high sensitivity, and large accessible maximum strain provide the potential of this stretchable strain sensor in the field of wearable electronics.

Declaration of Competing Interest

The Authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

X.G. acknowledges support from the National Natural Science Foundation of China (Grant No. 12072030 ).