Poly(vinyl alcohol)-Mediated Graphene Aerogels with Tailorable

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Polyvinyl Alcohol-Mediated Graphene Aerogels with Tailorable Architectures and Advanced Properties for Anisotropic Sensing Jingxia Huang, Zhangpeng Li, Xianzhang Wu, Jinqing Wang, and Shengrong Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11327 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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The Journal of Physical Chemistry

Polyvinyl Alcohol-Mediated Graphene Aerogels with Tailorable Architectures and Advanced Properties for Anisotropic Sensing Jingxia Huang

a, b,

Zhangpeng Li a,*, Xianzhang Wu

a, b,

Jinqing Wang a,*, and

Shengrong Yang a

a

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,

Chinese Academy of Sciences, Lanzhou 730000, China. b Center

of Materials Science and Optoelectronics Engineering, University of Chinese

Academy of Sciences, Beijing 100049, China

* Corresponding authors. Tel: +86-931-4968076; Fax: +86-931-4968019. E-mail address: [email protected] (J. Q. Wang); [email protected] (Z. P. Li)

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ABSTRACT: Three-dimensional graphene (3DG) aerogels possessing superior elasticity and conductivity are prerequisite to achive high sensitivity and reliability in flexible sensors. However, conventional 3DG with brittle structure and small recoverable deformation are often difficult to possess excellent sensing performance. Herein, 3DG-poly (vinyl alcohol) (3DG-PVA) composite aerogels with controlled structures and properties are designed and prepared by PVA-mediated self-assembly of graphene oxide nanosheets combining the hydrothermal reaction and freeze-drying processes. The resulting aerogels exhibit robust structure with a high porosity, superior elastic deformation, decent electrical conductivity, and outstanding eletromechanical performances including a high sensitivity of 0.34 kPa-1, a fast response time of 63.9 ms, and an excellent signal stability of over 5000 times of cycling. To demonstrate their potential applications in the field of piezoresistive sensing, the wearable sensors are designed and fabricated by using the 3DG-PVA aerogels as the sensing part, which exhibit the anisotropic sensing capabilities and ultrasensitive performances confirmed by the practical applications in monitoring the motions of human joints with different orientations.

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1. Introduction As newly emerging nanostructures, three-dimensional graphene (3DG) aerogels have become one of the most attractive bulk materials because of the large surface area (>850 m2 g-1),1 extremely low density (90%).4,

5

More importantly, aerogels in sponge, foam, petal arrays, microlattice

forms can convert the intrinsic superior performance of graphene into macroscopic applications, including bioscaffolds,6, devices,10,

11

7

pollutant treatment,8,

catalytic support,12 and sensors,13,

14

9

energy storage

etc. Thereinto, the compressible

graphene aerogels (GA) have drawn more attention recently. Even though graphene sheet has good toughness and high strengh, most of the GA possess stochastic porous network, general brittle structure, and small recoverable deformation before failure, which restrict their practical applications, especially in flexible sensors.15 To resolve the above problems, extensive efforts have been performed to assemble 3DG-based composites in large-scale level.16,

17

In particular, the

construction of 3DG-polymer composites has acquired an increasing attention ascribing to the outstanding structural stability induced by the strong interaction between graphene and polymers.18 For instance, super flexible and elastic 3DG-polyimide nanocomposite foam was prepared by Qin et al. using facile freeze-drying and thermal annealing processes,19 which present a desirable electrical conductivity of 2.2 × 10-2 S m-1 and a compression sensitivity of 0.22 kPa-1 within 1.5 kPa, as well as the excellent durable stability; nevertheless, its 3



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deep application is still limited due to the low strength of 3 kPa at 50% strain. As another example, the compressible GA-poly (dimethylsiloxane) (GA-PDMS) composite was fabricated by Hu et al. via an infiltration-evaporation-curing strategy,20 which possessed high electromechanical stability upon 50% strain for multicycles and good linear sensitivity, but the tedious operations were involved. Additionally, the 3D macroporous GA-polystyrene composite aerogel was constructed via the freezing-directed assembly of graphene under the assistance of surfactant by Zhang et al.21 And such kind of aerogel had a high strength of 80 kPa and could tolerate 80% of strain deformation; meanwhile, it displayed stable electrical resistance response under a strain of 50% for 100 cycles and high compressive sensitivity of 0.22 kPa-1 within 2 kPa, but its detection range of pressure is still restricted (< 8kPa). In fact, the sensing behavior is closely related to the mechanical performance of the materials, especially the elasticity. Therefore, the enhancement of the mechanical property can contribute to realizing the super-elasticity of graphene aerogels, which can effectively improve the sensing performance. Poly (vinyl alcohol) (PVA), one kind of abundant and biocompatible polymers, is a potential candidate material for the fabrication of aerogels because of its chemical stability, good mechanical property, and massive hydroxyl groups.22 Moreover, it can effectively reinforce the mechanical property of graphene through cross-linking reaction with GO nansheets.23,

24

For

example, as demonstrated by Park et al.,24 the cross-linked reduced graphene oxide (rGO)-based aerogels were prepared by combining PVA-assisted self-assembly and 4


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cross-linking of glutaraldehyde, which possesses reversible compressibility and high elastic modulus (186.6 kPa). Besides, 3DG-PVA composite hydrogel has also been prepared by Xiong et al. via the freeze-thaw method combining the γ -rays irradiation process,25 the result revealed that the covalent bonding between rGO sheets and PVA matrix endowed the hydrogel with high compressive strength. Unfortunately, the reported 3DG-PVA composites presented low electrical conductivity due to the massive oxygen-containing functional groups on GO nanosheets; meanwhile, the multistep preparation was not easy to be carried out. Therefore, to design and fabricate 3DG-PVA aerogel with high elasticity and decent electrical conductivity via a straightforward route is urgently needed to be developed. Herein, we highlight 3DG aerogels with tailorable architectures at the macroscale and properties, which were prepared by PVA-mediated self-assembly of GO nanosheets combining hydrothermal reaction and freeze-drying processes. The aerogels have multiple advantages including robust structure with a high porosity, superior elastic deformation, satisfactory electrical conductivity, and outstanding electromechanical performances including a high sensitivity of 0.34 kPa-1, fast response time of 63.9 ms, and reliable signal response over 5000 times of cycling. In addition, the wearable sensors fabricated using the resulted 3DG-PVA aerogels as the sensing part exhibit anisotropic sensing capabilities, high sensing reliability and ultrasensitive performance, which are confirmed by practical applications in monitoring human motions of finger, between fingers, opisthenar, and elbow joint bending with different orientations. 5



2. Experimental section 2.1. Synthesis of 3DG-PVA composite aerogels GO dispersion was prepared from natural graphite by a modified Hummers method,26 and then diluted to 5 mg mL-1 and ultrasonically treated for 30 min in reserve. As reported in our previous work,27 the obtained GO nanosheets possessed the average area of ~9.6 μm2 and thickness of 0.8 nm. PVA (Mw=10000 ~ 26000, 86 ~ 89 % hydrolyzed) purchased from Alfa Aesar was dissolved in 100 mL ultrapure water to acquire 5 wt. % PVA aqueous solution by stirring at 90 ºC for 3 h. After that, 0.25 mL H2SO4 (4 M) was added into 5 mL PVA solution by agitation, followed by the addition of various volumes of GO dispersion to form the mixture solutions, which have different volume ratios (rG/P) of GO to PVAfrom 5:1, 2:1, 1:1, 1:2, 1:5, to 1:10. After being stirred for 1 h, the blended solution was sealed to the glass bottle and transferred into a Teflon-lined autoclave for the subsequent hydrothermal reaction at 180 ºC for 12 h. Next, the resulting hydrogels were dialyzed in ultrapure water for 12 h and then freeze-dried under vacuum for 48 h to obtain the products, which are named as G-1, G-2, G-3, G-4, G-5, and G-6, respectively. For comparison, the pure 3DG and PVA aerogels were also prepared via the similar processes. 2.2. Characterization techniques X-ray diffraction (XRD) patterns, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectra (FTIR) and Raman spectra were acquired from Empyrean, Escalab 250Xi, IFS 66v/s and LabRAM HR Evolution, respectively. Scanning electron microscope (SEM) and transmission electron microscope (TEM) 6


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images were obtained by JSM-5601LV and TF20, respectively. The specific surface area (SSA) and total pore volume were respectively calculated by Brunauer-Emmer-Teller (BET) and Barret-Joyner-Halenda (BJH) via nitrogen absorption on Autosorb iQ Gas Sorption (ASAP 2020M, USA) at liquid temperature (77 K). The density (ρ) and porosity (P) of composite aerogels were calculated by equations: ρ=(

𝑚 ) 𝑉 𝜌

P = (1 ― 𝜌𝐶) , where 𝑚 and 𝑉 are the mass and volume of each aerogel, while 𝜌𝐶 is the density of the solid with the value of about 2160 mg cm-3 (2090 ~ 2230 mg cm-3). At least three samples are tested. 2.3. Mechanical and sensing performance tests The compressive testing was performed to measure the compressive strength of cylindrical 3DG-PVA composite aerogels using an electrical universal materials testing machine (AGS-x, Shimadzu) with a 500 N load cell, and the velocity was maintained at 2 mm min-1. The real-time electrical signals of pressure sensors were recorded by a digital sourcemeter (2450, Keithley). Before recording a test, the pressure sensor and wearable device were respectively constructed, where the aerogels bulks with ~11 mm of the thickness and diameter or slices with ~2 mm of the thickness were connected to the measurement system of the sourcemeter by the electrical wires using a thin silver paste layer. 3. Results and Discussion 7



3.1. Self-assembly process of 3DG-PVA composite aerogels The self-assembly process of 3DG-PVA composite aerogels is depicted in the Experimental section. Briefly, the composite aerogels are obtained by a successive two-step strategy including the hydrothermal reaction and subsequent freeze-drying process. The preparation process and the resultant aerogels exhibit significant advantages as outlined below: (1) high efficiencies in terms of cost, time and scale; (2) the simple and environmentally friendly preparation only using water as the solvent, while PVA is biocompatible and biodegradable polymer; (3) the morphology and performance of the composite aerogels are tunable by adjusting the volume ratio of both precursor solutions of GO to PVA, as described in the following sections. Due to the abundant functionalities of GO, such as carboxylic, hydroxyl, epoxy, and carbonyl groups, as well as the hydroxyl groups of PVA, the chemical crosslink and self-crosslink actions can be involved in the self-assembly process,23, 28 thusly the resulting 3DG-PVA aerogels are of high porosity and porous volume, and the related parameters are summarized in Table 1. Apparently, the high PVA content contributes to the mild decrease of porosity but notable increase of density of composite aerogels. Furthermore, the SSA gradually increases and then sharply decreases with raising the content of PVA. The maximum SSA value of 82.9 m2 g-1 for the G-3 composite aerogel is greatly higher than 29.9 m2 g-1 for the 3DG, attributing to the fact that the PVA, as inflating agent, expands the pore volume during the self-assembly process,24 which also can be verified by the optical photographs of 3DG and G-3 composite hydrogels (Fig. S1 in the Supporting Information, SI). 8


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Table 1. The porosity, SSA and density of the 3DG and G-x composite aerogels Samples

3DG

G-1

G-2

G-3

Porosity (%)

99.2

99.4

99.0

98.6

Density (mg cm-3)

18.0

13.0

20.8

29.9

SSA (m2 g-1)

29.9

55.5

64.8

Porous volume (cm3 g-1)

0.0488

0.2425

0.2430

G-4

G-5

G-6

96.6

95.9

51. 7

73. 9

88.5

82.9

43.4

14.9

6.5

0.3774

0.1118

0.0130

0.0374

97.6

3.2. Morphology and structure of the 3DG-PVA composite aerogels SEM observation was performed to investigate the influence of PVA content on the macroporous structure of the composite aerogels, as shown in Fig. 1, which indicates that the morphology of aerogels strongly depends on the value of rG/P. Compared with pure 3DG (Fig. S2a, SI), G-1 and G-2 aerogels with small amounts of PVA display the abundant hierarchical network structure with pore size within 50 μm (Fig. 1a-b). As PVA content increases, the G-3 exhibits compact network structure with the uniformly distributed pores of about 8 μm (Fig. 1c), but the G-4 presents the layered and stacked structure (Fig. 1d). Further improving content of PVA can result in the completely disordered morphology, and the large number of dendritic arborizations of PVA distribute within graphene sheets, as presented in Fig. 1e and 1f, demonstrating that the self-crosslinking of PVA molecules appears in the assembly process of composite with a high PVA content. For comparison, the SEM image of the self-assembly PVA aerogel is exhibited in Fig. S2b, SI, which possesses the 3D 9



network structure consisted of abundant dendritic arborizations.

Fig. 1. SEM images of (a) G-1, (b) G-2, (c) G-3, (d) G-4, (e) G-5, and (f) G-6, respectively. Insets represent the high-resolution SEM images of G-3, G-4, G-5, and G-6.

The TEM observation shows the 3DG possesses a typical wrinkled morphology (Fig. 2a). With improving the PVA content, the wrinkles decrease and even disappear for the composites along with the thickness increase of rGO nanosheets, as shown in Fig. 2b-2d. The possible reasons can be outlined as below: (1) the strong interactions 10


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between GO sheets and PVA chains prevent the aggregation of rGO nanosheets during the assembly.29 (2) the weak interactions between rGO nanosheets can also decrease its aggregation, because the sufficient PVA chains are wrapped on both sides of rGO nanosheets, thusly greatly reducing the contact area between GO and PVA.

Fig. 2. TEM images of (a) 3DG, (b) G-1, (c) G-3, and (d) G-5.

To verify the reaction mechanism between GO and PVA or among PVA molecules, various characterizations are performed and the results are shown in Fig. 3 and Fig. S3, SI. In Fig. 3a, a typical XRD peak of GO is observed at about 11.2o (7.76 Å) while that of 3DG appears at 24.8o (3.73 Å),30, 31 indicating GO sheets are reduced upon the hydrothermal reaction. For 3DG-PVA, the characteristic peak of graphene shifts to a smaller angle of 22.9o (3.86 Å) owing to the strong interaction between GO and PVA;23 moreover, the peak intensity becomes weak or even disappears, revealing the absence of direct graphene-graphene packing mode in these composites due to the graphene sheets wrapped by PVA layers. Inversely, another broad peak resulting from 11



PVA is clearly observed at 17.5o corresponding to its (101) crystal plane;30 and the intensity of the peak gradually becomes strong as rG/P >5:1, signifying the increase of interaction among PVA molecules. Such phenomenon is caused by the gradually intensive interactions of PVA chains.23

Fig. 3. (a) XRD patterns of GO, 3DG, PVA, and a series of composites. (b) FTIR spectra of 3DG, G-3, and PVA.

In Fig. 3b, the FTIR characteristic peak of the synthesized PVA locating at 1074 cm-1 can be attributed to the formation of the ether chemical bonds caused by the self-crosslink reaction of PVA chains;28, 32 Moreover, this bond also appears in G-3 sample and other composites (see Fig. S3a, SI), which is only assigned to the interaction between GO sheets and PVA chains because of the absence PVA self-crosslinking products with dendritic arborizations, as shown in SEM photograph of G-3 (see Fig. 1c). Besides, compared with PVA, the peak intensity of O-H stretching in G-3 decreases, suggesting the dehydration also occurs between PVA chains and GO sheets. The Raman characterization result for composites and 3DG is also provided in Fig. S3b, SI. 12


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3.3. Performances of the 3DG-PVA composite aerogels Since PVA content can significantly impact on the crosslinking points between GO sheets and PVA chains, the mechanical properties of the composite aerogels also vary with the content of PVA. Apparently, 3DG aerogel is fragile and possesses the maximal stress of 13.8 kPa at 50% strain (Fig. S4a, SI), the addition of PVA can significantly enhance the strength and the flexibility of 3DG aerogels (Fig. 4 and Fig. S4, SI). Moreover, with the increase of PVA content or density of the composite aerogels, the strength of the aerogels increases firstly and then decreases gradually (Fig. 4e). Among them, the supreme stresses of G-3 and G-4 at 50% strain amplitude can be respectively increased to 21 and 24 times compared with that of 3DG, as shown in Fig. 4b and 4c. As expected, the plasticity of the aerogels has an apparent improvement with increasing the PVA content, ascribing to the good flexibility of PVA aerogel, as exhibited in Fig. S4b, SI and Movie 1, SI. However, the instantaneous sensing response (△I/I0), where △I represents for the value of (I-I0), while I0 and I are the initial current and real-time current, respectively, under dynamic pressures for composite aerogels reduces with improving the PVA content or the density of aerogels (see Fig. 4f and S5, SI), attributing to the fact that PVA as a spacer can be used to regulate the performance of devices.33 As observed in the SEM and TEM images, the more content of PVA is, the denser network structure of composites owns, thusly leading to the stronger isolation effect. Clearly, only the composite with the proper density can realize excellent electromechanical properties; therefore, the optimal sample of G-3 is chosen for the subsequent performance tests. 13



Fig. 4. Cyclic stress-strain curves of (a) G-2, (b) G-3, (c) G-4, and (d) G-5. (e) Stress at 50% strain and (f) the corresponding sensing response (△I/I0) as a function of density for the composite aerogels.

In Fig. 5a, the stress-strain curves of the G-3 aerogel, under loading-unloading up to different strain amplitudes in the range of 0.5%-70%, exhibit the perfect strain memory effects. Moreover, the synchronous △I/I0 values gradually increase with raising the strain amplitudes (Fig. 5b), confirming the device possesses the outstanding sensing performance; namely, it possesses wide strain range of 0.5%-70% and can detect very small deformation of 0.5% strain. Meanwhile, the G-3 based 14


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sensor presents a linearity of about 27% strain (Fig. 5c) and the perfect Ohmic behavior, which are reflected in the linear current-voltage (I-V) curves under a set of strains (Fig. 5d). Furthermore, to evaluate the reliability of the flexible sensor based on the G-3 aerogel with the electrical conductivity of 0.12 S m-1, it is programed to run the long-term testing under a strain of 30% at a velocity of 2 mm min-1. The △I/I0 results from initial to ending stages show the regular, repeatable, and steady electrical signal response over 5000 times of loading-unloading cycling (Fig. 5e), indicating the excellent structural stability of the 3DG-PVA-based sensors for continual use. What’s more, the response time for the sensor remains constant of 63.9 ms under the resolution limit of measuring equipment (Fig. 5f), implying the fast response of the device to the external pressure. As summarized in the Table S1, SI, the response time is lower than the values of 1-50 ms reported in the literatures,34, 35 but faster than those of most recently reported values of 87-332 ms from other pressure sensors.36-38

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Fig. 5. The sensitivity, reliability, durability, and response time of the pressure sensor based on G-3 composite aerogel. (a) The stress-strain curves and (b) the corresponding △I/I0 of the sensor under loading-unloading up to different strain amplitudes in the range of 0.5% - 70%; inset shows the curves tested in the region with compression strains of 0.5% - 9%. (c) Variation of the sensing response (△I/I0) with raising strains. (d) I-V curves under different strains with range of 0% - 70%. (e) Durability and reliability experiment of the pressure sensor during >5000 loading-unloading cycles; the two insets at top show some typical cycles at the initial and ending stages of the testing process, respectively. (f) The response time of the pressure sensor. 16


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3.4. Applications of the 3DG-PVA pressure sensors The high sensitivity and excellent stability, as well as fast response time imply the constructed stress sensors based on the conductive 3DG-PVA aerogel can also be applied to detect electrical signals resulting from the motions of human joints. For demonstration, the fabricated wearable sensors with the thickness of about 2 mm are adhered to the different parts of human body using tape. The sensitivity of stress sensor is defined as S = (△I/I0)/P, where P is the applied pressure. As shown in Fig. 6a, the device based on G-3 composite aerogel presents the sensitivities of S1 = 0.34 kPa-1 within 5 kPa stress while S2 = 0.17 kPa-1 for stress exceeding 5 kPa, respectively, which are much higher than those of most of the previous works 21, 33, 39 (see Table S2, SI). For practical applications, the electrical signal resulting from the different finger bending angels can be clearly recorded and distinguished (in Fig. 6b); that is, with programed bending steps, the △I/I0 shows high consistency with bending angles, ascribing to the conductivity from the confined 3D porous microstructure which facilitates the transports of electrons and ions.40 Similarly, the △I/I0 of stress sensors attached onto the finger, between fingers, opisthenar, and elbow joint with different bending directions, respectively, demonstrate the high sensing reliability and ultrasensitive performance (Fig. 6c-f). Note that no red and swollen, or damage of skin, and allergic reactions are observed for the volunteers during the application testing, strongly confirming the biocompatibility of the 3DG-PVA composite aerogel sensors. These results indicate that our flexible composite aerogels as wearable pressure/strain sensors will cause great concern in developing the health diagnoses 17



and monitoring human motions.

Fig. 6. The wearable pressure sensors assembled from the G-3 composite aerogel for monitoring human motions. (a) The sensitivity of the pressure sensor with pressure in the range of 0 27.5 kPa. (b) The △I/I0 of the sensor adhered onto the finger with different bending angels. (c-f) The △I/I0 of the sensor attached to the finger, between fingers, opisthenar, elbow joint bending with different orientations, respectively. The insets illustrate the relative photographs of the sensors fixed on human body at different parts.

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Obviously, the excellent performances can be attributable to the porous microstructure of composite aerogel as well as the synergistic effects of graphene and PVA. Herein, the porous structure can improve the piezoresistive sensitivity, because the sufficient contact sites or conductive paths between graphene sheets can be created during the loading. Moreover, PVA can endow the composite aerogel with the good flexibility, leading to remarkable sensing reliability. Besides, the chemical ether bonding and covalent bonding actions between GO and PVA, as well as the self-crosslinking of PVA molecules can enhance the strength and structural stability of 3DG aerogel.29 4. Conclusions In summary, we fabricated 3DG-PVA composite aerogels with tailorable 3D architectures and performances via a facile and scalable method. The obtained aerogels present a high porosity, and a sequence of multifunctional performances, including robust structure, large elastic deformation, high strength, satisfactory electrical conductivity, and outstanding eletromechanical performances. Besides, the wearable sensors designed and fabricated by using the prepared aerogels as the sensing part exhibit anisotropic sensing capabilities, high sensing reliability, and ultrasensitive performance, which can find practical applications in monitoring motions of human joints with different orientations. It is believed that this kind of active material represents a promising candidate for flexible eletronics.

Supproting Information. 19



1. Video of the elasticity of the PVA (avi). 2. Optical photographs of G-3 composite and 3DG hydrogels; SEM photographs of 3DG and PVA; FTIR spectra of 3DG-PVA composite aerogels and Raman spectra of 3DG and G-3; Cyclic stress-strain curves of 3DG, PVA, G-1, and G-6; The sensing response (△I/I0) of the composite aerogels under dynamic loading-unloading test at 50% strain; Comparison of sensing response time for different pressure sensors; Comparison of sensitivity for different pressure sensors.

Conflicts of interest There are no conflicts to declare.

Acknowledgements This work has been supported by the National Natural Science Foundation of China (Grant Nos. 51502306, 51575507, and 51675514).

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Table of Contents Graphic

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Fig. 1. SEM images of (a) G-1, (b) G-2, (c) G-3, (d) G-4, (e) G-5, and (f) G-6, respectively. Insets represent the high-resolution SEM images of G-3, G-4, G-5, and G-6.


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Fig. 2. TEM images of (a) 3DG, (b) G-1, (c) G-3, and (d) G-5.



Fig. 3. (a) XRD patterns of GO, 3DG, PVA, and a series of composites. (b) FTIR spectra of 3DG, G-3, and PVA.


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Fig. 4. Cyclic stress-strain curves of (a) G-2, (b) G-3, (c) G-4, and (d) G-5. (e) Stress at 50% strain and (f) the corresponding sensing response (△I/I0) as a function of density for the composite aerogels.



Fig. 5. The sensitivity, reliability, durability, and response time of the pressure sensor based on G-3 composite aerogel. (a) The stress-strain curves and (b) the corresponding △I/I0 of the sensor under loading-unloading up to different strain amplitudes in the range of 0.5% - 70%; inset shows the curves tested in the region with compression strains of 0.5% - 9%. (c) Variation of the sensing response (△I/I0) with raising strains. (d) I-V curves under different strains with range of 0% - 70%. (e) Durability and reliability experiment of the pressure sensor during >5000 loading-unloading cycles; the two insets at top show some typical cycles at the initial and ending stages of the testing process, respectively. (f) The response time of the pressure sensor.


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Fig. 6. The wearable pressure sensors assembled from the G-3 composite aerogel for monitoring human motions. (a) The sensitivity of the pressure sensor with pressure in the range of 0 27.5 kPa. (b) The △I/I0 of the sensor adhered onto the finger with different bending angels. (c-f) The △I/I0 of the sensor attached to the finger, between fingers, opisthenar, elbow joint bending with different orientations, respectively. The insets illustrate the relative photographs of the sensors fixed on human body at different parts.


Poly(vinyl alcohol)-Mediated Graphene Aerogels with Tailorable

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