Multifunctional GlycerolâWater Hydrogel for Biomimetic Human Skin

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Functional Nanostructured Materials (including low-D carbon)

Multifunctional Glycerol-Water Hydrogel for Biomimetic Human Skin with Resistance Memory Function Yuanmeng Xia, Yuanpeng Wu, Tian Yu, Shishan Xue, meiling guo, Jing-Liang Li, and Zhenyu Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 28, 2019

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ACS Applied Materials & Interfaces

Multifunctional Glycerol-Water Hydrogel for Biomimetic Human Skin with Resistance Memory Function Yuanmeng Xia,a Yuanpeng Wu,*a Tian Yu,b Shishan Xue,a Meiling Guo,a Jingliang Li,*c and Zhenyu Li*ac a. School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China; State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China. b. College of Physical Science and Technology, Sichuan University, Chengdu 610064, China c. Deakin University, Institute for Frontier Materials, VIC 3220, Australia.

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ABSTRACT Biomimetic human skin-like materials with preferably self-healable ability, high sensitivity for external stimuli, and good adhesiveness against diverse substrates under a wide range of temperature, are of great importance in various applications such as wearable devices, human-motion devices, and soft robotics. However, most of the reported biomimetic human skin-like materials lack the memory function, i.e., they cannot memorize the external stimuli once the stimuli disappear. This drawback hinders their applications in mimicking the human skin in real-world. Here a polyacrylamide/Au@polydopamine glycerol-water (GW) hydrogel has been designed to address this challenge. The as-prepared GW-hydrogel exhibits fast self-healable efficiency and good adhesiveness against diverse substrates under a wide range of temperature (from -15 oC to 37 oC). Additionally, our GW-hydrogel also possesses good perceived ability for external stimuli and subtle/large human motions. Most importantly, resistance memory function has been realized based on our GW-hydrogel. These outstanding properties make it potentially significant in mimicking the human skin in real-world. KEYWORDS ：Anti-freezing, Self-healable ability, Bioinspired conductive hydrogel, Human-motion sensor, Resistance memory performance.


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1. INTRODUCTION Biocompatible,1-6 wearable,7-11 self-healable,12-13 and adhesive14-15 human skin-like materials have gained special attention, driven by their potential applications in artificial intelligence,16 human-machine interface,17-18 soft robotics,19-20 and personal healthcare.21 In general, devices based on those materials can transduce diverse external stimuli into detectable electronic signals to mimic the flexible and sensory performances of human skin.22-24 Among the diverse wearable devices, devices based on conductive hydrogels are of particular interest due to their suitable mechanical properties, good stretchability, and excellent biocompatibility.12, 25-27

For example, Huang and Dong fabricated a biocompatible and highly

stretchable pressure device based on a conductive hydrogel with a binary network of polyvinyl alcohol (PVA) and polyacrylamide (PAM).28 The hydrogel, with potassium chloride as a conductor, could precisely monitor dynamic pressure with frequency-dependent behaviour and durable stability. Recently, for practical applications, self-healable capacity, which can extend the lifetime and enhance the durability of hydrogel, has been widely explored in hydrogel based devices.29-30 For instance, Wan et al. reported highly stretchable hybrid hydrogels with good self-healable ability based on cellulose nanocrystals and ferric ion.31 The hydrogels could self-heal at room temperature, without any extra requirement for the ionic coordination between cellulose nanocrystals and ferric ions. Although many successes have been achieved with conductive hydrogels, extra binding tapes are usually used during the practical applications due to the poor adhesiveness of the 3 ACS Paragon Plus Environment


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hydrogels. Thus interfacial delamination and friction between the hydrogels and contacted substrates are hard to be eliminated,26 resulting in interference to signals during the detection. Inspired by the strong adhesiveness and self-healable ability of catechol groups in mussel protein, polydopamine (PDA) has been used in fabricating biomimetic, adhesive and conductive hydrogels.22,

26, 32-34

However, those designed conductive

hydrogels are usually limited by the low freezing temperature of water, making them not applicable at sub-zero temperatures. Thus, to fabricate a device that mimics the human skin, a hydrogel shall have several unique features, which include (i) good self-healable efficiency under a wide range of temperature from subzero to normal human body temperature; (ii) sufficient adhesive capacity against diverse organic or inorganic substrates; (iii) steady and linear perceived ability for external stimuli and subtle/large human motions. Besides those unique features, another important role of human skin is to transduce external stimuli to neuron and memorize them even if the external stimuli disappear.35 However, to our best knowledge, such a memory function of human skin, rich in biological systems, has never been achieved in hydrogel-based human skin-like devices. Herein, we present a facile route for creating bioinspired and conductive polyacrylamide /Au@polydopamine glycerol-water (PAM/Au@PDA GW) hydrogel to address the challenges mentioned above. Under a wide temperature range (from -15 oC

to 37 oC), the as-prepared GW hydrogel exhibits good stretchability (up to 600%) 4 ACS Paragon Plus Environment

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and self-healable capacity, as well as strong adhesiveness to diverse substrates. Furthermore, the hydrogel also shows linear sensitivity for physiological signals from subtle human motions and repeated large strains at 37 oC (human body) till broken. Surprisingly, resistance memory function has been also realized by our GW-hydrogel. 2. EXPERIMENTAL SECTION 2.1. Materials Acrylamide (AM), potassium persulfate (KPS), N,N-methylenebis(acrylamide) (BIS), sodium hydroxide (NaOH), glycerol, and dopamine hydrochloride (DA) were purchased

from

Sigma-Aldrich.

Tetrachloroauric

(III)

acid

tetrahydrate

(HAuCl4•4H2O) was purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2. Synthesis of hydrogels Typically, 0.8 mL of dopamine solution (10 mg/mL) was added into 4 mL of HAuCl4 solution under stirring. Then, 0.8 mL of NaOH solution (40 mg/mL) was injected into the solution quickly and the solution turned red fast. To obtain PDA functionalized Au nanoparticles (Au@PDA), the solution was kept stirring for about 30 minutes without heating. Secondly, a certain amount of AM monomer, K2S2O8, 0.8 mL of BIS solution (5 mg/mL) and 2 mL of glycerol were mixed with the Au@PDA solution for 5 min. The total volume of the solution was kept at 10 mL. Finally, the mixture was reacted at 65 oC for 6 h to polymerize. The compositions of the 5 ACS Paragon Plus Environment


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hydrogels were listed in Table S1, and the hydrogels without Au nanoparticles were also prepared as controls. 2.3. Characterizations The PDA-functionalized Au nanoparticles were characterized by a transmission electron microscope (TEM, Hitachi H600) and a UV-vis spectrophotometer (Shimadzu UV-2505). The size of the Au nanoparticles were characterized on the basis of 200 particles from different areas of the TEM picture with a Nano Measurer 1.2 software. Amperometric I-t curve was used to identify the movement of human finger by an electrochemical workstation (CHI660D, China). The structures of the bulk PAM/PDA GW and PAM/Au@PDA GW-hydrogel were examined by using a scanning electron microscope (SEM, Carl ZEISS AG, ZEISS EV0 MA15). Before examination, the hydrogels were treated with supercritical carbon dioxide and then were freeze-dried. The lowest temperature for anti-freezing properties of the PAM/Au@PDA GW- hydrogel was evaluated by a differential scanning calorimeter (TA Instruments, DSC Q20). The sample was equilibrated at 25 °C and then cooled at a rate of 5 °C min-1 to -50 °C. 2.4. The dynamic oscillatory frequency sweep test A rheometer (HAAKE MARS Ⅲ ) was used to study the performance of the prepared hydrogels. Frequency sweep measurements (0.1 to 100 rad s-1) were


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performed at a shear stress τ0 (0.1 Pa). All the samples were 11 mm in diameter and 6 mm in height. The gap was 1.0 mm and the temperature was set at 37 °C. 2.5. Mechanical property tests The prepared hydrogels were cut into strips for tensile test .The test were carried out with an INSTRON (5965: Instron, Norwood, MA, USA) at a strain speed of 50 mm/min with a 200 N load cell. To determine the dependence of resistance (R) of the conductive hydrogels on the strain, the resistance was measured with a digital multimeter (KEITHLEY 2000 multimeter). The nominal stress (σ) was calculated by the Equation 1 as below: σ = F/A

(1)

where F is the tensile load and A is the cross-sectional area. 2.6. Adhesion properties Our hydrogels were adhered to different materials, including glass, plastic, leaf and the porcine skin for tensile-adhesion test by a universal mechanical testing machine. We test our hydrogels with a crosshead speed of 5 mm/min with the bond area of 95.06 mm2. The adhesion strength was calculated by the measured maximum strength divided by the bonded area. 2.7. Self-healing properties



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The self-healing ability of the hydrogel was performed at different temperatures, one was at 37 oC and the other was at -15 oC. The hydrogels were cut into two pieces and the broken pieces were brought into contact. Particularly, the samples placed in a freezer (-15 oC) were tested immediately after they were taken out from the freezer. For the electrical self-healing ability, the samples were cut into two pieces and the fractured surfaces were brought into contact immediately. We used an electrical circuit of LED in series with our hydrogels to visually observe the electrical self-healing performance. 2.8. High stretch vs conductivity We measured the high strain-dependent conductivity of our hydrogels by recording their resistance when they were subjected to cyclic tension, the resistance ratio (ΔR/R0) was calculated by following Equation 2: ΔR/R0 = (R-R0)/R0

(2)

R0 and R were the original resistance and the resistance under different tensile strain, respectively. 2.9. Negative differential resistance and resistance memory test. The prepared conductive hydrogels with different thickness (1.7 mm, 3.2 mm) and 11 mm diameter were subjected to the cyclic voltammetry curve test (C-V) by Keithley semiconductor parameter analyzer (model 4200-SCS). The Write–read–erase cycle test was measured by an electrochemical workstation (CHI660D, China). 8 ACS Paragon Plus Environment

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3. RESULTS AND DISCUSSION The PAM/Au@PDA GW-hydrogel was prepared through the typical free radical polymerization, in which in situ synthesis of Au nanoparticles and in situ polymerization of AM were combined as shown schematically in Figure 1a Firstly, 0.8 mL of dopamine solution (10 mg/mL) was added into 4 mL of HAuCl4 solution (0.1944 mmol of Au3+) under stirring to form dopamine-liganded Au3+. Secondly, 0.8 mL of NaOH solution (40 mg/mL) was injected into the solution quickly to form PDA functionalized Au nanoparticles.36 Thirdly, 1.92 g of AM monomer was loaded into the solution, followed by adding oxidizer (potassium persulfate: KPS), 0.8 mL (5 mg/mL) of crosslinker (N,N-methylenebis (acrylamide): BIS) and 2 mL of anti-freezing agent glycerol. Finally, the mixture was heated for polymerization. The Au@PDA NPs were characterized by a UV-vis spectrometer and transmission electron microscopy (TEM) as shown in Figure 1b and c, respectively. A characteristic absorption peak at ~515 nm can be clearly detected due to surface plasmon resonance of the Au nanoparticles.37 From the TEM image (Figure 1c), a large quantity of Au@PDA NPs with spherical morphologies can be clearly found. The average diameter of the Au nanoparticles, calculated from the TEM image, is 5.6 nm (Figure 1d). Hydrogels formed without Au nanoparticles were also prepared as control.



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Figure 1. (a) Schematic illustration of the fabrication of PAM/Au@PDA GW-hydrogel, b), c) are UV-vis absorption spectrum, TEM image of Au@PDA nanoparticles, the inset of b) is a dispersion of Au@PDA nanoparticles. d) size distribution of AuNPs inside Au@PDA nanocomposites. The morphologies of freeze-dried PAM/PDA GW and PAM/Au@PDA GW-hydrogels were characterized by scanning electron microscopy (SEM) as shown in Figure 2. 3D interconnected porous network microstructures could be clearly observed in the PAM/PDA GW-hydrogel (Figure 2a and b). Compared with PAM/PDA GW-hydrogel, a hierarchical pore structure, consisting of large (first-level) pores around several micrometers in diameter (Figure 2 c) and small (second-level) pores inside the large pores, can be found within the PAM/Au@PDA GW-hydrogel (Figure 2d). Such hierarchical porous structure could provide both large open channels between the branches and nanoscale porosities within the structure,


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facilitating the transportation of water molecules.38 What is more, we also observed some interwoven microfibrils (Figure 2e) within the PAM/Au@PDA GW-hydrogel, due to the intermolecular interactions between PDA and PAM.26

Figure 2. a) SEM images of the freeze-dried PAM/PDA GW-hydrogel (a and b) and PAM/Au@PDA GW-hydrogel (c and d) with low and high magnifications, respectively. e) microfibril structures in the freeze-dried PAM/Au@PDA GW-hydrogel. Figure 3a displays the total reflection -Fourier transform infrared spectrum (ATR-FTIR) of PAM/Au@PDA GW-hydrogel. As shown in ATR-FTIR spectra, the PAM/Au@PDA GW-hydrogel exhibited a broad peak whose maximum is centered at 3295 cm-1 bands. The broad feature of this peak was due to the fundamental stretching vibration of the OH group and N-H which existed in PAM/Au@PDA GW-hydrogel. The peak at 1664 cm-1 was for C=O stretching and the band at 1620 cm-1 was for N-H deformation of primary amine. 1453 cm-1 ( CH2 in-plane scissoring), 1422 cm-1 (C-N stretching for primary amide), 1351 cm-1 (C-H deformation) were also detected. Enabled by the catechol groups from PDA, our hydrogel exhibited 11 ACS Paragon Plus Environment


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excellent binding of diverse substrates, such as leaf to glass (Figure 3b-i), plastic to plastic (Figure 3b-ii), glass to glass (Figure 3b-iii), glass to skin (Figure 3b-iv). Taking the anti-freezing performance of glycerol, good adhesive capacity of our hydrogels could be maintained under a wide temperature range (from -15 oC to 37 oC). To further evaluate the anti-freezing properties of our PAM/Au@PDA GW-hydrogel, DSC was used to characterize the GW-hydrogel, a sharp peak at -25.59 oC (Figure S1; Supporting Information) confirms that the lowest temperature of PAM/Au@PDA GW-hydrogel is -25.59 oC. Inspired by works of Yuk15 and Lu32, the adhesive strengths of the gel against diverse substrates under different temperatures have been measured, with an angle of 0 degree between the peeling force and the plane of the adhesion area. The results in Figure 3c show that the adhesive strengths at the different temperatures are all larger than 10 KPa, indicating our hydrogel can be directly attached onto skin and human organs for personal healthcare diagnosis even under subzero degree. The original data on stress versus adhesion strain can be found in Figure S2 (Supporting Information).


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Figure 3. a) ATR-FTIR spectrum of PAM/Au@PDA GW-hydrogel. b) Adhesiveness to diverse substrates at both original state and under stretching: (i) leaf to glass, (ii) plastic to plastic, (iii) glass to glass, and (iv) glass to skin. c) Comparisons of adhesive strength of PAM/Au@PDA GW-hydrogel under -15 oC and 37 oC, respectively. Rheometry was employed to gain insight into the properties of the hydrogels at 37 oC. Frequency dependent oscillatory rheology indicated that both the storage modulus (G′) and loss modulus (G′′) increased with frequency (Figure S3; Supporting Information). The mechanical performances of the GW-hydrogels were measured at -15 oC and 37 oC, respectively, as shown in Figure S4 (Supporting Information) and the results are summarized in Figure 4. Compared with the PAM GW-hydrogel, the tensile strength of PAM/PDA GW-hydrogel reduced from 73 + 5



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KPa to 23 + 5 KPa (Figure 4a). Meanwhile, the elongation increased from 500 + 50% to 900 + 50% (Figure 4b), indicating the addition of PDA could enhance the viscosity but reduce the elasticity of the GW-hydrogel.39 When AuNPs were introduced into the GW-hydrogel, the tensile strength was increased to 53 + 5 KPa and elongation was reduced to 600 + 40%. This could be due to the adsorption of PDA chains on surface of AuNPs that reduces the interactions between PDA and PAM.38,

40

Interestingly, we also find that their mechanical performances can be maintained at -15 oC, proving the robust mechanical stability under a broad range of temperature.

Figure 4. Tensile strength (a) and elongation at break (b) of PAM GW-hydrogel, PAM/PDA GW-hydrogel and PAM/Au@PDA GW-hydrogel at different temperatures, respectively. The self-healable capacities of PAM/Au@PDA GW-hydrogel at 37 oC and -15 oC

were evaluated by connecting two pieces of the hydrogels with a LED indicator

and checking whether the LED indicator can be lighted in the circuit as shown in 14 ACS Paragon Plus Environment

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Figure 5. To this end, the LED bulb was firstly lighted (Figure 5a-i), and then switched off by cleaving the hydrogel with a knife (Figure 5a-ii). As the two pieces of the hydrogel was put together, the LED bulb was lighten again immediately (Figure 5a-iii). To further verify the self-healable ability of the hydrogel, we stretched the final hydrogel and found the LED bulb was still on, after the two furcated hydrogel parts had been brought into contact for 40 s. Even with an elongation of 100%, no rupture happened, directly confirming that robust cross-linking reformed between the interfaces of the two pieces of hydrogels (Figure 5a-iv). Even at a subzero temperature (-15 oC), rapid self-healing also occurred, as shown in Figure 5b (i to iii), indicating the excellent self-healable ability of the hydrogel even at a low temperature (Figure 5b-iv). Besides that, we also measured the mechanical properties of PAM/Au@PDA GW-hydrogel before and after self-healing at 37 oC (Figure S5; Supporting Information).

Such an excellent self-healable ability lies in

the water-locking effect by glycerol. The PAM/Au@PDA GW-hydrogel exhibits excellent anti-freezing ability since glycerol could form hydrogen bonds with H2O to inhibit ice formation.41 Furthermore, the glycerol-water binary solvent can also provide enough noncovalent interactions with PAM and PDA chains.35 To further prove our hypothesis, water and glycerol-water (with the same content of glycerol to water within GW-hydrogel) solution were stored at -15 oC for 3 d. Figure 5c shows that water had been totally frozen while the glycerol-water solution was still liquid.



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Figure 5. The electrical recovery of PAM/Au@PDA GW-hydrogel at (a) 37 oC and (b) -15 oC. The photos of original hydrogels (a-i and b-i), cut hydrogels (a-ii and b-ii), broken hydrogel pieces in contact immediately (a-iii and b-iii), and hydrogels self-healed for 40 s under stretching (a-iv and b-iv); (c) photos of water and glycerol-water solution stored at -15 oC for 3 d. For practical applications, a linear and detectable variation of electrical signals (sensitivity) against full-range human motions (from subtle to large human motions/external stimuli) is very important. To investigate such a potential, we firstly tested the ability of the PAM/Au@PDA GW-hydrogel based skin sensor against real-time and subtle human motion by attaching the hydrogel to a finger at 37 oC. The sensing sensitivity was defined as S (%) = (I - I0)/I0 × 100% = ΔI/I0 × 100%, 16 ACS Paragon Plus Environment

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where I0 and I are the original current and the current under different finger bending angles, respectively. Figure 6a, b and c display the current variation as a function of bending angle of the PAM/Au@PDA GW-hydrogel slab (19.27 mm length × 9.88 mm width × 2.81 mm thickness). The regular “wave” shape of the curve corresponds to the bending process of the index finger and the magnitude of current variation was related to the degree of finger bending/recovery. As the bending angle was increased from 30° to 50° and to 90°, the linear and detectable S increases from 30%, to 40%, and to 50%, respectively, due to the variations in the distance between adjacent Au nanoparticles induced by bending. We also found that the reliability and sensitivity of our PAM/Au@PDA GW-hydrogel could be maintained not only after it had been stored for at least 60 days (Figure S6 a-c; Supporting Information) but also after at least 200 repeated uses (Figure S6-d; Supporting Information) without any obvious changes, confirming the high reliability and sensitivity of the sensor for subtle human motions. Such good properties is due to the introduction of glycerol in the hydrogel, which significantly improves the water retaining property and enhances the stability of hydrogels as demonstrated in Figure S7 (Supporting Information). Apart from subtle human motions, the hydrogel could also be used as a good perceived device (diameter: 11 mm, height: 10 mm) against external pressure as shown in Figure 6d. Increasing the pressure (mass of the object on the hydrogel), the adjacent AuNPs were brought closer so that the conductivity of the hydrogel was increased, indicating the good reliability of the sensor. Besides those modest stimuli, our hydrogel could also act as a highly stretchable sensor to detect large deformation (tensile strain 17 ACS Paragon Plus Environment


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changes between 250% and 500%). In this case, the sensing sensitivity is defined as S (%) = (R - R0)/R0 × 100% = ΔR/R0 × 100%, where R0 and R are the original resistance and the resistance under different tensile strains, respectively. From Figure 6e, good linear sensitivity on signal-to-deformation ratios could be clearly detected. After each circle, the resistance recovered to its initial value, confirming the robust sensitivity of this hydrogel. Furthermore, we also found that such linear and detectable variation of electrical signals could be maintained when the hydrogel was under stretch until broken (tensile strain is 600%) as shown in Figure S8 (Supporting Information), surpassing these reported physical devices based on conductive hydrogels

(e.g.,

PANI/P(AAm-co-HEMA),42

rGO-PAA,43

Ppy/PAAm,44

PAA-co-DMAPS,45 M-hydrogel,46 DN hydrogel,47 PIC/Ani.48)(Figure 6f).

Figure 6. Recorded current variations of the PAM/Au@PDA GW-hydrogel slab in response to finger bending with different angles: 30° (a), 50° (b) and 90° (c), respectively. The insets are corresponding photos of the devices in work; (d) current 18 ACS Paragon Plus Environment

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change of the hydrogel vs pressure during cyclic deformation; (e) resistance variation of the hydrogel vs large tensile strain (250% and 500%); (f) comparisons of linear and detectable variation of electrical signals on conductive hydrogel based devices. It’s well known that human skin cannot only detect external stimuli but also transduce the induced signals to brain for memorizing even if the external stimuli disappear. However, devices based on hydrogels suffer the volatility of the hydrogels (namely, lack of sensing−memorizing−integrating capacity), which hinders their potential to mimic the real human skin. Interestingly, resistance memory function has been realized based on our PAM/Au@PDA GW-hydrogel (1.7 mm thickness) by its negative differential resistance (NDR) property (Figure 7a).49 A non-linear close loop with central symmetry is evident within the cyclic voltammetry (CV) curve, indicating that the resistance of the PAM/Au@PDA GW-hydrogel depends on its history of bias voltage applied. More specifically, starting from the origin as we enlarge the amplitude of the bias voltage ǀVǀ, the PAM/Au@PDA GW-hydrogel shows a low resistance (ON state) before ǀVǀ reaches 0.16 V (1 & 4), while a relative high resistance (OFF state) is obtained as we continuously increase ǀVǀ (3 & 6). Under the same bias voltage, the resistance of the OFF state can be ~ 103 times larger than that of the ON state (@ ǀVǀ= 72 mV). Thus the negative differential resistance (dV/dI, NDR), in which current decreases with enlarging amplitude of the bias voltage, appears when the bias voltage is within a range of 0.16 V

Multifunctional GlycerolâWater Hydrogel for Biomimetic Human Skin

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Multifunctional GlycerolâWater Hydrogel for Biomimetic Human Skin