Received: 25 Feb 2019
Revised: 29 Mar 2019
Accepted: 03 Apr 2019
Published online: 09 Apr 2019
Hailong Huang #, Lu Han #, Yangling Wang, Zhongli Yang, Feng Zhu and Min Xu*
School of Physics and Materials Science & Shanghai Key Laboratory of Magnetic Resonance, East China Normal University, No. 3663 North Zhongshan Road, Shanghai 200062, China
# These authors contributed equally to this work
Wearable health monitoring smart systems are considered to be the next generation artificial intelligence devices for real-time tracking down human body motion. However, due to the stress relaxation and viscosity, the existed stretching sensor, especially most ultra-stretching ones, are hard to recover to their original shape after cycles repeatedly stretching, leading to inspection hysteresis and shorter service life. Herein, we reported an intelligent ionic conductive hydrogel (SAMA) as stretching sensor with good mechanical property and controllable thermal-response shape memory property. The obtained hydrogel sensor exhibited outstanding sensitivity to human body motion. Besides, it had a controllable upper critical solution temperature (UCST) from 15 oC to 58 oC through adjusting the concentration of Li+ in the hydrogel. When temperature above UCST, the deformed hydrogel could quickly recover to original shape within 30 seconds and regain sensitive inspection ability. Moreover, the shape memory ability of SAMA hydrogel exhibited a good reuseability at least 50 cycles. The unique stretching hydrogel sensor with controllable thermal-response shape memory could effectively solve the deformation problem of stretching sensor and prolong the service life, demonstrating great potential in the flexible wearable electronics and biologic devices.
Table of Content
An intelligent ionic conductive hydrogel sensor is prepared with outstanding sensitivity to body motion and controllable thermal-response shape memory property.
Stretchable sensor Ionic conductive hydrogel Body motion detection Shape memory Thermal response
During the past decade, flexible electronic devices have earned great achievements and attracted much interests,1-7 especially the stretching sensor which has been greatly developed in wearable electronics devices,8-14 flexible luminescence devices15-18 and soft robotics.19-25 Among different kinds of stretching sensor, ionic conductive hydrogel stretching sensor is considered to be the best materials for the human body motion sensor, due to the good electrochemical performance26-34 and controllable mechanical properties.35-37 Many studies have been focused on the hydrogel stretching sensors. Alshareef38 reported a MXene-based hydrogel (M-hydrogel) exhibited good sensitivity for detecting body motions with excellent elongation of nearly 3400 %. Sui39 mimicked dermis structures to prepared a double network ionic hydrogel with natural polymer sodium alginate nanofibrillar, which shown good flexibility to a broad strain window for the sports monitoring. Wu40 fabricated multiple sensations ionic hydrogel sensor with good sensations towards strain, stress, touch, humidity and temperature. Under the remote control of a NIR laser, the polyionic sensor could gradually transform from bending to stretching and showed good repeatability.
However, there exists a serious shortage limiting the development of stretching sensor as wearable electronic detection, which is the stress relaxation. At present, there are two types of stretching sensors for the human body motion. One is ultra-stretching sensor with low tensile stress and high tensile strain. It cannot recover to the original shape after overstretching. The other is flexible stretching sensor with high tensile stress and low tensile strain. It can recover to original shape only in small strain. Attributed to these problems, the existed stretching hydrogel sensors cannot satisfy the needs of ideal stretching sensor for the human body motion detection. Importantly, the unrecoverable deformation of stretching hydrogel sensor could not make the hydrogel sensor tightly stick on the human body, and then, resulting in response hysteresis and service life degradation. Although lots of researchers work in stretching hydrogel sensor, few work pay attention to solve stress relaxation for the stretchable sensor.
In this work, we designed a series of stretching hydrogel (SAMA) with excellent mechanical property and shape memory ability. Functional monomer and metal ion were introduced into sodium alginate framework to fabricate a double network conductive hydrogel. Due to the goog flexibility and conductivity, the SAMA hydrogel sensor exhibited excellent sensitivity to inspect body motion with different strength, frequency and gesture synchronously. Besides, based on the thermal-response shape memory performance, the hydrogel sensor revealed excellent shape recovery ability. It could easily escape from the unrecoverable deformation and quickly recover to the original shape through thermal-response ability. More importantly, the recovered stretching hydrogel sensor maintained good flexibility and sensitivity with good reuseability. Hence, the SAMA exhibited greatly potential applications in wearable electronics devices with sensitive detection and reversible shape recovery properties.
Sodium alginate was obtained from Aladdin Chemical Co. Acrylic acid (AA), acrylamide (AM), sodium chloride (LiCl), ammonium persulfate (APS) were purchased from Aladdin Chemical Co and used as received.
Preparation of SA/P(AA-AM)/LiCl hydrogels (SAMA)
The SAMA shape memory hydrogels were prepared by free radical polymerization. 1.0 g AM, 2.0 g AA and 5.0 g water were put into 4.5 g SA aqueous solution under stirring at room temperature, until the monomer of AM and AA completely dissolved. Then 0.1 g LiCl and 6 % (wt) APS were added into the mixture and kept stirring for 10 mins. After that, the reaction was carried out at 55 °C for 8 h. The different concentration of LiCl in SAMA hydrogels were prepared as shown in Table 1.
Table 1 Contents of SAMA hydrogels
The chemical structure was measured by Fourier transform infrared spectroscopy (FTIR) spectra on Nicolet-Nexus 670 spectrophotometer at room temperature. XRD data were recorded using X-ray diffractometer (Holland Panalytical PRO PW 3040/60, V = 35 kV, I = 25 mA, λ=1.5418 Å)，in the 2θ range 10-80 °, at a scanning rate of 10 ° min-1. The concentration of Li+ in the SAMA was measured by ICP41 (Varian 720-ES). The microtopography of SAMA hdyrogel was tested using Hitachi S-4800 filed emission scanning electron microspoce. The mechanical property of SAMA hydrogel was measured on Instron 5967 tensile machine. The stress-strain speeds for the compression and stretching were 2 mm/min and 20 mm/min at 25 oC, respectively. The thermal response ability was studied by turbidimetry with UV-Vis spectrometer (from 5 oC to 65 oC). The temperature at 50 % transmittance of thermal transition was taken as the UCST.
Body motion detection
In order to study the performance of SAMA hydrogel sensor,. the SAMA hydrogel were connected with PARSTAT4000A via copper wires to record the relative resistance changes. To monitor the subtle movement, a SAMA-3 hydrogel frame (length × width × thickness is 60 mm × 10 mm × 1 mm) was fabricated and adhered to the index finger. For the detecting human body motion, the same SAMA-3 hydrogel frame was adhered to the elbow and knee. The conductive tape was used to protect the electrodes to prevent unstable contact and electrode motion.
Shape memory property
The helical shaped SAMA hydrogel was prepared to study the deformation and resistance changes during the shape memory process. An as-prepared helical SAMA-3 hydrogel with length of 4.35 cm was stretched repeatedly more than 30 times until unrecoverable deformation occurred. Then, the deformed SAMA-3 hydrogel was put into hot water of 65 oC and the deformation process was recorded. After the SAMA-3 hydrogel recovered to its original shape, and then cooled to 25 oC. The changes of the relative resistance and length were recorded by continuous 50 helical-stretch-helical cycles.
Results and discussion
For the stretching sensor, the unrecoverable deformation always is a serious problem due to the viscoelasticity and stress relaxation, resulting in detection inefficiency and service life degradation. Although many efforts are performed were developed, such as improving the mechanical property, introducing self-healing ability and so on, the unrecoverable deformation for the stretching sensor is not effectively solved. In order to solve the deformation, we fabricated an ionic conductive hydrogel (SAMA) with thermal-response shape memory property. Based on the framework of SA, functional monomers and metal ion were introduced to build a double network structure conductive hydrogel. The microstructure of SAMA hydrogel is shown in Fig. 1A.