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Researcher Li Zhou's group at the Nano Energy Institute of the Chinese Academy of Sciences has made new progress in the field of conductive hydrogel sensing
2020-01-10 Source: Polymer Technology

Hydrogel is a "water-retaining" material formed by the dispersion of hydrophilic polymers in an aqueous phase environment. Generally, it has excellent biocompatibility, mechanical flexibility, and environmental friendliness. Through various physical and chemical methods, the mechanical properties of the hydrogel, such as flexibility and viscoelasticity, can be enhanced, thereby realizing the application potential of the hydrogel in wearable sensors. Although there are many reports about gel-type mechanical sensors, a series of problems exist at the same time. For example, in order to enhance the stretchability or strength of the gel, scientists have designed and synthesized a variety of complex polymer materials. By introducing special functional chemical groups in the polymer chain to enhance the hydrogen bonding in the hydrogel network, Electrostatic attraction, metal chelation or dynamic covalent interaction. The way to improve the hydrogel performance through molecular design is like a "double-edged sword". On the one hand, the mechanical properties of the gel are significantly improved; however, the production cost and the difficulty of synthesis are accompanied by this, which makes the gel's The difficulty of large-scale industrial production has increased.

How to make engineered hydrogel materials with ideal performance in a more economical way is a problem in multidisciplinary fields. Recently , the research group of Li Zhou, a researcher of the Institute of Nano Energy and Systems, Chinese Academy of Sciences, and collaborators have made new progress in the field of engineering hydrogel preparation and sensing applications. The achievement "internal regulation instead of functional group diversity" changed the key link of gel performance optimization from tedious chemical synthesis to the regulation of the components in the gel, thereby greatly reducing the production cost and process. The researchers first systematically screened the common hydrogel-forming polymer materials (such as chitosan, sodium alginate, and polyvinyl alcohol). They examined the mechanical properties of eight hydrogels and found that Hydrogels of vinyl alcohol (PVA) and polyethylene diamine (PEI) show the best mechanical properties. Based on the PVA / PEI system, the researchers conducted detailed investigation and optimization of the hydrogel. Considering that PVA and PEI show opposite charges, they changed the ratio of PVA and PEI to maximize the electrostatic attraction between molecules. The results show that when the ratio of PVA to PEI changes from 1: 2 to 2: 1, the tensile strain and stress increase by two and six times, respectively.

Figure 1. Conceptual drawing and mechanical properties testing of elastomers based on PVA / PEI hydrogels. a) Conceptual diagram, optical image of molecular structure and hydrogel. b) Stress-strain curves of hydrogels with different ratios (mass) of PVA to PEI. c) Histogram showing elongation at break (%) (I) tensile strength (kPa) (II) and Young's modulus (kPa) (III) of different gels. d) When the elastomer (PVA2PEI1) is stretched to 100%, 200%, 300%, and 400%, there is an uninterrupted sequential extension-contraction cycle curve.

Next, using PVA2PEI1 as a model, the effect of water content on gel performance was investigated. When the water content in the gel changed from 85% to 75%, the tensile stress of the gel changed more than 6 times. In the end, the form of the gel with the best performance was: PVA2PEI1-75. The breaking strain of the gel is as high as 500%, and the corresponding breaking stress is 0.6 MPa.

1)的性能。 Figure 2. Study of the performance of elastomers with different water contents (PVA / PEI mass ratio of 2: 1). a) Conceptual drawing of hydrogels with water content of 75%, 80% and 85%. b) Stress-strain curves and f) Optical images of hydrogels with different water contents. c) The water content of hydrogels in the atmosphere varies with the day. d) Stress-strain curves of hydrogels exposed to air for different days. e) The storage modulus (G '') and loss modulus (G '' '') of the hydrogel (PVA2PEI1-75).

Based on the obtained hydrogel, researchers explore its application in piezoresistive sensors. Researchers have improved the gel's electrical and mechanical properties. The calculated sensitivity factor of the gel is 9-22, which is significantly higher than the reported sensor of the same type. Later, they used the manufactured sensors to monitor the movements of different parts of the human body, including: finger / elbow / knee extension and flexion, raising the corners of the mouth, blinking movements, vocal movements, etc. All results show that the PVA-PEI-based sensor has excellent sensitivity and accuracy. Finally, cell experiments and antibacterial experiments show that the gel has good biocompatibility and antibacterial properties.

Figure 3. Application of a stretchable elastomer in a piezoresistive sensor. i)原始状态; ii)弯曲30; iii)弯曲60; iv)弯曲90。 a) When the fingers are at different bending angles, the resistance change curve: i) the original state; ii) bending 30 ; iii) bending 60 ; iv) bending 90 . b) There is a linear relationship between resistance change and bending angle. i)弯曲; ii)原始状态; iii)弯下腰。 c) Store resistance changes when the wrist is bent: i) bent; ii) original state; iii) bent down. i)原始状态; ii)弯曲状态。 d) Resistance change when the arm is bent: i) original state; ii) bent state. e) The resistance changes when the leg is bent. i)面无表情,ii)微笑。 f) Resistance changes with different facial expressions: i) expressionless, ii) smile. g) Resistance changes when frowning. h) When volunteers say "ni hao" (hello), the relative resistance changes. The right illustration shows a single pulse of i) "ni" and ii) "hao".

This research proposed a new method to optimize the performance of the gel by “complexing and simplifying”, and obtained an elastic hydrogel based on PVA-PEI. Because the gel meets the strict requirements of wearable flexible electronic devices, it will have a wide range of applications in the field of strain gauge pressure sensors in the future.

The above results were recently published in the academic journal Small (Impact Factor: 10.8). The title of the paper is "Customization of Conductive Elastomer Based on PVA / PEI for Stretchable Sensors". Dr. Hu Kuan, National Institute of Quantitative Research, Corresponding author is Researcher Li Zhou .

Paper link: http://onlinelibrary.wiley.com/doi/10.1002/smll.201904758

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