Starch Composite Hydrogels And Their Applications In Flexible Sensing

Wearable sensors convert various external stimuli into electronic signals,which can be used in the fields of health surveillance,disease diagnosis,and artificial intelligence.At present,hydrogels are ideal materials for constructing wearable sensors in consideration of remarkable flexibility and stretchability.However,the majority of hydrogels possess weak mechanical strength,low electrical conductivity,Poor biocompatibility and antibacterial properties,hindering the application of hydrogel-based sensors.In order to improve the conductivity,temperature resistance,water retention,biocompatibility and antibacterial properties of hydrogels,a series of studies were carried out.In view of the existing problems,this paper starts from the selection of biological materials.Based on the structural characteristics of starch and its modified starch,a multi-functional hydrogel wearable sensor with toughness,conductivity,frost resistance,water retention,antibacterial and biocompatibility was developed.The main research is as follows:(1)The simultaneous improvement of mechanical properties and conductivity of ionic conductive hydrogels is contradictory.The improvement of mechanical properties mainly depends on the increase of crosslinking density of hydrogel network,but the increase of crosslinking density will limit the migration of ions in the network.Thus,it is a challenge to simultaneously enhance the mechanical property and conductivity of hydrogels.Herein,a simple strategy was proposed for concurrently enhancing the mechanical property and conductivity of the wearable hydrogel sensors by introducing carboxymethyl starch sodium(CMS).The introduction of CMS not only dramatically enhanced the mechanical performance of the hydrogel due to hydrogen bonding and electrostatic interaction,but also improved the conductivity of the hydrogel owing to the existence of sodium ions.More importantly,the hydrogel displayed fast response,outstanding sensitivity and excellent flexibility,which was assembled into wearable strain sensors for distinguishing and detecting both large-scale and subtle human movement,such as speaking,trampling,and walking.Meanwhile,the hydrogel sensors were assembled into a sensor array for detecting the three-dimensional distribution of stress and strain.Furthermore,the hydrogel sensor could be used to detect EMG signals of the human epidermis and ECG signals of the human heart.The multifunctional hydrogel presented memorably potential for applications in human medical diagnosis,healthy detection and artificial intelligence.(2)However,most hydrogels exhibited poor freezing resistance and weak water-holding ability,thus hindering the working range of hydrogel-based sensors in extreme environments.Here,a conductive hydrogel featuring conspicuous anti-freezing and water retention abilities was prepared by integrating amylose(AMY)into polyvinyl alcohol(PVA)/glycerol/NaCl hydrogel.AMY possessed amounts of hydroxyl groups,which could interact with PVA and glycerin via hydrogen bonds,enhancing the toughness of the hydrogel.Meanwhile,based on the strong hydrogen bonding between glycerol and water molecules,the hydrogel displayed water-retaining property after 7 days of storage in an open environment.The synergistic action of NaCl and glycerol prevented the crystallization of the water at low temperatures,endowing the hydrogel with outstanding conductivity and stretchability at-20 °C.Furthermore,the hydrogel-based wearable strain sensor exhibited excellent sensitivity(GF=2.55),fast response/recovery time,good durability and biocompatibility,which accurately detected joint movements and physiological signals at room temperature or under extreme conditions.(3)The hydrogel sensor needs to stop bacteria growing as it attaches to human skin.However,it is an enormous challenge to fabricate conductive hydrogel sensor with biocompatibility,antibacterial properties,and toughness.Here,a highly conductive hydrogel with excellent toughness,good biocompatibility and strong antibacterial properties was prepared by incorporating acetylated distarch phosphate(ADSP)into the polyvinyl alcohol(PVA)/ Polyhexamethylene biguanide hydrochloride(PHMG).The addition of ADSP not only ionized sodium ions to make the hydrogel conductive,but also contained abundant hydroxyl groups to form hydrogen bonds with PVA to improve the toughness of the hydrogel.Furthermore,the PHMG endowed the hydrogel with antibacterial property to E.coli(Escherichia coli,Gram-negative bacteria)and S.aureus(Staphylococcus aureus,Grampositive bacteria).Meanwhile,the hydrogel was implanted in mice for 14 days,and the surrounding tissue remained in good condition.More importantly,hydrogel sensors can detect different movement signals,such as finger,wrist,knee and other joint movements,breathing patterns,and minor ECG signals.(4)When the hydrogel sensor is attached to the skin for a long time,symptoms such as inflammation and redness will appear.Biocompatibility plays an important role in hydrogel sensors.PGLA hydrogels with high toughness and antifreeze performance were prepared by mixing polyvinyl alcohol,amylopectin,glycerin and lithium chloride in one pot.Because its unique design strategy,it shown high conductivity,toughness and good fatigue resistance.At the same time,the binary solvent composed of glycerol and water gives the hydrogel good water retention and freezing resistance.Based on the design strategy of ionic hydration and hydrogen bond synergism,the tensile fracture strain and stress of PGLA hydrogel gradually increased to 608% and 581 k Pa,respectively.The PGLA hydrogel exhibited good sensitivity(GF= 4.82),faster response time(53 ms)and recovery time(50 ms).The PGLA hydrogel displayed repeatability and remarkable stability with almost no significant electrical signal damage during 70 consecutive stretches at 50% strain.After being stored at room temperature for 7 days,the water retention rate of PGLA hydrogel was up to 66%.In addition,PGLA hydrogels have frost resistance,and PGLA hydrogels at-20 °C also have electrical conductivity.The hydrogel sensor can detect the movement of the human body under large and small strains and convert it into electrical signals.