"Casein micelle -nanoparticle double cross-linking" triggered stable adhesive, tough CA/MWCNT/PAAm hydrogel wearable strain sensors, for human motion monitoring

A Flexible Leather-Based Sensor via the Dual In Situ Growth of Conductive Materials for Human Motion Detection

Natural leather, with its mechanical strength, flexibility, and wearing comfort, is an ideal substrate material for wearable sensors. Currently, leather-based piezoresistive sensors still suffer from poor uniformity of conductive networks and weak interfacial interactions between conductive materials and leather, which limit their performance. Herein, it was innovatively proposed that polypyrrole (PPy) and silver nanoparticles (AgNPs) were sequentially grown in situ on the surface of collagen fibers (CFs). Then, the tanning process was carried out to produce a leather-based flexible wearable sensor (PPy/AgNPs-LBPS) with excellent conductivity, hydrothermal, and environmental stability. Specifically, the dual in situ growth and tanning process ensured the uniform penetration and distribution of conductive materials in leather substrates. Meanwhile, the hydrogen bonds between the conductive materials and CFs provided a firm combination to prevent the dropping of conductive materials. The synergistic effect of PPy and AgNPs enhanced the sensing performance of PPy/AgNPs-LBPS. It exhibited high sensitivity (0.65 kPa-1 and 3.76), a wide detection range (0-80 kPa and 0-100%), and fast response capability. These characteristics enabled PPy/AgNPs-LBPS to monitor subtle activities and large-scale movements of the human body in real time, as well as tactile perception. This thesis provides a new idea for the intelligent design of traditional leather materials, multidimensional perception in electronic skin, and advancements in artificial intelligence.

A natural gelatin/casein hydrogel with on-demand adhesion via chitosan solution for wound healing

Adhesive hydrogels have been widely studied as wound dressings due to their excellent biocompatibility and biological activity. However, most designed hydrogels still exist limitations including potentially toxic monomer, complex preparation process and non-degradable property. Here, a natural and degradable gelatin/casein hydrogel was prepared by enzymatic cross-linking. The hydrogel could adhere to tissue on-demand with the mediation of chitosan (CS) solution. It was found that the adhesion strength of hydrogel could be controlled by adjusting gelatin/casein ratio, EDC&NHS concentration, CS concentration, glycerol content and crosslinking degree. To further expand the applicability of hydrogels, the degradation and drug release rate of hydrogels could be modulated by changing transglutaminase (TG) concentration. Moreover, tetracycline hydrochloride (TH) was loaded into hydrogel as a drug model, which endowed hydrogel with good antibacterial properties. It was shown that the 0.03 % TH hydrogel had excellent blood compatibility and cell compatibility, and can promote the healing of infected wounds in mice. This research provides a new natural adhesive hydrogel for biomedical engineering field.

Progress in Protein-Based Hydrogels for Flexible Sensors: Insights from Casein

In recent years, the rapid advancement of flexible sensors as the cornerstone of flexible electronics has propelled a flourishing evolution within the realm of flexible electronics. Unlike traditional flexible devices, hydrogel flexible sensors have characteristic advantages such as biocompatibility, adhesion, and adjustable mechanical properties and have similar properties to human skin. Especially, biobased hydrogels have become the preferred substrate material for flexible sensors due to increased environmental pressures caused by the scarcity of petrochemical resources. In this regard, proteins possess advantages such as diverse amino acid compositions, adjustable advanced structures, chemical modifiability, the application of protein engineering techniques, and the ability to respond to various external stimuli. These enable the hydrogels constructed from them to have greater designability, flexibility, and adaptability. As a result, their applications in manufacturing various types of sensors have experienced rapid growth. This work systematically reviews the sensing mechanism of protein-based hydrogels, focusing on the preparation of protein-based hydrogels and the optimization of flexible sensors mainly from the perspective of a typical type of animal-derived protein casein. In addition, while the potential of casein is recognized, the limitations of casein-based hydrogels in flexible sensor applications are explored, and insights are provided into the development trends of next-generation sensors based on casein-based hydrogel materials.

Casein-based hydrogels: Advances and prospects

Casein-based hydrogels (Casein Gels) possess advantageous properties, including mechanical strength, stability, biocompatibility, and even adhesion, conductivity, sensing capabilities, as well as controlled-releasing behavior of drugs. These features are attributed to their gelation methods and functionalization with various polymers. Casein Gels is an important protein-based material in the food industry, in terms of dairy and functional foods, biological and medicine, in terms of carrier for bioactive and sensitive drugs, wound healing, and flexible sensors and wearable devices. Herein, this review aims to highlight the importance of the features mentioned above via a comprehensive investigation of Casein Gels through multiple directions and dimensional applications. Firstly, the composition, structure, and properties of casein, along with the gelation methods employed to create Casein Gels are elaborated, which serves as a foundation for further exploration. Then, the application progresses of Casein Gels in dairy products, functional foods, medicine, flexible sensors and wearable devices, are thoroughly discussed to provide insights into the diverse fields where Casein Gels have shown promise and utility. Lastly, the existing challenges and future research trends are highlighted from an interdisciplinary perspective. We present the latest research advances of Casein Gels and provide references for the development of multifunctional biomass-based hydrogels.

Anti-freezing hydrogel regulated by ice-structuring proteins/cellulose nanofibers system as flexible sensor for winter sports

Conductive hydrogels are widely used as sensors in wearable devices. However, hydrogels cannot endure harsh low-temperature environments. Herein, a new regulatory system based on natural ice-structuring proteins (ISPs) and cellulose nanofibers (CNFs) is introduced into hydrogel network consisting of chemically crosslinked network of copolymerized acrylamide and 2-acrylamide-2-methylpropanesulfonic acid, and physically crosslinked polyvinyl alcohol chains, affording an anti-freezing hydrogel with high conductivity (2.63 S/m). These hydrogels show excellent adhesion behavior to various matrices (including aluminum, glass, pigskin, and plastic). Their mechanical properties are significantly improved with the increase in CNF content (tensile strength of 106.4 kPa, elastic modulus of 133.8 kPa). In addition, ISPs inhibit the growth of ice. This endows the hydrogels with anti-freezing property and allows them to maintain satisfactory mechanical properties, conductivity and sensing properties below zero degrees. Moreover, this hydrogel shows high sensitivity to tensile and compressive deformation (GF = 5.07 at 600-800 % strain). Therefore, it can be utilized to develop strain-type pressure sensors that can be attached directly to human skin for detecting various body motions accurately, reliably, and stably. This study proposes a simple strategy to improve the anti-freezing property of hydrogels, which provides new insights for developing flexible hydrogel electronic devices for application in winter sports.

Conductive and Eco-friendly Biomaterials-based Hydrogels for Noninvasive Epidermal Sensors: A Review

As noninvasive wearable electronic devices, epidermal sensors enable continuous, real-time, and remote monitoring of various human physiological parameters. Conductive biomaterials-based hydrogels as sensor matrix materials have good biocompatibility, biodegradability, and efficient stimulus response capabilities and are widely applied in motion monitoring, healthcare, and human-machine interaction. However, biomass hydrogel-based epidermal sensing devices still need excellent mechanical properties, prolonged stability, multifunctionality, and extensive practicality. Therefore, this paper reviews the common biomass hydrogel materials for epidermal sensing (proteins, polysaccharides, polyphenols, etc.) and the various types of noninvasive sensing devices (strain/pressure sensors, temperature sensors, glucose sensors, electrocardiograms, etc.). Moreover, this review focuses on the strategies of scholars to enhance sensor properties, such as strength, conductivity, stability, adhesion, and self-healing ability. This work will guide the preparation and optimization of high-performance biomaterials-based hydrogel epidermal sensors.