Nanochitin and poly(N-isopropylacrylamide) interpenetrating network hydrogels for temperature sensor applications

Preparation and properties of hydrogel photonic crystals assembled by biodegradable nanogels

Thermally induced physical hydrogels formed through the sol-gel transition of nanogels usually lose structural color above phase transition temperature (Tp). Herein, temperature/pH/redox-responsive nanogels that undergo sol-gel transition still keep structural colors above the Tp have been synthesized and studied. N-isopropylacrylamide (NIPAm) was copolymerized with N-tert-butylacrylamide (TBA) and N-acrylamido-l-phenylalanine (Aphe) to form P(NIPAm/TBA/Aphe) nanogel crosslinked with N,N'-bis(acryloyl)cystine (BISS) (referred to as PNTA-BISS). PNTA-BISS nanogel with a broad range of biodegradable crosslinker BISS content can achieve a reversible sol-gel transition above the Tp, surprisingly, while PNTA nanogels with a comparable content of biodegradable N,N'-Bis(acryloyl)cystam (BAC) crosslinker (referred to as PNTA-BAC) didn't form sol-gel transition. Although BISS and BAC possess same disulfide bonds with redox properties, BISS, unlike BAC, is water-soluble and features two carboxyl groups. The mechanism by which PNTA-BISS nanogels form hydrogel photonic crystals has been deeply explored with temperature-variable NMR. The results showed the introduction of Aphe with both steric hindrance and carboxyl groups greatly slowed down the shrinkage of PNTA-BISS nanogels. Therefore, PNTA-BISS nanogels can form sol-gel transition and further structural color of hydrogel photonic crystals due to carboxyl groups above the Tp. Furthermore, the properties of biodegradable hydrogel photonic crystals above the Tp were investigated for the first time, attributed to the presence of the strong reducing agent 1,4-dithiothreitol (DTT). When loaded with doxorubicin (DOX), PNTA-BISS exhibited favorable degradation properties under the influence of DTT. In summary, the PNTA-BISS nanogel, in addition to its in-situ gelation capabilities, demonstrated degradability, potentially providing a novel nanoplatform for applications in drug delivery, biotechnology, and related fields.

Enhancing the mechanical strength of 3D printed GelMA for soft tissue engineering applications

Gelatin methacrylate (GelMA) hydrogels have gained significant traction in diverse tissue engineering applications through the utilization of 3D printing technology. As an artificial hydrogel possessing remarkable processability, GelMA has emerged as a pioneering material in the advancement of tissue engineering due to its exceptional biocompatibility and degradability. The integration of 3D printing technology facilitates the precise arrangement of cells and hydrogel materials, thereby enabling the creation of in vitro models that simulate artificial tissues suitable for transplantation. Consequently, the potential applications of GelMA in tissue engineering are further expanded. In tissue engineering applications, the mechanical properties of GelMA are often modified to overcome the hydrogel material's inherent mechanical strength limitations. This review provides a comprehensive overview of recent advancements in enhancing the mechanical properties of GelMA at the monomer, micron, and nano scales. Additionally, the diverse applications of GelMA in soft tissue engineering via 3D printing are emphasized. Furthermore, the potential opportunities and obstacles that GelMA may encounter in the field of tissue engineering are discussed. It is our contention that through ongoing technological progress, GelMA hydrogels with enhanced mechanical strength can be successfully fabricated, leading to the production of superior biological scaffolds with increased efficacy for tissue engineering purposes.

Preparation of Janus film for fog water collection via layer-by-layer assembling of nanocellulose and nanochitin on PLA

In order to explore the possibility of natural carbohydrate polymers as a biodegradable and sustainable fog water harvesting material, this work proposed an efficient substrate (hydrophobic)-transition layer (amphoteric)-coating (hydrophilic) sandwich spin-coating strategy to form all biomass-based Janus film. The oxalic acid hydrolyzed nanochitin (OAChN) was applied as a transition layer that enabled successful spin-coating of the hydrophilic nanocellulose (TEMPO-oxidized cellulose nanofiber, TOCN) and nanochitin (partially deacetylated chitin nanofibers, DEChN) on the hydrophobic polylactic acid (PLA) film substrate. In which a layer-by-layer (LBL) assembling of TOCN (carboxyl-rich negative surface charge) and DEChN (amino-rich positive surface charge) was designed to form a thickness and surface property controllable polysaccharide coating on PLA. The finally formed PLA-OAChN-TOCN/DEChN (LBL) film showed hydrophilic and hydrophobic heteromeric faces at the opposite sides and thus had improved fog water collection capacity of 90.85 mg·cm-2·h-1 (30 layers of TOCN/DEChN spin-coated on PLA), which was 276 % higher than the pure PLA film. The transition layer engaged sandwich spin-coating strategy, together with LBL assembling method proposed in this study provided a feasible fabrication of all biomass-based fog water collectors (FWC) that could contribute to alleviating water shortage.

Assembling PNIPAM-Capped Gold Nanoparticles in Aqueous Solutions

Employing small-angle X-ray scattering (SAXS), we explore the conditions under which assembly of gold nanoparticles (AuNPs) grafted with the thermosensitive polymer poly(N-isopropylacrylamide) (PNIPAM) emerges. We find that short-range order assembly emerges by combining the addition of electrolytes or polyelectrolytes with raising the temperature of the suspensions above the lower-critical solution temperature (LCST) of PNIPAM. Our results show that the longer the PNIPAM chain is, the better organization in the assembled clusters. Interestingly, without added electrolytes, there is no evidence of AuNPs assembly as a function of temperature, although untethered PNIPAM is known to undergo a coil-to-globule transition above its LCST. This study demonstrates another approach to assembling potential thermosensitive nanostructures for devices by leveraging the unique properties of PNIPAM.

Hydrogel-Based Sensors for Human-Machine Interaction

In the past decades, remarkable progress has been made in the field of human-machine interaction. The need for accurate sensing devices with satisfactory user experiences has propelled the development of flexible, stretchable, biocompatible, and imperceptible hydrogel-based interfaces. These innovative interfaces facilitate direct interactions between humans and machines while receiving detected input signals from sensors and giving output commands to controllers, thus motivating accurate real-time responsiveness. This Perspective discusses the sensing mechanisms for the two categories of hydrogel-based sensors and summarizes the recent progress in the development of different representations of human-machine interactions, including intelligent identification, information secrecy, interactive control, and virtual reality and augmented reality technologies. The advantages of hydrogel-based systems over conventionally used rigid electrical components are explicitly discussed. The conclusion provides a perspective on current challenges and outlines a future roadmap for the realization of state-of-the-art hydrogel-based smart systems.

Hydrogel Micropillar Array for Temperature Sensing in Fluid

Localized temperature sensing and control on a micron-scale have diverse applications in biological systems. We present a micron-sized hydrogel pillar array as potential temperature probes and actuators by exploiting sensitive temperature dependence of their volume change. Soft lithography-based molding processes were presented to fabricate poly N-isopropyl acrylamide (p-NIPAAm)-based hydrogel pillar array on a glass substrate. Au nanorods as light-induced heating elements were embedded within the hydrogel pillars, and near-infrared (NIR) light was used to modulate temperature in a local area. First, static responses of the micro-pillar array were characterized as a function of its temperature. It was shown that the hydrogel had a sensitive volume transition near its low critical solution temperature (LCST). Furthermore, we showed that LCST could be readily adjusted by utilizing copolymerizing with acrylamide (AAM). To demonstrate the feasibility of spatiotemporal temperature mapping and modulation using the presented pillar array, pulsed NIR light was illuminated on a local area of the hydrogel pillar array, and its responses were recorded. Dynamic temperature change in water was mapped based on the abrupt volume change characteristics of the hydrogel pillar, and its potential actuation using NIR light was successfully demonstrated. Considering that the structure can be arrayed in a two-dimensional pixel format with high spatial resolution and high sensitive temperature characteristics, the presented method and the device structure can have diverse applications to change and sense local temperatures in liquid. This is particularly useful in biological systems, where their physiological temperature can be modulated and mapped with high spatial resolution.

Nano-chitin reinforced agarose hydrogels: Effects of nano-chitin addition and acidic gas-phase coagulation

Hydrogels based on natural polymers such as agarose usually show low applicability due to their weak mechanical properties. In this work, we developed a dual cross-linked agarose hydrogel by adding different amounts of TEMPO-oxidized nano-chitin (0-0.2 %) to agarose hydrogel matrices and then physically cross-linked under acidic gas-phase coagulation. The prepared hydrogels were characterized by FTIR, XRD, TGA, and SEM. The effects of nano-chitin addition and acidic gas-phase coagulation on the properties of agarose hydrogels, such as gel strength, swelling degree, rheological properties, and methylene blue (MB) adsorption capacity, were also studied. Structural characterizations confirmed that nano-chitin was successfully introduced into agarose hydrogels. The gel strength, storage modulus, and MB adsorption capacity of agarose hydrogels gradually increased with the increasing nano-chitin addition, whereas the swelling degree decreased. After acidic gas-phase coagulation, agarose/nano-chitin nanocomposite hydrogels exhibited improved gel strength and storage modulus, while the swelling degree and MB adsorption capacity were slightly reduced. The combination of oxidized nano-chitin and acidic gas-phase coagulation is expected to be an effective way to improve the properties of natural polymer hydrogels.

Implantable and in-vivo shape-recoverable nanocellulose-hyaluronic acid composite hydrogel

Hydrogels have been used as a filling material in medical cosmetology, but current injection hydrogels have poor shaping ability due to its fluidity, while the hydrogels with fixed shape are easy to cause large wound size, resulting in rarely used in clinical practice. An implantable and in-vivo shape-recoverable hyaluronic acid (HA) based hydrogel is developed for tissue filling. In this work, complexes were made by hydrogen bonding between two natural polysaccharides: HA and TEMPO-oxidation cellulose nano-fiber. The elastic modulus of the HA/TOCN physical crosslinking hydrogel was maintained at 2500 G' in Pa, while, when ethylene glycol diglycidyl ether was introduced in the hydrogel, the elastic modulus could reach 60,000 G' in Pa. The volume of shrunk hydrogel reduced 80 ± 6 % of initial state, importantly, it can recover the shape in vivo inducing by extracellular moisture environment. Facts have proved that these shape recovery hydrogels were non-toxic to mammalian cells.

Highly conductive thermoresponsive silver nanowire PNIPAM nanocomposite for reversible electrical switch

Highly conductive nanocomposite hydrogels have been challenging to produce due to their high water volumes inhibiting the incorporation of an essential amount of conductive nanofillers. Furthermore, the most common fillers used, typically for easy integration, display small aspect ratios. Thus, the formation of interparticle pathways for electronic travel is limited, resulting in low conductivities. Here, we introduce ultralong silver nanowires (ULAgNWs) into a thermoresponsive, volume changing PNIPAM gel to form a nanocomposite that shows switchable electronic performance. The produced nanocomposite surpasses other PNIPAM nanocomposites by expressing the largest electrical switch ratio and the highest peak conductivity. The PNIPAM matrix possesses an interconnected microporous structure that offers a spacious network for the dispersion of nanowires while still maintaining a high volume switch ratio and excellent elastic behavior under extreme compression cycles (98% compression). The ULAgNWs significantly enhance the probability of more numerous connections forming during shrinking cycles. The high swellability displayed by the PNIPAM gel provides the ability to separate the embedded nanowires by many lengths. Together, they form a nanocomposite that can thermo-modulate its electrical properties. Moreover, the conductive PNIPAM maintains the electrical switch of 4.3-4.4 orders of magnitude with thermo-responsive cycles. Because of their high electrical conductivity and outstanding elastic behavior, these stimuli-responsive nanocomposite hydrogels may expand the prospects for conductive hydrogel applications and provide greater performance in their applications.