Thermo-Responsive Hydrogels Coupled with Photothermal Agents for Biomedical Applications

Intelligent hydrogels are materials with abilities to change their chemical nature or physical structure in response to external stimuli showing promising potential in multitudinous applications. Especially, photo-thermo coupled responsive hydrogels that are prepared by encapsulating photothermal agents into thermo-responsive hydrogel matrix exhibit more attractive advantages in biomedical applications owing to their spatiotemporal control and precise therapy. This work summarizes the latest progress of the photo-thermo coupled responsive hydrogel in biomedical applications. Three major elements of the photo-thermo coupled responsive hydrogel, i.e., thermo-responsive hydrogel matrix, photothermal agents, and construction methods are introduced. Furthermore, the recent developments of these hydrogels for biomedical applications are described with some selected examples. Finally, the challenges and future perspectives for photo-thermo coupled responsive hydrogels are outlined.

Phase transition and potential biomedical applications of thermoresponsive compositions based on polysaccharides, proteins and DNA: A review

Smart thermoresponsive polymers have long attracted attention as materials of a great potential for biomedical applications, mainly for drug delivery, tissue engineering and wound dressing, with a special interest to injectable hydrogels. Poly-N-isopropylacrylamide (PNIPAM) is the most important synthetic thermoresponsive polymer due to its physiologically relevant transition temperature. However, the use of unmodified PNIPAM encounters such problems as low biodegradability, low drug loading capacity, slow response to thermal stimuli, and insufficient mechanical robustness. The use of natural polysaccharides and proteins in combinations with PNIPAM, in the form of grafted copolymers, IPNs, microgels and physical mixtures, is aimed at overcoming these drawbacks and creating dual-functional materials with both synthetic and natural polymers' properties. When developing such compositions, special attention should be paid to preserving their key property, thermoresponsiveness. Addition of hydrophobic and hydrophilic fragments to PNIPAM is known to affect its transition temperature. This review covers various classes of natural polymers - polysaccharides, fibrous and non-fibrous proteins, DNA - used in combination with PNIPAM for the prospective biomedical purposes, with a focus on their phase transition temperatures and its relation to the natural polymer's structure.

Network architecture dependent mechanical response in temperature responsive collagen-PNIPAM composites

Collagen is the most abundant protein in the mammalian extracellular matrix. In-vitro collagen-based materials with specific mechanical properties are important for various bio-medical and tissue-engineering applications. Here, we study the reversible mechanical switching behaviour of a bio-compatible composite formed by collagen networks seeded with thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) microgel particles, by exploiting the swelling/de-swelling of the particles across the lower critical solution temperature (LCST). Interestingly, we find that the shear modulus of the system reversibly enhances whenever the diameter of the microgel particles is changed from that corresponding to the polymerization temperature of the composite, irrespective of swelling or, de-swelling. However, the degree of such enhancement significantly depends on the temperature-dependent collagen network architecture quantified by the mesh size of the network. Furthermore, confocal imaging of the composite during the temperature switching reveals that the reversible clustering of microgel particles above LCST plays a crucial role in the observed switching response.

Development of a multifunctional injectable temperature-sensitive gelatin-based adhesive double-network hydrogel

Gelatin-based bioadhesives are suitable for the treatment of wounds due to their inherent biocompatibility, lack of immunogenicity, and potential for modification. However, common limitations with such adhesives include their adhesive strength and versatility. In the present study, a multifunctional injectable temperature-sensitive gelatin-based adhesive double-network hydrogel (DNGel) was engineered using facile dual-syringe methodology. An integrative crosslinking strategy utilized the complexation of catechol-Fe³⁺ and NIPAAm-methacryloyl. As anticipated, the DNGel exhibited multifunctional therapeutic properties, namely temperature-sensitivity, mechanical flexibility, good adhesive strength, injectability, self-healing capability, antibacterial activity, and the capability to enable hemostasis and wound healing. The bioinspired dynamic double-network was stabilized by a number of molecular interactions between components in the DNGel, providing multifunctional therapeutic performance. In addition, comprehensive in vitro and in vivo testing confirmed that the adhesive hydrogel exhibited effective antihemorrhagic properties and accelerated wound healing by the promotion of revascularization, representing considerable potential as a next-generation multifunctional smart adhesive patch.

Antioxidant and multi-sensitive PNIPAAm/keratin double network gels for self-stripping wound dressing application

Hydrogel is a potential wound dressing material due to its ability to maintain a humid environment, the strong absorptive capacity of exuded tissue fluid, and gas exchange function. Herein, poly(N-isopropyl acrylamide)/keratin double network (PNIPAAm/keratin DN) gels were fabricated through covalent and ionic double cross-linking strategy. The effects of PNIPAAm/keratin ratios on the morphology and swelling rate of gels were characterized. The DN gels could swell up from 2600% to 4600% in proportion to the keratin content, demonstrating their great ability to absorb tissue fluid. The gels possessed thermo-sensitiveness, imparting self-stripping property. Moreover, the antibacterial chlorhexidine acetate (CHX) was loaded into gels with a post-fabrication drug-loading strategy. The release behavior showed that CHX-loaded DN gels exhibited multiple responsive characteristics (temperature, pH, and ROS). Furthermore, the drug-loaded gels showed greater antibacterial activity than free CHX due to the sustained drug release effect. Meanwhile, the antioxidant efficiency of PNIPAAm/keratin DN gels was ca. 33.1%, while the PNIPAAm gel was just ca. 18.2%, indicating the strong oxidation resistance of DN gels. In the Sprague Dawley (SD) rat skin defect model, the hydrogel had better tissue regeneration ability than the commercial film. Taken together, the multifunctional PNIPAAm/keratin DN gels are potential candidates for clinical wound treatment.

Electro-responsive hydrogel-based microfluidic actuator platform for photothermal therapy

Electrical stimuli play an important role in regulating the delivery of plasmonic nanomaterials with cancer targeting peptides. Here, we developed an electro-responsive hydrogel-based microfluidic actuator platform for brain tumor targeting and photothermal therapy (PTT) applications. The electro-responsive hydrogels consisted of highly conductive silver nanowires (AgNWs) and biocompatible collagen I gels. We confirmed that an electrically conductive hydrogel could be used as an effective actuator by applying an electrical signal in the microfluidic platform. Furthermore, we successfully demonstrated PTT efficacy for brain tumor cells using targetable Arg-Gly-Asp (RGD) peptide-conjugated gold nanorods (GNRs). Therefore, our electro-responsive hydrogel-based microfluidic actuator platform could be useful for electro-responsive intelligent nanomaterial delivery and PTT applications.