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In this study, pH-sensitive gelatin hydrogels reinforced with cellulose nanocrystals (CNCs) from rice husk were fabricated by introducing gamma irradiation to induce green cross-linking method hydrogels. The properties of the hydrogels were studied at varied CNCs and pH. The hydrogels were characterized by fourier-transform infrared (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), rheology, swelling studies, scanning electron microscopy (SEM), drug loading and release analysis, as well as cytotoxicity tests. The addition of CNC improved both the thermal and dynamic mechanical properties of the hydrogels. The hydrogels were sensitive to changes in pH, and the highest swelling degree was observed at pH 3. The optimum swelling performance was obtained with gelatin/4CNC hydrogels. SEM images revealed that incorporation of CNC content up to 4% improved the porosity of the hydrogels. The drug release analysis using riboflavin as a drug model showed that gelatin/4CNC hydrogels released more than 70% of riboflavin in 12 h. In addition, hydrogels showed non-cytotoxic properties, suggesting their potential application in drug delivery systems.

Smart hydrogels which can respond to external stimuli have been widely focused with increasing interest. Thereinto, magnetic-responsive hydrogels that are prepared by embedding magnetic nanomaterials into hydrogel networks are more advantageous in biomedical applications due to their rapid magnetic response, precisely temporal and spatial control and non-invasively remote actuation. Upon the application of an external magnetic field, magnetic hydrogels can be actuated to perform multiple response modes such as locomotion, deformation and thermogenesis for therapeutic purposes without the limit of tissue penetration depth. This review summarizes the latest advances of magnetic-responsive hydrogels with focus on biomedical applications. The synthetic methods of magnetic hydrogels are firstly introduced. Then, the roles of different response modes of magnetic hydrogels played in different biomedical applications are emphatically discussed in detail. In the end, the current limitations and future perspectives for magnetic hydrogels are given.

Introduction This review aims at describing different types of hydrogels in context to their composition, fabrication techniques and other specific features along with an insight into the latest advancements including smart hydrogels, 3D printed, programmable, shape memory and self-healing hydrogels for their applicability as scaffold in maxillofacial bone and cartilage tissue regeneration. Methods Electronic database searches were undertaken on PubMed, Ovid, Medline, Embase, ProQuest and science direct for English language literature, published for application of hydrogels in maxillofacial bone and cartilage tissue engineering. The search items used in this article were hydrogel, bone and cartilage tissue engineering, maxillofacial, clinical trials. Reviews and in vitro studies were excluded. Results Search for injectable hydrogel showed 4955 articles, when restricted to bone tissue engineering results were reduced to 463 and for cartilage engineering to 335; when we limited it to maxillofacial bone and cartilage tissue engineering, search results showed 49 articles to which 9 additional articles were included from references, after exclusion of in-vitro studies and duplicates 16 articles were obtained for our study. Similarly, for 3D printed hydrogels, result showed 1126 articles, which got restricted to 19 when searched for maxillofacial bone and cartilage engineering, then 2 additional articles were included directly from references, and finally after exclusion of the invitro studies and duplicates, a total of 5 articles were obtained. Conclusion Modifications in hydrogel can improve the mechanical properties, biocompatibility and unique chemistries for its use in bone and cartilage tissue engineering for future research

Smart hydrogels are regarded as novel sensors due to large reversible volume change in response to external stimuli. Here we prepared a novel visual multi-triggered sensor based on inverse opal hydrogel (IOH) that responses to pH, temperature and light, respectively. The hydrogel was composed of hydrophilic pH and temperature responsive poly(dimethylaminoethyl methacrylate) (PDMAEMA), and light responsive spiropyran-containing segment. When the pH decreased, the sensor changed its structural color from blue to orange-red, and the structural color was reversibly recovered by increasing pH. In addition, the color of sensor can be controlled to change from green to violet while applying a temperature field. Furthermore, the reflection peak of IOH exhibited obvious shifted behavior by UV/visible irradiation. Repeated condition changes revealed that the sensor has a long lifetime.

This review discusses basic aspects used to control the architecture and functional properties of smart hydrogels. The introduction briefly outlines what has been accomplished regarding smart hydrogels and explores historical aspects and the fundamental understanding of these systems. Then, a short discussion on the chemical interactions and the main variables involved in architectural construction is exhibited. Further analysis provides the basis for optimizing biological responses through system modulation. Finally, we outline perspectives and challenges for building smart hydrogels into functionalized and modulated delivery systems.

Smart hydrogels have received increasing attention for many applications. Herein, a novel supra-rapid formation supramolecular hydrogel with excellent antimicrobial activity was successfully constructed by GA-O-09, which was obtained by modification from none antimicrobial activity compound of 18β-glycyrrhetinic acid (GA). Interestingly, both GA-O-09 and the supra-rapid formation supramolecular hydrogel showed excellent performance in antimicrobial for Staphylococcus aureus (SA). Meanwhile, this supra-rapid formation supramolecular hydrogel also presented outstanding thermodynamics stability and showed performance in sustained released. This study implied that this super-rapid formation super-supramolecular GA-O-09 hydrogel showed excellent antimicrobial activity and had the potential for antibacterial material agent.

The emergence of smart biomaterials has significantly developed the boundaries of modern medicine. This class of biomaterials can operate as an ‘on–off’ switch for multiple applications from nanometer to macrometer scales because of their unique characteristics responsive to the environmental changes. As for illustration, smart biomaterials can guide stem cells to enhance tissue regeneration, resulting in numerous applications in tissue engineering. They can also self-fold or self-unfold to encapsulate and release cells or drugs in a controlled fashion, leading to a smart drug delivery system. Besides, the combination of stimuli-responsive biomaterials with additive manufacturing introduces smart constructs that may closely resemble natural tissues. Therefore, progress in smart biomaterials can trigger significant changes in the field of biomedical engineering. This article will briefly review the current advances and applications in smart biomaterials with emphasis on smart hydrogels, nanomaterials, bioconjugates, and shape-memory biomaterials. Furthermore, smart constructs, as well as intelligent biomaterials and systems, are summarized.

Synthesis of environmentally-sensitive, and therefore smart hydrogels of micro- and nanosize allowed substantial shortening of time of response of these gels to a change in environmental conditions. This made the hydrogels much more attractive and opened new possibilities of their applications. In this paper we present recent developments in ways of construction of micro- and nanogels, their adaptation to particular needs and possibilities of use. The focus was given to individual, spherical particles and very thin layers of gels on solid supports, including electrodes.

Hydrogels prepared from natural polymers have received immense considerations over the past decade due to their safe nature, biocompatibility, hydrophilic properties, and biodegradable nature. More recently, when treated with electroactive materials, these hydrogels were endowed with high electrical conductivity, electrochemical redox properties, and electromechanical properties; consequently, forming a smart hydrogel. The biological properties of these smart hydrogels, classified as electroconductive hydrogels, can be combined with electronics. Thus, they are considered as good candidates for some potential uses, which include bioconductors, biosensors, electro-stimulated drug delivery systems, as well as neuron-, muscle-, and skin-tissue engineering. However, there is lacking comprehensive information on the current state of these electroconductive hydrogels which complicates our understanding of this new type of biomaterials as well as their potential applications. Hence, this review provides a summary on the current development of electroconductive natural polymer-based hydrogels (ENPHs). We have introduced various types of ENPHs, with a brief description of their advantages and shortcomings. In addition, emerging technologies regarding their synthesis developed during the past decade are discussed. Finally, two attractive potential applications of ENPHs, cell culture and biomedical devices, are reviewed, along with their current challenges.

Development of polymer-based smart materials, which can autonomously alter their physical and/or chemical properties when exposed to external stimuli, is a thriving research frontier in contemporary advanced functional materials science. Poly(N-isopropylacrylamide) (PNIPAM)-based smart hydrogels are known to exhibit distinct thermo-responsive properties near a lower critical solution temperature (LCST), which have found diverse promising applications such as smart coating, drug delivery, tissue regeneration, and artificial muscles. In this review, we provide an up-to-date account on the recent developments in advanced functional PNIPAM-based smart hydrogels and their emerging technological applications in the fields of smart actuators, photonic crystals, smart windows and novel biomedical applications. The fundamental design and synthetic strategies of PNIPAM-based smart hydrogels are discussed. Their unique properties, underlying mechanisms and potential applications in different fields are highlighted. Finally, this review provides a brief conclusion and enumerates the challenges and opportunities in this rising area of research and development involving these intriguing polymer-based advanced smart systems rooted in chemistry and materials science. It is expected that this review would provide significant insights for the development of reconfigurable and programmable advanced smart materials with numerous possibilities, prompting the rapid advancement of this highly interdisciplinary area, which encompasses materials science, polymer science, synthetic chemistry, device engineering, physics, biology, nanoscience and nanotechnology.

During the past few decades, thiomers have attracted interest as responsive-polymers in the design of smart hydrogels, due to thiol reactivity to physical stimuli (i.e., pH, oxygen, UV/Vis irradiation). However, thiomer potentialities have been limited by their low thiolation degree and lack of multi-responsiveness to physical cues. In this work, a new class of thiomers with tailored thiolation degree and sensitivity to multiple physical stimuli was developed. To this aim, a polyurethane (33,000 Da) was synthesised from Poloxamer® 407, 1,6-diisocyanatohexane and N-Boc diethanolamine. Secondary amino groups were exposed through Boc-removal (yield 80%, assessed by 1H NMR) giving up to 4.5 × 1020 ± 1.8 × 1019 –NH units/gpolymer (Orange II Sodium Salt colorimetric assay) without polymer chemical degradation. Polymer thermo-responsiveness was proved by the increase in micelle hydrodynamic diameter upon system heating up to 45 °C and through estimated critical micellar temperatures. Thioglycolic acid grafting to exposed amines via carbodiimide chemistry was optimised to give up to 1.73 × 1019 thiols/gpolymer (Ellman's method), while minimising disulphide bond formation. Finally, the best storage conditions against oxidation were investigated by quantifying free thiols at different time points.

Hydrogels were the first biomaterials developed for human use. The state-of-the-art and potential for the future are discussed. Recently, new designs have produced mechanically strong synthetic hydrogels. Protein based hydrogels and hybrid hydrogels containing protein domains present a novel advance; such biomaterials may self-assemble from block or graft copolymers containing biorecognition domains. One of the domains, the coiled-coil, ubiquitously found in nature, has been used as an example to demonstrate the developments in the design of smart hydrogels. The application potential of synthetic, protein-based, DNA-based, and hybrid hydrogels bodes well for the future of this class of biomaterials.

Smart hydrogels show phase transition behavior. This property can be used to operate sensors and actuators in a reversible and reproducible mode. Ignoring phenomena occurring at volume phase transition can result in a malfunction or even failure of the system. In this paper, the influence of volume phase transition phenomena on the behavior of hydrogel-based valves are discussed. Furthermore, the operation mode of such valves in dependence of various design parameters and hydrogel properties are described.

Environmentally sensitive hydrogels have enormous potential in various applications. Some environmental variables, such as low pH and elevated temperatures, are found in the body. For this reason, either pH-sensitive and/or temperature-sensitive hydrogels can be used for site-specific controlled drug delivery. Hydrogels that are responsive to specific molecules, such as glucose or antigens, can be used as biosensors as well as drug delivery systems. Light-sensitive, pressure-responsive and electro-sensitive hydrogels also have the potential to be used in drug delivery and bioseparation. While the concepts of these environment-sensitive hydrogels are sound, the practical applications require significant improvements in the hydrogel properties. The most significant weakness of all these external stimuli-sensitive hydrogels is that their response time is too slow. Thus, fast-acting hydrogels are necessary, and the easiest way of achieving that goal is to make thinner and smaller hydrogels. This usually makes the hydrogel systems too fragile and they do not have mechanical strength necessary in many applications. Environmentally sensitive hydrogels for drug delivery applications also require biocompatibility. Synthesis of new polymers and crosslinkers with more biocompatibility and better biodegradability would be essential for successful applications. Development of environmentally sensitive hydrogels with such properties is a formidable challenge. If the achievements of the past can be extrapolated into the future, however, it is highly likely that responsive hydrogels with a wide array of desirable properties can be made.