Superporous thermo-responsive hydrogels by combination of cellulose fibers and aligned micropores

Strategies for Anisotropic Fibrillar Hydrogels: Design, Cell Alignment, and Applications in Tissue Engineering

Efficient cellular alignment in biomaterials presents a considerable challenge, demanding the refinement of appropriate material morphologies, while ensuring effective cell-surface interactions. To address this, biomaterials are continuously researched with diverse coatings, hydrogels, and polymeric surfaces. In this context, we investigate the influence of physicochemical parameters on the architecture of fibrillar hydrogels that significantly orient the topography of flexible hydrogel substrates, thereby fostering cellular adhesion and spatial organization. Our Review comprehensively assesses various techniques for aligning polymer fibrils within hydrogels, specifically interventions applied during and after the cross-linking process. These methodologies include mechanical strains, precise temperature modulation, controlled fluidic dynamics, and chemical modulators, as well as the use of magnetic and electric fields. We highlight the intrinsic appeal of these methodologies in fabricating cell-aligning interfaces and discuss their potential implications within the fields of biomaterials and tissue engineering, particularly concerning the pursuit of optimal cellular alignment.

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.

The effect of the printing temperature on 4D DLP printed pNIPAM hydrogels

Additive manufacturing (AM), in its little more than 40 years of existence, has already established itself as a technology with enormous potential for several fields, especially the ones that require complex, high resolution, small structures, such as tissue engineering. This field has been especially attracted to the most recent AM evolution, 4D printing, due to its ability to create structures responsive to external stimuli. Among the range of materials that are simultaneously suitable for 4D printing and biological uses, poly(N-isopropylacrylamide) (pNIPAM) stands out. pNIPAM presents exceptional characteristics such as a low critical solution temperature (LCST) close to the human physiological temperature and biocompatibility with several cell types. However, these characteristics are greatly affected by processing parameters. In this work, pNIPAM hydrogels were manufactured by AM using digital light processing; the printing temperature was varied between 5, 10 and 15 °C to analyze how it affects the hydrogels' final properties. The impact on hydrogels was analyzed by differential scanning calorimetry (DSC), swelling, deswelling and reswelling analyses, scanning electron microscopy (SEM) images, and compression tests. Based on our results increasing the production temperature of the hydrogels by 10 °C led to a decrease of more than 50% in the maximum swelling capacity, approximately 10% increase in water retention, and 6.5 °C variation in the LCST. The justification for such behaviour lies in the increase of the crosslinking rate and thickening of the external layer of hydrogels, which prevents the free movement of water from its interior.

Photopolymerization of Bio-Based Polymers in a Biomedical Engineering Perspective

Photopolymerization is an effective method to covalently cross-link polymer chains that can be shaped into several biomedical products and devices. Additionally, polymerization reaction may induce a fluid-solid phase transformation under physiological conditions and is ideal for in vivo cross-linking of injectable polymers. The photoinitiator is a key ingredient able to absorb the energy at a specific light wavelength and create radicals that convert the liquid monomer solution into polymers. The combination of photopolymerizable polymers, containing appropriate photoinitiators, and effective curing based on dedicated light sources offers the possibility to implement photopolymerization technology in 3D bioprinting systems. Hence, cell-laden structures with high cell viability and proliferation, high accuracy in production, and good control of scaffold geometry can be biofabricated. In this review, we provide an overview of photopolymerization technology, focusing our efforts on natural polymers, the chemistry involved, and their combination with appropriate photoinitiators to be used within 3D bioprinting and manufacturing of biomedical devices. The reviewed articles showed the impact of different factors that influence the success of the photopolymerization process and the final properties of the cross-linked materials.

Photopolymerized Porous Hydrogels

Hydrogels are polymeric networks highly swollen with water. Because of their versatility and properties mimicking biological tissues, they are very interesting for biomedical applications. In this aim, the control of porosity is of crucial importance since it governs the transport properties and influences the fate of cells cultured onto or into the hydrogels. Among the techniques allowing for the elaboration of hydrogels, photopolymerization or photo-cross-linking are probably the most powerful and versatile synthetic routes. This Review aims at giving an overview of the literature dealing with photopolymerized hydrogels for which the generation or characterization of porosity is studied. First, the materials (polymers and photoinitiating systems) used for synthesizing hydrogels are presented. The different ways for generating porosity in the photopolymerized hydrogels are explained, and the characterization techniques allowing adequate study of the porosity are presented. Finally, some applications in the field of controlled release and tissue engineering are reviewed.

Temperature-Responsive On-Off Control over Water Evaporation Achieved via Sweat-Gland-Mimetic Composites

Responsive cooling materials that mimic sweat glands have gained popularity because they are efficient and do not require artificial energy sources. Temperature-responsive hydrogels sweat above their volume transition temperature through the release of water and exhibit excellent cooling ability. However, thus far, practical applications have not been possible because the water in these materials cannot be preserved in cool environments. To address this issue, this paper presents a simple composite of poly(N-isopropylacrylamide) and polydimethylsiloxane that offers excellent on-off control over water evaporation and can be used repeatedly; the proposed composite features an evaporation rate of 2.97 g/h above the lower critical solution temperature (LCST) and 0.08 g/h below the LCST. This 35.7-fold change in the water evaporation rate is comparable to that in mammalian sweat glands. The responsive on-off control relies on the structures of the composite and the dry layers formed on the surface of the composite in cool environments. The proposed material effectively regulates water evaporation and offers a novel, low-cost cooling strategy suitable for numerous applications.

Nanocellulose: Recent Fundamental Advances and Emerging Biological and Biomimicking Applications

In the effort toward sustainable advanced functional materials, nanocelluloses have attracted extensive recent attention. Nanocelluloses range from rod-like highly crystalline cellulose nanocrystals to longer and more entangled cellulose nanofibers, earlier denoted also as microfibrillated celluloses and bacterial cellulose. In recent years, they have spurred research toward a wide range of applications, ranging from nanocomposites, viscosity modifiers, films, barrier layers, fibers, structural color, gels, aerogels and foams, and energy applications, until filtering membranes, to name a few. Still, nanocelluloses continue to show surprisingly high challenges to master their interactions and tailorability to allow well-controlled assemblies for functional materials. Rather than trying to review the already extensive nanocellulose literature at large, here selected aspects of the recent progress are the focus. Water interactions, which are central for processing for the functional properties, are discussed first. Then advanced hybrid gels toward (multi)stimuli responses, shape-memory materials, self-healing, adhesion and gluing, biological scaffolding, and forensic applications are discussed. Finally, composite fibers are discussed, as well as nanocellulose as a strategy for improvement of photosynthesis-based chemicals production. In summary, selected perspectives toward new directions for sustainable high-tech functional materials science based on nanocelluloses are described.

Interpenetrating polymer networks in polyvinyl alcohol/cellulose nanocrystals hydrogels to develop absorbent materials

Polyvinyl alcohol/cellulose nanocrystals/poly(2-Hydroxyethyl methacrylate) (PVA/CNC/polyHEMA) and PVA/CNC/poly(N'-methylenebisacrylamide) (PVA/CNC/polyMBA) hydrogels were prepared by photo-crosslinking followed by freezing/thawing (F-T) cycle and this novel preparation method was reported. The formation of interpenetrating polymer networks (IPN) resulted from the addition of crosslinking HEMA or MBA monomers displayed improved interfacial adhesion. The produced hydrogels were measured by scanning electron microscopy (SEM), real-time fourier transform infrared (RTIR), thermogravimetric analysis (TGA), mechanical, swelling and adsorption tests. The results showed both PVA/CNC/polyHEMA with semi-IPN and PVA/CNC/polyMBA with dual network (DN) hydrogels had higher thermal stability, lower water loss rate and better swelling and reswelling and mechanical properties, comparing to PVA and PVA/CNC hydrogels. The adsorption behaviors of hydrogels using xylenol orange (XO) and methylene blue (MB) as model dyes were evaluated, indicating that PVA/CNC/polyHEMA and PVA/CNC/polyMBA hydrogels could hold some dyes. Overall, this work provided a good way for increasing mechanical, swelling, reswelling, thermal, and adsorption properties of PVA/CNC, which will be a promising water-manageable material for agriculture application and a candidate for dye carrier.

Fast and Efficient Water Absorption Material Inspired by Cactus Root

Analogous to the morphological and functional features of cactus root, a novel cactus root-inspired material (CRIM) was fabricated by integrating cellulose fibers, microparticles, and agarose-based cryogels. Without undergoing sophisticated chemical synthesis or surface modification, the CRIM exhibited efficient water absorption and retention ability with high structural stability. 82% of the total water absorption capacity was recovered within 1 min, with a swelling rate nearly 930-fold faster than the evaporation rate, while only about 17% of the length extension occurred. Given that efficient water absorption and storage without physical change is crucial to the design and fabrication of water management devices, the CRIM is a promising material for various applications, including cosmetics or healthcare products, functional fabrics, and drug delivery devices.

Structured Macroporous Hydrogels: Progress, Challenges, and Opportunities

Structured macroporous hydrogels that have controllable porosities on both the nanoscale and the microscale offer both the swelling and interfacial properties of bulk hydrogels as well as the transport properties of "hard" macroporous materials. While a variety of techniques such as solvent casting, freeze drying, gas foaming, and phase separation have been developed to fabricate structured macroporous hydrogels, the typically weak mechanics and isotropic pore structures achieved as well as the required use of solvent/additives in the preparation process all limit the potential applications of these materials, particularly in biomedical contexts. This review highlights recent developments in the field of structured macroporous hydrogels aiming to increase network strength, create anisotropy and directionality within the networks, and utilize solvent-free or additive-free fabrication methods. Such functional materials are well suited for not only biomedical applications like tissue engineering and drug delivery but also selective filtration, environmental sorption, and the physical templating of secondary networks.

3D Cocontinuous Composites of Hydrophilic and Hydrophobic Soft Materials: High Modulus and Fast Actuation Time

Hydrogels in nature seldom form a single phase, more often forming structured phases with other soft phases, allowing nature to develop responsive and adaptive strategies. Based on knowledge of how hydrogels are utilized in nature, we developed novel 3D cocontinuous composites from soft materials with extremely different properties, a hydrogel and a silicone. These were successfully prepared by infiltrating liquid polydimethylsiloxane (PDMS) into poly(N-isopropylacrylamide) (PNIPAm) frameworks of aligned pores prepared by directional melt crystallization. The composites had outstanding modulus and swelling ratio compared to other mechanically strong hydrogels. More interestingly, the deswelling kinetics were dramatically accelerated (by a factor of 1000), possibly due to the aligned microchannels and the hydrophobic nature of PDMS. As a result, an actuator movement mimicking flowering could be completed in less than 20 s. This novel and versatile cocontinuous composite strategy could overcome the current limitations of soft materials.

Collagen-Poly(N-isopropylacrylamide) Hydrogels with Tunable Properties

There is a lack of hydrogel materials whose properties can be tuned at the point of use. Biological hydrogels, such as collagen, gelate at physiological temperatures; however, they are not always ideal as scaffolds because of their low mechanical strength. Their mechanics can be improved through cross-linking and chemical modification, but these methods still require further synthesis. We have demonstrated that by combining collagen with a thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAM), the mechanical properties can be improved while maintaining cytocompatibility. Furthermore, different concentrations of this polymer led to a range of hydrogels with shear moduli ranging from 10(5) Pa down to less than 10(2) Pa, similar to the soft tissues in the body. In addition to variable mechanical properties, the hydrogel blends have a range of micron-scale structures and porosities, which caused adipose-derived stromal cells (ADSCs) to adopt different morphologies when encapsulated within and may therefore be able to direct cell fate.