Novel Trends in Hydrogel Development for Biomedical Applications: A Review

Nowadays, there are still numerous challenges for well-known biomedical applications, such as tissue engineering (TE), wound healing and controlled drug delivery, which must be faced and solved. Hydrogels have been proposed as excellent candidates for these applications, as they have promising properties for the mentioned applications, including biocompatibility, biodegradability, great absorption capacity and tunable mechanical properties. However, depending on the material or the manufacturing method, the resulting hydrogel may not be up to the specific task for which it is designed, thus there are different approaches proposed to enhance hydrogel performance for the requirements of the application in question. The main purpose of this review article was to summarize the most recent trends of hydrogel technology, going through the most used polymeric materials and the most popular hydrogel synthesis methods in recent years, including different strategies of enhancing hydrogels’ properties, such as cross-linking and the manufacture of composite hydrogels. In addition, the secondary objective of this review was to briefly discuss other novel applications of hydrogels that have been proposed in the past few years which have drawn a lot of attention.

Hipster microcarriers: exploring geometrical and topographical cues of non-spherical microcarriers in biomedical applications

Structure and organisation are key aspects of the native tissue environment, which ultimately condition cell fate via a myriad of processes, including the activation of mechanotransduction pathways. By modulating the formation of integrin-mediated adhesions and consequently impacting cell contractility, engineered geometrical and topographical cues may be introduced to activate downstream signalling and ultimately control cell morphology, proliferation, and differentiation. Microcarriers appear as attractive vehicles for cell-based tissue engineering strategies aiming to modulate this 3D environment, but also as vehicles for cell-free applications, given the ease in tuning their chemical and physical properties. In this review, geometry and topography are highlighted as two preponderant features in actively regulating interactions between cells and the extracellular matrix. While most studies focus on the 2D environment, we focus on how the incorporation of these strategies in 3D systems could be beneficial. The techniques applied to design 3D microcarriers with unique geometries and surface topographical cues are covered, as well as specific tissue engineering approaches employing these microcarriers. In fact, successfully achieving a functional histoarchitecture may depend on a combination of fine-tuned geometrically shaped microcarriers presenting intricately tailored topographical cues. Lastly, we pinpoint microcarrier geometry as a key player in cell-free biomaterial-based strategies, and its impact on drug release kinetics, the production of steerable microcarriers to target tumour cells, and as protein or antibody biosensors.

Anodic Alumina Membranes: From Electrochemical Growth to Use as Template for Fabrication of Nanostructured Electrodes

The great success of anodic alumina membranes is due to their morphological features coupled to both thermal and chemical stability. The electrochemical fabrication allows accurate control of the porous structure: in fact, the membrane morphological characteristics (pore length, pore diameter and cell density) can be controlled by adjusting the anodizing parameters (bath, temperature, voltage and time). This article deals with both the fabrication and use of anodic alumina membranes. In particular, we will show the specific role of the addition of aluminum ions to phosphoric acid-based anodizing solution in modifying the morphology of anodic alumina membranes. Anodic alumina membranes were obtained at −1 °C in aqueous solutions of 0.4 M H3PO4 added with different amounts of Al(OH)3. For sake of completeness, the formation of PAA in pure 0.4 M H3PO4 in otherwise identical conditions was also investigated. We found that the presence of Al(OH)3 in solution highly affects the morphology of the porous layer. In particular, at high Al(OH)3 concentration (close to saturation) more compact porous layers were formed with narrow pores separated by thick oxide. The increase in the electric charge from 20 to 160 C cm−2 also contributes to modifying the morphology of porous oxide. The obtained anodic alumina membranes were used as a template to fabricate a regular array of PdCo alloy nanowires that is a valid alternative to Pt for hydrogen evolution reaction. The PdCo alloy was obtained by electrodeposition and we found that the composition of the nanowires depends on the concentration of two metals in the deposition solution.

Bioinspired Microstructure Platform for Modular Cell-Laden Microgel Fabrication

Cell-laden microgels have attracted increasing interest in various biomedical fields, as living building blocks to construct spatially organized multicellular structures or complex tissue features (e.g., cell spheroids and aligned cells/fibers). Although numerous approaches have been developed to tailor cell-laden microgels, there is still an unmet need for modular, versatile, convenient, and high-throughput methods. In this study, as inspired by the phenomena of water droplet manipulation from natural microstructures, a novel platform is developed to manipulate microscale hydrogel droplets and fabricate modular cell-laden microgels. First, taking antenna-like trichome as a template, catcher-like bioinspired microstructures are fabricated and hydrogel droplets are manipulated modularly in a versatile, convenient, and high-throughput manner, which is compatible with various types of hydrogels (e.g., photo-cross-linking, thermal-cross-linking, and ion-cross-linking). It is demonstrated that this platform can manipulate cell-laden microgels as modular units, such as two or more cell-laden microgels on one single catcher-like structure and different structures on one single chip. The authors also demonstrate the application of this platform on constructing complex tissue features like myocardial fibrosis tissue models to study cardiac fibrosis. The developed platform will be a powerful tool for engineering various in vitro tissue models for widespread biomedical applications.

Microfluidic Approach to Generate a Tadpole-Egg-Shaped Alginate Fiber and Its Application in Tissue Engineering

Herein, we introduce a facile microfluidic technique to produce a hybrid alginate fiber with a tadpole-egg shape. A triple-flow polydimethylsiloxane microfluidic device was constructed to allow the formation of oil droplets inside the alginate stream and was instantaneously gelated with the coaxially adjacent CaCl₂. The fiber entrapping the uniform oil droplets was dehydrated, leading to the formation of a distinct tadpole-egg-shaped structure. A series of diverse fiber architectures was fabricated in a controlled manner based on the flow rates of the relevant flows. The tadpole-egg-shaped alginate fibers were employed as building blocks to create a three-dimensional microwell template for cell cultures. First, the tadpole-egg-shaped alginate fibers containing the oil droplets were half-dipped into a melted agarose solution. After the solidification of the agarose gel, the alginate fibers were degraded by an ethylenediaminetetraacetic acid (EDTA) solution to generate the hemispherical microwells. Mesenchymal stem cells (MSCs) were cultured in the microwells to generate spheroids, which were induced into chondrocytes using transforming growth factor-β3. The formed MSC spheroids exhibited a relatively high ratio of cell viability with more than 95% live cells after 14 days of culture. The success of the chondrogenic differentiation was proven based on staining (Safranin O) and the glycosaminoglycan levels. The latter was significantly higher in spheroids that were induced to form chondrocytes compared to those that were not induced after 21 days of differentiation. Second, we investigated the potential of the tadpole-egg-shaped alginate fibers as microcarriers for applications in drug delivery and implantable technologies. It was revealed that the degradation of the Ca-alginate wall of the hybrid fibers to release the oil droplets required an EDTA solution with a concentration of 500 mM for a 15 min period. This result can be used to further develop the tadpole-egg-shaped alginate fibers as uniform microcarriers with multiple compartments.

Methods for producing microstructured hydrogels for targeted applications in biology

Hydrogels have been broadly studied for applications in clinically motivated fields such as tissue regeneration, drug delivery, and wound healing, as well as in a wide variety of consumer and industry uses. While the control of mechanical properties and network structures are important in all of these applications, for regenerative medicine applications in particular, matching the chemical, topographical and mechanical properties for the target use/tissue is critical. There have been multiple alternatives developed for fabricating materials with microstructures with goals of controlling the spatial location, phenotypic evolution, and signaling of cells. The commonly employed polymers such as poly(ethylene glycol) (PEG), polypeptides, and polysaccharides (as well as others) can be processed by various methods in order to control material heterogeneity and microscale structures. We review here the more commonly used polymers, chemistries, and methods for generating microstructures in biomaterials, highlighting the range of possible morphologies that can be produced, and the limitations of each method. With a focus in liquid-liquid phase separation, methods and chemistries well suited for stabilizing the interface and arresting the phase separation are covered. As the microstructures can affect cell behavior, examples of such effects are reviewed as well. STATEMENT OF SIGNIFICANCE: Heterogeneous hydrogels with enhanced matrix complexity have been studied for a variety of biomimetic materials. A range of materials based on poly(ethylene glycol), polypeptides, proteins, and/or polysaccharides, have been employed in the studies of materials that by virtue of their microstructure, can control the behaviors of cells. Methods including microfluidics, photolithography, gelation in the presence of porogens, and liquid-liquid phase separation, are presented as possible strategies for producing materials, and their relative advantages and disadvantages are discussed. We also describe in more detail the various processes involved in LLPS, and how they can be manipulated to alter the kinetics of phase separation and to yield different microstructured materials.

Magnetothermal heating facilitates the cryogenic recovery of stem cell–laden alginate–Fe3O4 nanocomposite hydrogels

A report on the self-heating enabled cryopreservation of stem cell–laden magnetic nanocomposite hydrogels.