Review: Hydrogels for cell immobilization

Injectable polyoxazoline grafted hyaluronic acid thermoresponsive hydrogels for biomedical applications

Injectable thermosensitive hydrogels based on hyaluronic acid (HA) grafted with lower critical solution temperature (LCST) polyoxazoline (copolymers of poly(isopropyl-co-butyl oxazoline)) or P(iPrOx-co-BuOx) have been elaborated with tunable solution/gel temperature transitions and gel state elastic modulus. A suitable HA-g-P(iPrOx-co-BuOx-67/33)-0.10 sample with an iPrOx/BuOx ratio of 67/33, a polymerization degree (DP) of 25, a substitution degree (DS) of 10%, and displaying thermally induced gelling character with elastic (G') and viscous (G'') moduli crossover points at 25 °C and a G' at 37 °C around 80 Pa has been chosen for medical application. Hydrogels obtained with HA-g-P(iPrOx-co-BuOx-67/33)-0.10 exhibited high stability at 37 °C and excellent injectability properties with full and quick reversibility. The incorporation of a secondary network (HA), until 35 wt%, into the thermosensitive hydrogel also demonstrated very good stability and injectability.

Micro- and Nanostructured Fibrous Composites via Electro-Fluid Dynamics: Design and Applications for Brain

The brain consists of an interconnected network of neurons tightly packed in the extracellular matrix (ECM) to form complex and heterogeneous composite tissue. According to recent biomimicry approaches that consider biological features as active components of biomaterials, designing a highly reproducible microenvironment for brain cells can represent a key tool for tissue repair and regeneration. Indeed, this is crucial to support cell growth, mitigate inflammation phenomena and provide adequate structural properties needed to support the damaged tissue, corroborating the activity of the vascular network and ultimately the functionality of neurons. In this context, electro-fluid dynamic techniques (EFDTs), i.e., electrospinning, electrospraying and related techniques, offer the opportunity to engineer a wide variety of composite substrates by integrating fibers, particles, and hydrogels at different scales-from several hundred microns down to tens of nanometers-for the generation of countless patterns of physical and biochemical cues suitable for influencing the in vitro response of coexistent brain cell populations mediated by the surrounding microenvironment. In this review, an overview of the different technological approaches-based on EFDTs-for engineering fibrous and/or particle-loaded composite substrates will be proposed. The second section of this review will primarily focus on describing current and future approaches to the use of composites for brain applications, ranging from therapeutic to diagnostic/theranostic use and from repair to regeneration, with the ultimate goal of providing insightful information to guide future research efforts toward the development of more efficient and reliable solutions.

Mapping Nanocellulose- and Alginate-Based Photosynthetic Cell Factory Scaffolds: Interlinking Porosity, Wet Strength, and Gas Exchange

To develop efficient solid-state photosynthetic cell factories for sustainable chemical production, we present an interdisciplinary experimental toolbox to investigate and interlink the structure, operative stability, and gas transfer properties of alginate- and nanocellulose-based hydrogel matrices with entrapped wild-type Synechocystis PCC 6803 cyanobacteria. We created a rheological map based on the mechanical performance of the hydrogel matrices. The results highlighted the importance of Ca2+-cross-linking and showed that nanocellulose matrices possess higher yield properties, and alginate matrices possess higher rest properties. We observed higher porosity for nanocellulose-based matrices in a water-swollen state via calorimetric thermoporosimetry and scanning electron microscopy imaging. Finally, by pioneering a gas flux analysis via membrane-inlet mass spectrometry for entrapped cells, we observed that the porosity and rigidity of the matrices are connected to their gas exchange rates over time. Overall, these findings link the dynamic properties of the life-sustaining matrix to the performance of the immobilized cells in tailored solid-state photosynthetic cell factories.

Polymeric DNA Hydrogels and Their Applications in Drug Delivery for Cancer Therapy

The biomolecule deoxyribonucleic acid (DNA), which acts as the carrier of genetic information, is also regarded as a block copolymer for the construction of biomaterials. DNA hydrogels, composed of three-dimensional networks of DNA chains, have received considerable attention as a promising biomaterial due to their good biocompatibility and biodegradability. DNA hydrogels with specific functions can be prepared via assembly of various functional sequences containing DNA modules. In recent years, DNA hydrogels have been widely used for drug delivery, particularly in cancer therapy. Benefiting from the sequence programmability and molecular recognition ability of DNA molecules, DNA hydrogels prepared using functional DNA modules can achieve efficient loading of anti-cancer drugs and integration of specific DNA sequences with cancer therapeutic effects, thus achieving targeted drug delivery and controlled drug release, which are conducive to cancer therapy. In this review, we summarized the assembly strategies for the preparation of DNA hydrogels on the basis of branched DNA modules, hybrid chain reaction (HCR)-synthesized DNA networks and rolling circle amplification (RCA)-produced DNA chains, respectively. The application of DNA hydrogels as drug delivery carriers in cancer therapy has been discussed. Finally, the future development directions of DNA hydrogels in cancer therapy are prospected.

Application of poly (vinyl alcohol)-cryogels to immobilizing nitrifiers: Enhanced tolerance to shear stress-induced destruction and viability control

The hardness of poly (vinyl alcohol)-cryogels (PVA-CGs) was improved under three parameter conditions: 7.5 %-12.5 % PVA, 1-5 freezing-thawing cycles (FTCs), and the addition of 0 %-10 % glycerol as a cryoprotectant. This study investigated the effects of shear stress-induced destruction (SSID) on mechanical strength by inducing rapid erosion with a high frictional force. Tolerance to SSID (Tol-SSID) exhibited different sensitivities and trends depending on the above three fabrication parameters. The measured Tol-SSID exhibited consistent and inconsistent trends with tensile strength and swelling, respectively. Tol-SSID evaluation provides new insights into the practically meaningful mechanical strength of PVA-CGs against strong friction, which simulates extreme shear stress in a bioreactor. A PVA-CG with a PVA concentration of 10 % and in two FTCs resulted in Tol-SSID and tensile strength of 88.3 % and 0.59 kPa, respectively. Here, 5 % glycerol was added to maintain the bacterial respiration activity of immobilized nitrifiers of 0.097 mg-O₂/g-VSS·min and survival of 88.6 %. The continuous mode of nitrification using the optimized PVA-CG for 10 days resulted in an ammonia removal rate of 0.2173 kg-N/m³·d, which is an improvement over cases without glycerol addition: 0.1426 and 0.1472 kg-N/m³·d for PVA-CGs in two and three FTCs, respectively.

Soft hydrogel-shell confinement systems as bacteria-based bioactuators and biosensors

Genetically engineered (GE) bacteria were utilized for developing functional systems upon confinement within a restricted space. Use of natural soft hydrogel such as alginate, gelatin, and agarose, have been investigated as promising approaches to design functional architectures. Nevertheless, a challenge is to develop functional microenvironments that support biofilm-like confinement in a relevant three-dimensional (3D) format for long-term studies. We demonstrate a natural soft hydrogel bioactuator based on alginate core-shell structures (0.25-2 mm core and 50-300 μm shell thickness) that enables 3D microbial colonization upon confinement with high cell density. Specially, our study evaluates the efficiency of bacteria-functional system by recapitulating various GE bacteria which can produce common reporter proteins, to demonstrate their actuator functions as well as dynamic pair-wise interactions. The structural design of the hydrogel can endure continued growth of various bacteria colonies within the confined space for over 10 days. The total amount of cellular biomass upon hydrogel-shell confinement was increased 5-fold compared to conventional techniques without hydrogel-shell. Furthermore, the enzymatic activity increased 3.8-fold and bioluminescence signal by 8-fold compared to the responses from conventional hydrogel systems. The conceptualized platform and our workflow represent a reliable strategy with core-shell structures to develop artificial hydrogel habitats as bacteria-based functional systems for bioactuation.

Development of Growth Factor Releasing Hyaluronic Acid-Based Hydrogel for Pulp Regeneration: A Preliminary Study

Growth factors play essential roles as signaling molecules in pulp regeneration. We investigated the effect of a hyaluronic acid (HA)-collagen hybrid hydrogel with controlled release of fibroblast growth factor (FGF)-2 and platelet-derived growth factor (PDGF)-BB on human pulp regeneration. The cell interaction and cytotoxicity of the HA-collagen hybrid hydrogel, the release kinetics of each growth factor, and the effects of the released growth factors on pulp cell proliferation were examined. The vitality of pulp cells was maintained. The amounts of FGF-2 and PDGF-BB released over 7 days were 68% and 50%, respectively. Groups with a different concentration of growth factor (FGF-2: 100, 200, 500, and 1000 ng/mL; PDGF-BB: 10, 50, 100, 200, and 500 ng/mL) were experimented on days 1, 3, 5, and 7. Considering FGF-2 concentration, significantly increased pulp cell proliferation was observed on days 1, 3, 5, and 7 in the 100 ng/mL group and on days 3, 5, and 7 in the 200 ng/mL group. In the case of PDGF-BB concentration, significantly increased pulp cell proliferation was observed at all four time points in the 100 ng/mL group and on days 3, 5, and 7 in the 50, 200, and 500 ng/mL groups. This indicates that the optimal concentration of FGF-2 and PDGF-BB for pulp cell proliferation was 100 ng/mL and that the HA-collagen hybrid hydrogel has potential as a controlled release delivery system for FGF-2 and PDGF-BB.

Encapsulation of Bacillus thuringiensis in an inverse Pickering emulsion for pest control applications

Here, we present an inverse Pickering emulsion-based formulation for Bacillus thuringiensis serovar aizawai (BtA) encapsulations utilized towards pest control applications. The emulsification was carried out by high shear homogenization process via ULTRA-TURRAX®. The water-in-mineral oil emulsions were stabilized by commercial hydrophobic silica. Different silica contents and water/oil ratios were studied. Stable emulsions were obtained at 2 and 3 wt% silica at 30% and 20% water volumes, respectively. The structure of the Pickering emulsions were characterized by laser scanning confocal microscopy and cryogenic scanning electron microscopy. The BtA cells, spores and crystals were encapsulated in the water droplets of the inverse Pickering emulsions. An emulsion composed of 3 wt% silica and 30% water was found to be the most suitable for encapsulation. The pest control efficiency of the encapsulated BtA against Spodoptera littoralis first instar larvae was tested. The studied BtA/emulsion system exhibited a mortality rate of 92%. However, the non-formulated BtA has shown 71% mortality, and the emulsion alone resulted in only 9% mortality. These findings confirm that an emulsion with encapsulated BtA can function as an efficient formulation for biopesticides.

Super-Adsorptive Biodegradable Hydrogel from Simply Treated Sugarcane Bagasse

There is a great demand for biodegradable hydrogel, and cellulose enriched wastes materials are widely used to serve this purpose for various advance applications (e.g., biomedical and environmental). Sugarcane bagasse is cellulose-enriched agro-waste, abundantly grown in Bangladesh. This study aimed to treat sugarcane bagasse-based agro-waste using a sustainable and ecofriendly approach to produce hydrogel with super-swelling capacity for adsorption of copper, chromium, iron ions, methylene blue and drimaren red dyes. To increase the swelling property of hydrogels, copolymerization of hydrophilic monomers is an effective technique. Therefore, this study aimed to prepare hydrogel via free radical graft-copolymerization reaction among acrylamide, methyl methacrylate and treated bagasse in the presence of N,N-methylene-bis-acrylamide as a crosslinker and potassium persulphate as an initiator. To obtain maximum yield, reaction conditions were optimized. It was found that hydrogel obtained from chemically treated sugarcane bagasse showed maximum water absorption capacity of 228.0 g/g, whereas untreated bagassebased hydrogel could absorb ~50 g/g of water. Maximum adsorption capacity of 247.0 mg/g was found for copper ion. In addition, organic pollutant removal from industrial effluent also showed good performance, removing >90% of methylene blue and 62% of drimaren red dye, with shorter kinetics. The biodegradability study showed that after 90 days of exposure, the hydrogels degraded to about 43% of their own mass. Therefore, the produced hydrogel could be an alternative adsorbent to remove pollutants and also for other potential applications.

Nanostructured Lipid Carriers-Hydrogels System for Drug Delivery: Nanohybrid Technology Perspective

Advanced hybrid component development in nanotechnology provides superior functionality in the application of scientific knowledge for the drug delivery industry. The purpose of this paper is to review important nanohybrid perspectives in drug delivery between nanostructured lipid carriers (NLC) and hydrogel systems. The hybrid system may result in the enhancement of each component's synergistic properties in the mechanical strength of the hydrogel and concomitantly decrease aggregation of the NLC. The significant progress in nanostructured lipid carriers-hydrogels is reviewed here, with an emphasis on their preparation, potential applications, advantages, and underlying issues associated with these exciting materials.

Tailoring Therapeutic Responses via Engineering Microenvironments with a Novel Synthetic Fluid Gel

This study reports the first fully synthetic fluid gel (SyMGels) using a simple poly(ethylene glycol) polymer. Fluid gels are an interesting class of materials: structured during gelation via shear-confinement to form microparticulate suspensions, through a bottom-up approach. Structuring in this way, when compared to first forming a gel and subsequently breaking it down, results in the formation of a particulate dispersion with particles "grown" in the shear flow. Resultantly, systems form a complex microstructure, where gelled particles concentrate remaining non-gelled polymer within the continuous phase, creating an amorphous-like interstitial phase. As such, these materials demonstrate mechanical characteristics typical of colloidal glasses, presenting solid-like behaviors at rest with defined yielding; likely through intrinsic particle-particle and particle-polymer interactions. To date, fluid gels have been fabricated using polysaccharides with relatively complex chemistries, making further modifications challenging. SyMGels are easily functionalised, using simple click-chemistry. This chemical flexibility, allows the creation of microenvironments with discrete biological decoration. Cellular control is demonstrated using MSC (mesenchymal stem cells)/chondrocytes and enables the regulation of key biomarkers such as aggrecan and SOX9. These potential therapeutic platforms demonstrate an important advancement in the biomaterial field, underpinning the mechanisms which drive their mechanical properties, and providing a versatile delivery system for advanced therapeutics.

Research progress on enhancing the performance of autotrophic nitrogen removal systems using microbial immobilization technology

The autotrophic nitrogen removal process has great potential to be applied to the biological removal of nitrogen from wastewater, but its application is hindered by its unstable operation under adverse environmental conditions, such as those presented by low temperatures, high organic matter concentrations, or the presence of toxic substances. Granules and microbial entrapment technology can effectively retain and enrich microbial assemblages in reactors to improve operating efficiency and reactor stability. The carriers can also protect the reactor's internal microorganisms from interference from the external environment. This article critically reviews the existing literature on autotrophic nitrogen removal systems using immobilization technology. We focus our discussion on the natural aggregation process (granulation) and entrapment technology. The selection of carrier materials and entrapment methods are identified and described in detail and the mechanisms through which entrapment technology protects microorganisms are analyzed. This review will provide a better understanding of the mechanisms through which immobilization operates and the prospects for immobilization technology to be applied in autotrophic nitrogen removal systems.

Oxi-HA/ADH Hydrogels: A Novel Approach in Tissue Engineering and Regenerative Medicine

Hyaluronic acid (HA) is a natural polyelectrolyte abundant in mammalian connective tissues, such as cartilage and skin. Both endogenous and exogenous HA produced by fermentation have similar physicochemical, rheological, and biological properties, leading to medical and dermo-cosmetic products. Chemical modifications such as cross-linking or conjugation in target groups of the HA molecule improve its properties and in vivo stability, expanding its applications. Currently, HA-based scaffolds and matrices are of great interest in tissue engineering and regenerative medicine. However, the partial oxidation of the proximal hydroxyl groups in HA to electrophilic aldehydes mediated by periodate is still rarely investigated. The introduced aldehyde groups in the HA backbone allow spontaneous cross-linking with adipic dihydrazide (ADH), thermosensitivity, and noncytotoxicity to the hydrogels, which are advantageous for medical applications. This review provides an overview of the physicochemical properties of HA and its usual chemical modifications to better understand oxi-HA/ADH hydrogels, their functional properties modulated by the oxidation degree and ADH concentration, and the current clinical research. Finally, it discusses the development of biomaterials based on oxi-HA/ADH as a novel approach in tissue engineering and regenerative medicine.

Single-Step Synthesis of Alginate Microgels Enveloped with a Covalent Polymeric Shell: A Simple Way to Protect Encapsulated Cells

Microgels of biopolymers such as alginate are widely used to encapsulate cells and other biological payloads. Alginate is an attractive material for cell encapsulation because it is nontoxic and convenient: spherical alginate gels are easily created by contacting aqueous droplets of sodium alginate with divalent cations such as Ca²⁺. Alginate chains in the gel become cross-linked by Ca²⁺ cations into a 3-D network. When alginate gels are placed in a buffer, however, the Ca²⁺ cross-links are eliminated by exchange with Na⁺, thereby weakening and degrading the gels. With time, encapsulated cells are released into the external solution. Here, we describe a simple solution to the above problem, which involves forming alginate gels enveloped by a thin shell of a covalently cross-linked gel. The shell is formed via free-radical polymerization using conventional monomers such as acrylamide (AAm) or acrylate derivatives, including polyethylene glycol diacrylate (PEGDA). The entire process is performed in a single step at room temperature (or 37 °C) under mild, aqueous conditions. It involves combining the alginate solution with a radical initiator, which is then introduced as droplets into a reservoir containing Ca²⁺ and monomers. Within minutes of either simple incubation or exposure to ultraviolet (UV) light, the droplets are converted into alginate-polymer microcapsules with a core of alginate and a shell of the polymer (AAm or PEGDA). The microcapsules are mechanically more robust than conventional alginate/Ca²⁺ microgels, and while the latter swell and degrade when placed in buffers or in chelators like sodium citrate, the former remain stable under all conditions. We encapsulate both bacteria and mammalian cells in these microcapsules and find that the cells remain viable and functional over time. Lastly, a variation of the synthesis technique is shown to generate multilayered microcapsules with a liquid core surrounded by concentric layers of alginate and AAm gels. We anticipate that the approaches presented here will find application in a variety of areas including cell therapies, artificial cells, drug delivery, and tissue engineering.

A Constitutive Model for Alginate-Based Double Network Hydrogels Cross-Linked by Mono-, Di-, and Trivalent Cations

In this contribution, a micro-mechanically based constitutive model is proposed to describe the nonlinear inelastic rubber-like features of alginate-based double network hydrogel cross-linked via various counterions. To this end, the lengthening of the polysaccharide polymer chain after a fully stretched state is characterized. A polymer chain is firstly considered behaving entropically up to the fully stretched state. Then, enthalpic behavior is accounted for concerning the following lengthening. To calculate enthalpic behavior, the macroscopic material properties, such as elastic modulus, are integrated into the proposed model. Thus, a new energy concept for a polymer chain is proposed. The model is constituted by the proposed energy concept, the network decomposition model, the Arruda–Boyce eight chain model and the network alteration theory. The model is compared against the cyclic tensile test data of alginate-based double network hydrogels cross-linked via mono-, di-, and trivalent cations. Good agreement between the model and experiments is obtained.

Medical Applications Based on Supramolecular Self-Assembled Materials From Tannic Acid

Polyphenol, characterized by various phenolic rings in the chemical structure and an abundance in nature, can be extracted from vegetables, grains, chocolates, fruits, tea, legumes, and seeds, among other sources. Tannic acid (TA), a classical polyphenol with a specific chemical structure, has been widely used in biomedicine because of its outstanding biocompatibility and antibacterial and antioxidant properties. TA has tunable interactions with various materials that are widely distributed in the body, such as proteins, polysaccharides, and glycoproteins, through multimodes including hydrogen bonding, hydrophobic interactions, and charge interactions, assisting TA as important building blocks in the supramolecular self-assembled materials. This review summarizes the recent immense progress in supramolecular self-assembled materials using TA as building blocks to generate different materials such as hydrogels, nanoparticles/microparticles, hollow capsules, and coating films, with enormous potential medical applications including drug delivery, tumor diagnosis and treatment, bone tissue engineering, biofunctional membrane material, and the treatment of certain diseases. Furthermore, we discuss the challenges and developmental prospects of supramolecular self-assembly nanomaterials based on TA.

Technologies and Formulation Design of Polysaccharide-Based Hydrogels for Drug Delivery

Polysaccharide-based hydrogel particles (PbHPs) are very promising carriers aiming to control and target the release of drugs with different physico-chemical properties. Such delivery systems can offer benefits through the proper encapsulation of many drugs (non-steroidal and steroidal anti-inflammatory drugs, antibiotics, etc) ensuring their proper release and targeting. This review discusses the different phases involved in the production of PbHPs in pharmaceutical technology, such as droplet formation (SOL phase), sol-gel transition of the droplets (GEL phase) and drying, as well as the different methods available for droplet production with a special focus on prilling technique. In addition, an overview of the various droplet gelation methods with particular emphasis on ionic cross-linking of several polysaccharides enabling the formation of particles with inner highly porous network or nanofibrillar structure is given. Moreover, a detailed survey of the different inner texture, in xerogels, cryogels or aerogels, each with specific arrangement and properties, which can be obtained with different drying methods, is presented. Various case studies are reported to highlight the most appropriate application of such systems in pharmaceutical field. We also describe the challenges to be faced for the breakthrough towards clinic studies and, finally, the market, focusing on the useful approach of safety-by-design (SbD).

Hydrogel Nanoparticle as a Functional Coating Layer in Biosensing, Tissue Engineering, and Drug Delivery

The development of functional coating materials has resulted in many breakthroughs in the discovery of energy, environmental, and biomedical applications. Responsive polymeric hydrogels are an example of smart coating materials due to their stimuli-responsive characteristics upon changes in their local environment. This review focuses on the introduction of hydrogel nanoparticles and their applications in functional layers as responsive coating materials. Hydrogels are explained by the composition of cross-links and monomers used for preparation. In particular, an important class of responsive hydrogels, that is, nanosized hydrogel particles (nanogels), are described for thee synthesis, modification, and application in assembly of functional coating layers. Finally, nanogel functional layers for biological applications will be discussed with recent advances in biosensing, tissue engineering, and drug delivery.

Microfluidic Generation of All-Aqueous Double and Triple Emulsions

Higher order emulsions are used in a variety of different applications in biomedicine, biological studies, cosmetics, and the food industry. Conventional droplet generation platforms for making higher order emulsions use organic solvents as the continuous phase, which is not biocompatible and as a result, further washing steps are required to remove the toxic continuous phase. Recently, droplet generation based on aqueous two-phase systems (ATPS) has emerged in the field of droplet microfluidics due to their intrinsic biocompatibility. Here, a platform to generate all-aqueous double and triple emulsions by introducing pressure-driven flows inside a microfluidic hybrid device is presented. This system uses a conventional microfluidic flow-focusing geometry coupled with a coaxial microneedle and a glass capillary embedded in flow-focusing junctions. The configuration of the hybrid device enables the focusing of two coaxial two-phase streams, which helps to avoid commonly observed channel-wetting problems. It is shown that this approach achieves the fabrication of higher-order emulsions in a poly(dimethylsiloxane)-based microfluidic device, and controls the structure of the all-aqueous emulsions. This hybrid microfluidic approach allows for facile higher-order biocompatible emulsion formation, and it is anticipated that this platform will find utility for generating biocompatible materials for various biotechnological applications.

Cell-free protein synthesis in hydrogel materials

Fabrication of macro-scale polysaccharide, proteinaceous, micellular and covalently crosslinked hydrogels for housing cell-free protein synthesis reactions.

Current applications of Colloidal Liquid Aphrons: Predispersed solvent extraction, enzyme immobilization and drug delivery

Colloidal Liquid Aphrons (CLAs) are micron sized discrete spherical solvent droplets formed by the dispersion of polyaphrons into a bulk aqueous phase at a low phase volume ratio where they can be kept homogenously suspended with only minimal agitation. CLAs have high stability due to the presence of a surfactant 'shell' surrounding the solvent core, and possess large surface areas per unit volume for mass transfer due to their small size. Therefore, CLAs are well suited for applications in pre-dispersed solvent extraction (PSE), enzyme immobilization, and have the potential to be used as a drug delivery system. Using PSE, CLAs have been used to remove metals such as Ni²⁺, Cu²⁺, Fe³⁺, Cr³⁺ and Mg²⁺ from dilute streams, separate organic dyes such as Yellow 1 from wastewater, extract succinic and lactic acid, reactively extract phenylalanine, and separate suspensions. CLAs have also been used to immobilize enzymes such as lipase, lysozyme and albumins with cases of superactivity being reported due to the influence of surfactant and solvent interactions with the enzyme. Furthermore, due to their similarity to current drug delivery systems such as microemulsions and hydrogels, and other advantages, CLA systems have the potential to be adapted for drug delivery systems also. This article provides a complete list of the current applications of Colloidal Liquid Aphrons (CLAs) in PSE and enzyme immobilization, and also presents insight into how CLAs can be utilized as a drug delivery method in the future. Finally, this review ends by summarizing potentially interesting research areas to pursue in this field.

Robotic Extrusion of Algae-Laden Hydrogels for Large-Scale Applications

A bioprinting technique for large-scale, custom-printed immobilization of microalgae is developed for potential applications within architecture and the built environment. Alginate-based hydrogels with various rheology modifying polymers and varying water percentages are characterized to establish a window of operation suitable for layer-by-layer deposition on a large scale. Hydrogels formulated with methylcellulose and carrageenan, with water percentages ranging from 80% to 92.5%, demonstrate a dominant viscoelastic solid-like property with G' > G″ and a low phase angle, making them the most suitable for extrusion-based printing. A custom multimaterial pneumatic extrusion system is developed to be attached on the end effector of an industrial multiaxis robot arm, allowing precision-based numerically controlled layered deposition of the viscous hydrogel. The relationship between the various printing parameters, namely air pressure, material viscosity, viscoelasticity, feed rate, printing distance, nozzle diameter, and the speed of printing, are characterized to achieve the desired resolution of the component. Printed prototypes are postcured in CaCl2 via crosslinking. Biocompatibility tests show that cells can survive for 21 days after printing the constructs. To demonstrate the methodology for scale-up, a 1000 × 500 mm fibrous hydrogel panel is additively deposited with 3 different hydrogels with varying water percentages.

Applications of Highly Stretchable and Tough Hydrogels

Stretchable and tough hydrogels have drawn a lot of attention recently. Due to their unique properties, they have great potential in the application in areas such as mechanical sensing, wound healing, and drug delivery. In this review, we will summarize recent developments of stretchable and tough hydrogels in these areas.

Continuous Manufacturing of Recombinant Therapeutic Proteins: Upstream and Downstream Technologies

Continuous biomanufacturing of recombinant therapeutic proteins offers several potential advantages over conventional batch processing, including reduced cost of goods, more flexible and responsive manufacturing facilities, and improved and consistent product quality. Although continuous approaches to various upstream and downstream unit operations have been considered and studied for decades, in recent years interest and application have accelerated. Researchers have achieved increasingly higher levels of process intensification, and have also begun to integrate different continuous unit operations into larger, holistically continuous processes. This review first discusses approaches for continuous cell culture, with a focus on perfusion-enabling cell separation technologies including gravitational, centrifugal, and acoustic settling, as well as filtration-based techniques. We follow with a review of various continuous downstream unit operations, covering categories such as clarification, chromatography, formulation, and viral inactivation and filtration. The review ends by summarizing case studies of integrated and continuous processing as reported in the literature.

Channeled ECM-Based Nanofibrous Hydrogel for Engineering Vascularized Cardiac Tissues

Hydrogels are widely used materials for cardiac tissue engineering. However, once the cells are encapsulated within hydrogels, mass transfer to the core of the engineered tissue is limited, and cell viability is compromised. Here, we report on the development of a channeled ECM-based nanofibrous hydrogel for engineering vascularized cardiac tissues. An omentum hydrogel was mixed with cardiac cells, patterned to create channels and closed, and then seeded with endothelial cells to form open cellular lumens. A mathematical model was used to evaluate the necessity of the channels for maintaining cell viability and the true potential of the vascularized hydrogel to form a viable cardiac patch was studied.

Rational Design of Microfabricated Electroconductive Hydrogels for Biomedical Applications

Electroconductive hydrogels (ECHs) are highly hydrated 3D networks generated through the incorporation of conductive polymers, nanoparticles, and other conductive materials into polymeric hydrogels. ECHs combine several advantageous properties of inherently conductive materials with the highly tunable physical and biochemical properties of hydrogels. Recently, the development of biocompatible ECHs has been investigated for various biomedical applications, such as tissue engineering, drug delivery, biosensors, flexible electronics, and other implantable medical devices. Several methods for the synthesis of ECHs have been reported, which include the incorporation of electrically conductive materials such as gold and silver nanoparticles, graphene, and carbon nanotubes, as well as various conductive polymers (CPs), such as polyaniline, polypyrrole, and poly(3,4-ethylenedioxyythiophene) into hydrogel networks. Theses electroconductive composite hydrogels can be used as scaffolds with high swellability, tunable mechanical properties, and the capability to support cell growth both in vitro and in vivo. Furthermore, recent advancements in microfabrication techniques such as three dimensional (3D) bioprinting, micropatterning, and electrospinning have led to the development of ECHs with biomimetic microarchitectures that reproduce the characteristics of the native extracellular matrix (ECM). In addition, smart ECHs with controlled structures and healing properties have also been engineered into devices with prolonged half-lives and increased durability. The combination of sophisticated synthesis chemistries and modern microfabrication techniques have led to engineer smart ECHs with advanced architectures, geometries, and functionalities that are being increasingly used in drug delivery systems, biosensors, tissue engineering, and soft electronics. In this review, we will summarize different strategies to synthesize conductive biomaterials. We will also discuss the advanced microfabrication techniques used to fabricate ECHs with complex 3D architectures, as well as various biomedical applications of microfabricated ECHs.

Photolithographically assembled polyelectrolyte complexes as shape-directing templates for thermoreversible gels

Preparation of soft materials with diverse, customized shapes has been a topic of intense research interest. To this end, we have recently demonstrated photolithographic directed assembly as a strategy for customizing polyelectrolyte complex (PEC) shape. This process uses in situ photopolymerization of an anionic monomer in the presence of a cationic polymer, which drives localized PEC formation at the irradiation sites. Here, we show how such photolithographically assembled PECs can serve as structure-directing templates for tailoring the shapes of other soft materials; namely, thermoreversible gels. These templated hydrogels are prepared by adding a thermogelling polymer (agarose) to the anionic monomer/cationic polymer/photoinitiator precursor solutions so that, upon irradiation, custom-shaped PECs form within agarose gel matrices. Once these PECs are formed, the surrounding agarose gels are melted (through heating) and washed away which, upon returning the samples to room temperature, produces interpenetrating PEC/agarose gel networks with photopatterned shapes and dimensions. Dissolution of these sacrificial PEC templates in concentrated NaCl solutions then generates photolithographically templated agarose gels, whose shapes and dimensions match those of their PEC templates. Besides tuning their shapes and sizes, the mechanical properties of these gels can be easily tailored by varying the initial agarose concentrations used. Moreover, this PEC-templated gel synthesis appears to not adversely affect hydrogel cytocompatibility, suggesting its potential suitability for biological and biomedical applications. Though the present study uses only agarose as the model gel system, this PEC-based strategy for customizing gel shape can likely also be applied to other thermoreversible gel networks (e.g., those based on methylcellulose, poloxamers or thermoresponsive chitosan derivatives) and could have many attractive applications, ranging from drug delivery and tissue engineering, to sensing and soft robotics.

Uptake and release of photosensitizers in a hydrogel for applications in photodynamic therapy: the impact of structural parameters on intrapolymer transport dynamics

In this study a hydrogel is presented that can be used as a carrier and release system for photosensitizers. Because of the high structural variety of photosensitizers, four different substances were analysed. Two porphyrins, 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluene-sulfonate) and sodium meso-tetraphenylporphine-4,4',4'',4'''-tetrasulfonat, eosin y and methylene blue were selected. Uptake and release of these photosensitizers were studied. All photosensitizers were taken up by the hydrogel not depending significantly on the structure of the photosensitizer, and it was possible to load the hydrogels in the μmol g-1 range. Nevertheless, size and pK a value were shown to influence the release behaviour. Finally, the singlet oxygen generation of the photosensitizer after release was demonstrated. The photosensitizer was still highly active and produced a sufficient amount of singlet oxygen.

Acrylic acid grafted tamarind kernel polysaccharide-based hydrogel for bone tissue engineering in absence of any osteo-inducing factors

PURPOSE

With increased life expectancy, disorders in lifestyle and other clinical conditions, and the changes in the connective tissues such as in bone, impose diverse biomedical problems. Cells belong to osteogenic lineages are extremely specific for their surface requirements. Therefore, suitable surfaces are the critical bottle neck for successful bone tissue engineering. This study involves assessment of polysaccharide-based hydrogel which effectively allows growth, differentiation and mineralisation of osteogenic cells even in the absence of osteogenic inducing factors.

MATERIALS AND METHODS

Tamarind Kernel Polysaccharide was grafted with acrylic acid at different mole ratio. The critical parameter, surface morphology for bio application was assessed by SEM. MTT assay has been performed with hydrogels on Saos-2 cells. The biocompatibility and adhesion of different cell lines (F-11, Saos-2, Raw 264.7 and MSCs) on hydrogel surface was performed by Phalloidin and DAPI staining. Further the differentiation, mineralization and expression of different osteogenic markers, ALP assay, Alizarin Red staining and q-PCR was performed.

RESULTS

The hydrogels show highly porous and interconnected pores. MTT assay demonstrates the hydrogel have no cytotoxicity towards Saos-2 cells and are suitable for proliferation of different lineage of cell lines. ALP, Alizarin red staining and q-PCR assay shows that the hydrogel surface enhances the differentiation, mineralization and expression of different osteogenic genes in Saos-2 cells in the absence of any osteogenic inducing factors. Conclusion Synthesized hydrogel surface triggers signalling events towards osteogenesis even in the absence of added growth factors. We proposed that this material can be used for effective bone tissue engineering in vitro at low cost.

Hydrogel and Organogel Formation by Hierarchical Self-Assembly of Cyclic Peptides Nanotubes

Breaking away from the linear structure of previously reported peptide-based gelators, this study reports the first example of gel formation based on the use of cyclic peptides made of alternating d- and l-amino acids, known to self-assemble in solution to form long nanotubes. Herein, a library of cyclic peptides was systemically studied for their gelation properties in various solvents, uncovering key parameters driving both organogel and hydrogel formation. The hierarchical nature of the self-assembly process in water was characterised by a combination of electron microscopy imaging and small-angle X-ray scattering, revealing a porous network of entangled nanofibres composed by the aggregation of several cyclic peptide nanotubes. Rheology measurements then confirmed the formation of soft hydrogels.

Thermoplastic PCL-b-PEG-b-PCL and HDI Polyurethanes for Extrusion-Based 3D-Printing of Tough Hydrogels

Novel tough hydrogel materials are required for 3D-printing applications. Here, a series of thermoplastic polyurethanes (TPUs) based on poly(ɛ-caprolactone)-b-poly(ethylene glycol)-b-poly(ɛ-caprolactone) (PCL-b-PEG-b-PCL) triblock copolymers and hexamethylene diisocyanate (HDI) were developed with PEG contents varying between 30 and 70 mol%. These showed excellent mechanical properties not only when dry, but also when hydrated: TPUs prepared from PCL-b-PEG-b-PCL with PEG of Mn 6 kg/mol (PCL₇-PEG₆-PCL₇) took up 122 wt.% upon hydration and had an E-modulus of 52 ± 10 MPa, a tensile strength of 17 ± 2 MPa, and a strain at break of 1553 ± 155% in the hydrated state. They had a fracture energy of 17976 ± 3011 N/mm² and a high tearing energy of 72 kJ/m². TPUs prepared using PEG with Mn of 10 kg/mol (PCL₅-PEG10-PCL₅) took up 534% water and were more flexible. When wet, they had an E-modulus of 7 ± 2 MPa, a tensile strength of 4 ± 1 MPa, and a strain at break of 147 ± 41%. These hydrogels had a fracture energy of 513 ± 267 N/mm² and a tearing energy of 16 kJ/m². The latter TPU was first extruded into filaments and then processed into designed porous hydrogel structures by 3D-printing. These hydrogels can be used in 3D printing of tissue engineering scaffolds with high fracture toughness.

On the electrical conductivity of alginate hydrogels

Hydrogels have been extensively used in the field of biomedical applications, offering customizable natural, synthetic or hybrid materials, particularly relevant in the field of tissue engineering. In the bioelectronics discipline, hydrogels are promising mainly as sensing platforms with or without encapsulated cells, showing great potential in healthcare and medicine. However, to date there is little data in the literature which characterizes the electrical properties of tissue engineering materials which are relevant to bioelectronics. In this work, we present electrical characterization of alginate hydrogels, a natural polysaccharide, using a four-probe method similar to electrical impedance spectroscopy. The acquired conductance data show distinct frequency-dependent features that change as a function of alginate and crosslinker concentration reflecting ion kinetics inside the measured sample. Furthermore, the presence of NIH 3T3 fibroblasts encapsulated in the hydrogels matrix was found to alter the artificial tissue's electrical properties. The method used provides valuable insight to the frequency-dependent electrical response of the resulting systems. It is hoped that the outcome of this research will be of use in the development of cell/electronic interfaces, possibly toward diagnostic biosensors and therapeutic bioelectronics.

Carrageenan based hydrogels for drug delivery, tissue engineering and wound healing

Carrageenan is a class of naturally occurring sulphated polysaccharides, which is currently a promising candidate in tissue engineering and regenerative medicine as it resemblances native glycosaminoglycans. From pharmaceutical drug formulations to tissue engineered scaffolds, carrageenan has broad range of applications. Here we provide an overview of developing various forms of carrageenan based hydrogels. We focus on how these fabrication processes has an effect on physiochemical properties of the hydrogel. We outline the application of these hydrogels not only pertaining to sustained drug release but also their application in bone and cartilage tissue engineering as well as in wound healing and antimicrobial formulations. Administration of these hydrogels through various routes for drug delivery applications has been critically reviewed. Finally, we conclude by summarizing the current and future outlook that promotes the seaweed-derived polysaccharide as versatile, promising biomaterial for a variety of bioengineering applications.

Photo-Cross-Linked Self-Assembled Poly(ethylene oxide)-Based Hydrogels Containing Hybrid Junctions with Dynamic and Permanent Cross-Links

Homogeneous hydrogels were formed by self-assembly of triblock copolymers via association of small hydrophobic end blocks into micelles bridged by large poly(ethylene oxide) central blocks. A fraction of the end blocks were photo-cross-linkable and could be rapidly cross-linked covalently by in situ UV irradiation. In this manner networks were formed with well-defined chain lengths between homogeneously distributed hybrid micelles that contained both permanent and dynamically cross-linked end blocks. Linear rheology showed a single relaxation mode before in situ irradiation intermediate between those of the individual networks. The presence of transient cross-links decreased the percolation threshold of the network rendered permanent by irradiation and caused a strong increase of the elastic modulus at lower polymer concentrations. Large amplitude oscillation and tensile tests showed significant increase of the fracture strain caused by the dynamic cross-links.

A Cellulose/Laponite Interpenetrated Polymer Network (IPN) Hydrogel: Controllable Double-Network Structure with High Modulus

Laponite XLS™, which is a synthetic clay of nanometric dimensions containing a peptizing agent, has been associated with silanized hydroxypropylmethylcellulose (Si-HPMC) to form, after crosslinking, a novel composite hydrogel. Different protocols of sample preparation were used, leading to different morphologies. A key result was that the storage modulus of Si-HPMC/XLS composite hydrogel could be increased ten times when compared to that of pure Si-HPMC hydrogel using 2 wt % of Laponite. The viscoelastic properties of the composite formulations indicated that chemical and physical network structures co-existed in the Si-HPMC/XLS composite hydrogel. Images that were obtained from confocal laser scanning microscopy using labelled Laponite XLS in the composite hydrogels show two co-continuous areas: red light area and dark area. The tracking of fluorescent microspheres motions in the composite formulations revealed that the red-light area was a dense structure, whereas the dark area was rather loose without aggregated Laponite. This novel special double-network structure facilitates the composite hydrogel to be an adapted biomaterial for specific tissue engineering. Unfortunately, cytotoxicity’s assays suggested that XLS Laponites are cytotoxic at low concentration. This study validates that the hybrid interpenetrated network IPN hydrogel has a high modulus that has adapted for tissue engineering, but the cell’s internalization of Laponites has to be controlled.

Journey into Bone Models: A Review

Bone is a complex tissue with a variety of functions, such as providing mechanical stability for locomotion, protection of the inner organs, mineral homeostasis and haematopoiesis. To fulfil these diverse roles in the human body, bone consists of a multitude of different cells and an extracellular matrix that is mechanically stable, yet flexible at the same time. Unlike most tissues, bone is under constant renewal facilitated by a coordinated interaction of bone-forming and bone-resorbing cells. It is thus challenging to recreate bone in its complexity in vitro and most current models rather focus on certain aspects of bone biology that are of relevance for the research question addressed. In addition, animal models are still regarded as the gold-standard in the context of bone biology and pathology, especially for the development of novel treatment strategies. However, species-specific differences impede the translation of findings from animal models to humans. The current review summarizes and discusses the latest developments in bone tissue engineering and organoid culture including suitable cell sources, extracellular matrices and microfluidic bioreactor systems. With available technology in mind, a best possible bone model will be hypothesized. Furthermore, the future need and application of such a complex model will be discussed.