Preparation of cell-enclosing microcapsules through photopolymerization of methacrylated alginate solution triggered by irradiation with visible light

Sodium alginate-hydrogel coatings on extracorporeal membrane oxygenation for anticoagulation

Thromboembolism caused by the use of extracorporeal membrane oxygenation (ECMO) remains common among patients with existing heart diseases and contributes to significant morbidity and mortality during the COVID-19 pandemic. Various surface modification strategies have been proposed, showing that the methacrylated alginate (MA-SA) hydrogel layer is transparent, which aids the observation of the thromboembolism from the inner wall of the tubing. In the combined dynamic and static blood of ECMO tubing inner surface in vitro experiments, it was also demonstrated that the adhesion of blood clots to the surface of vessels was remarkably reduced, and the MA-SA-based hydrogel coating could significantly prolong the activated partial thrombin time and block the endogenous coagulation. The favorable properties of natural polysaccharides of hydrogel coatings make them the best surface material choices to be applied for blood-contacting medical devices and significantly improve anticoagulant performance.

Alginate Hydrogels with Tuneable Properties

Alginate is a material that has many biomedical applications due to its low toxicity and a variety of favourable physical properties. In particular, the ease in which hydrogels are formed from alginate and the variety of mechanical behaviours that can be imparted on the hydrogels, by understanding alginate chemistry and intuitive design, has made alginate the most widely investigated polysaccharide used for tissue engineering. This chapter provides an overview of alginate, from how the source and natural variations in composition can influence mechanical properties of alginate hydrogels, through to some innovative techniques used to modify and functionalise the hydrogels designed specifically for cell-based therapies. The main focus is on how these strategies of understanding and controlling the chemistry of alginates have resulted in the development of hydrogels that can be tuned to deliver the physical behaviours required for successful application. This will also highlight how research on the physicochemical properties has helped alginate evolve from a structural polysaccharide in brown seaweed into a highly tuneable, multifunctional, smart biomaterial, which is likely to find further biomedical applications in the future.

Dual Crosslinked Methacrylated Alginate Hydrogel Micron Fibers and Tissue Constructs for Cell Biology

As an important natural polysaccharide biomaterial from marine organisms, alginate and its derivatives have shown great potential in the fabrication of biomedical materials such as tissue engineering, cell biology, drug delivery, and pharmaceuticals due to their excellent biological activity and controllable physicochemical properties. Ionic crosslinking is the most common method used in the preparation of alginate-based biomaterials, but ionic crosslinked alginate hydrogels are prone to decompose in physiological solution, which hinders their applications in biomedical fields. In this study, dual crosslinked alginate hydrogel microfibers were prepared for the first time. The ionic crosslinked methacrylated alginate (Alg-MA) hydrogel microfibers fabricated by Microfluidic Fabrication (MFF) system were exposed to ultraviolet (UV) light and covalent crosslink between methacrylate groups avoided the fracture of dual crosslinked macromolecular chains in organizational environment. The chemical structures, swelling ratio, mechanical performance, and stability were investigated. Cell-encapsulated dual crosslinked Alg-MA hydrogel microfibers were fabricated to explore the application in tissue engineering for the first time. The hydrogel microfibers provided an excellent 3D distribution and growth conditions for cells. Cell-encapsulated Alg-MA microfibers scaffolds with functional 3D tissue structures were developed which possessed great potential in the production of next-generation scaffolds for tissue engineering and regenerative medicine.

Gelatin/Hyaluronic Acid Content in Hydrogels Obtained through Blue Light-Induced Gelation Affects Hydrogel Properties and Adipose Stem Cell Behaviors

Composite hydrogels of hyaluronic acid and gelatin attract great attention in biomedical fields. In particular, the composite hydrogels obtained through processes that are mild for cells are useful in tissue engineering. In this study, hyaluronic acid/gelatin composite hydrogels obtained through a blue light-induced gelation that is mild for mammalian cells were studied for the effect of the content of each polymer in the precursor solution on gelation, properties of resultant hydrogels, and behaviors of human adipose stem cells laden in the hydrogels. Control of the content enabled gelation in less than 20 s, and also enabled hydrogels to be obtained with 0.5-1.2 kPa Young's modulus. Human adipose stem cells were more elongated in hydrogels with a higher rather than lower content of hyaluronic acid. Stem cell marker genes, Nanog, Oct4, and Sox2, were expressed more in the cells in the composite hydrogels with a higher content of hyaluronic acid compared with those in the hydrogel composed of gelatin alone and on tissue culture dishes. These results are useful for designing conditions for using gelatin/hyaluronic acid composite hydrogels obtained through blue light-induced gelation suitable for tissue engineering applications.

A straightforward method for quantification of vinyl functionalized water soluble alginates via 13C-NMR spectroscopy

Alginates are fairly abundant in nature and possess many interesting properties, including their biocompatibility and ability to absorb large amounts of water. Hence, increasing interest in their derivatization has been observed and the determination of the number of newly introduced functionalities has become a key issue. For this purpose, literature generally reports on conventional ¹H-NMR spectra, typically recorded at elevated temperatures and/or after hydrolysis of the alginate to circumvent line broadening effects resulting from the high viscosity. The present work reports on the modification of alginate with methacrylate functionalities and determination of the resulting degree of substitution (DS), i.e. the number of introduced methacrylate moieties relative to the initial amount of hydroxyl groups along the alginate backbone, via NMR spectroscopy. Freeze-drying and low power water presaturation were applied to improve the quality of the ¹H NMR spectra. Nevertheless, it remains a qualitative method, to be used only for mutual comparisons of samples. A new and accurate method for DS determination of methacrylated alginates, based on ¹³C-NMR spectroscopy, is proposed. Quantitative ¹³C-NMR spectra were recorded with reduced measuring times by addition of a paramagnetic relaxation agent. The proposed method will also be applicable for other water-soluble functionalized alginates and polysaccharides in general.

Recent Strategies in Extrusion-Based Three-Dimensional Cell Printing toward Organ Biofabrication

Reconstructing human organs is one of the ultimate goals of the medical industry. Organ printing utilizing three-dimensional cell printing technology to fabricate artificial living organ equivalents has shed light on the advancement of this field into a new era. Among three currently applied techniques (inkjet, laser-assisted, and extrusion-based), extrusion-based cell printing (ECP) has evoked the majority of interest due to its low cost, wide range of applicable materials, and ease of spatial and depositional controllability. Major challenges in organ reconstruction include difficulties in precisely fabricating complex structural features, creating perfusable and functional vasculatures, and mimicking biophysical and biochemical characteristics in the printed constructs. In this review, we describe the merits and limitations of ECP for organ fabrication and discuss its recent advances aimed at overcoming these challenges. In addition, we delineate the expected future techniques for printing live tissue or organ substitutes.

Dual-crosslinked homogeneous alginate microspheres for mesenchymal stem cell encapsulation

A smart hydrogel material was used in combination with custom microfluidic devices (MFDs) to create microspheres for human mesenchymal stem cell (MSC) encapsulation. Methods for fabricating homogeneous stimuli-responsive microspheres for MSC encapsulation and cell delivery have gained interest to increase viability and manipulate microencapsulation within microspheres 10-1000 µm in diameter. Herein, MFDs were combined with non-toxic smart hydrogel materials to tune both the size and mechanics of the microspheres. Traditional hydrogels have a single input/stimulus for crosslinking, utilize potentially toxic ultraviolet radiation, and fail to mimic surrounding musculoskeletal tissue mechanics. Thus, it is highly beneficial to encapsulate MSCs inside a mechanically-stable microsphere made from naturally-derived materials. The objectives of this research were to optimize microsphere fabrication techniques using custom microfluidic devices (MFDs), and to encapsulate viable MSCs within visible-light crosslinked smart-alginate microspheres, with tunable mechanical properties. Microsphere production was characterized optically, and MSC viability, post-encapsulation, was verified using a standard florescence assay. Cell viability was maintained in chemically-modified alginate homogenous microspheres post encapsulation, and after subsequent crosslinking via green light exposure.

An in-situ photocrosslinking microfluidic technique to generate non-spherical, cytocompatible, degradable, monodisperse alginate microgels for chondrocyte encapsulation

Alginate microgels are widely generated by ionic crosslinking methods, but this method has limitations in controlling the microgel degradation and generating non-spherical microgels. By employing oxidized methacrylated alginate (OMA) that is degradable and photocrosslinkable, we have successfully photocrosslinked monodisperse OMA microgels and demonstrated the feasibility to generate discoid alginate microgels. However, several technical issues obstructed our opto-microfluidic method from being a useful technique. Here, we further characterized and optimized this method. Monodisperse discoid OMA microgels with good shape consistency were, for the first time, generated. The curability of OMA microgels was characterized as the macromer concentration varied from 2% to 10%, and the minimum required photoinitiator (VA-086) concentrations were determined. The effects of crosslinking density and the presence of ions in the storage solution on swelling of OMA hydrogels were identified to give insights into accurate controlling of the microgel size. A much quicker degradation rate (within three weeks) compared to ionically crosslinked alginate hydrogels was indirectly identified by quantifying the elastic modulus using atomic force microscopy. The viability of encapsulated chondrocytes in OMA microgels formed by this method was higher than those from other existing methods, demonstrating its favorable cytocompatibility. It was found that the oxygen tension played a critical role in both the curability of microgels and the cytocompatibility of this technique. We also summarize common practical issues and provide related solutions and/or operational suggestions. By this method, OMA microgels are expected to be valuable alternatives to traditional ionically crosslinked alginate microgels in drug delivery, tissue engineering, and single cell analysis areas due to their multiple favorable properties.

Synthesis and evaluation of dual crosslinked alginate microbeads

Alginate hydrogels have been investigated for a broad variety of medical applications. The ability to assemble hydrogels at neutral pH and mild temperatures makes alginate a popular choice for the encapsulation and delivery of cells and proteins. Alginate has been studied extensively for the delivery of islets as a treatment for type 1 diabetes. However, poor stability of the encapsulation systems after implantation remains a challenge. In this paper, alginate was modified with 2-aminoethyl methacrylate hydrochloride (AEMA) to introduce groups that can be photoactivated to generate covalent bonds. This enabled formation of dual crosslinked structure upon exposure to ultraviolet light following initial ionic crosslinking into bead structures. The degree of methacrylation was varied and in vitro stability, long term swelling, and cell viability examined. At low levels of the methacrylation, the beads could be formed by first ionic crosslinks followed by exposure to ultraviolet light to generate covalent bonds. The methacrylated alginate resulted in more stable beads and cells were viable following encapsulation. Alginate microbeads, ionic (unmodified) and dual crosslinked, were implanted into a rat omentum pouch model. Implantation was performed with a local injection of 100 µl of 50 µg/ml of Lipopolysaccharide (LPS) to stimulate a robust inflammatory challenge in vivo. Implants were retrieved at 1 and 3 weeks for analysis. The unmodified alginate microbeads had all failed by week 1, whereas the dual-crosslinked alginate microbeads remained stable up through 3 weeks. The modified alginate microbeads may provide a more stable alternative to current alginate-based systems for cell encapsulation.

STATEMENT OF SIGNIFICANCE

Alginate, a naturally occurring polysaccharide, has been used for cell encapsulation to prevent graft rejection of cell transplants for people with type I diabetes. Although some success has been observed in clinical trials, the lack of reproducibility and failure to reach insulin dependence for longer periods of time indicates the need for improvements in the procedure. A major requirement for the long-term function of alginate encapsulated cells is the mechanical stability of microcapsules. Insufficient mechanical integrity of the capsules can lead to immunological reactions in the recipients. In this work, alginate was modified to allow photoactivatable groups in order to allow formation of covalent crosslinks in addition to ionic crosslinking. The dual crosslinking design prevents capsule breakdown following implantation in vivo.

Anticancer Therapeutic Alginate-Based Tissue Sealants for Lung Repair

Injury to the connective tissue that lines the lung, the pleura, or the lung itself can occur from many causes including trauma or surgery, as well as lung diseases or cancers. To address current limitations for patching lung injuries, to stop air or fluid leaks, an adherent hydrogel sealant patch system was developed, based on methacrylated alginate (AMA) and AMA dialdehyde (AMA-DA) blends, which is capable of sealing damaged tissues and sustaining physiological pressures. Methacrylation of alginate hydroxyl groups rendered the polysaccharide capable of photo-cross-linking when mixed with an eosin Y-based photoinitiator system and exposed to visible green light. Oxidation of alginate yields functional aldehyde groups capable of imine bond formation with proteins found in many tissues. The alginate-based patch system was rigorously tested on a custom burst pressure testing device. Blending of nonoxidized material with oxidized (aldehyde modified) alginates yielded patches with improved burst pressure performance and decreased delamination as compared with pure AMA. Human mesothelial cell (MeT-5A) viability and cytotoxicity were retained when cultured with the hydrogel patches. The release and bioactivity of doxorubicin-encapsulated submicrospheres enabled the fabrication of drug-eluting adhesive patches and were effective in decreasing human lung cancer cell (A549) viability.

In-situ photopolymerization of monodisperse and discoid oxidized methacrylated alginate microgels in a microfluidic channel

We present a simple microfluidic technique to in-situ photopolymerize (by 365 nm ultraviolet) monodisperse oxidized methacrylated alginate (OMA) microgels using a photoinitiator (VA-086). By this technique, we generated monodisperse spherical OMA beads and discoid non-spherical beads with better shape consistency than ionic crosslinking methods do. We found that a high monomer concentration (8 w/v %), a high photoinitiator concentration (1.5 w/v %), and absence of oxygen are critical factors to cure OMA microgels. This photopolymerizing method is an alternative to current methods to form alginate microgels and is a simpler approach to generate non-spherical alginate microgels.