Bioreactor-free tissue engineering: directed tissue assembly by centrifugal casting

Construction of PLGA Porous Microsphere-Based Artificial Pancreatic Islets Assisted by the Cell Centrifugation Perfusion Technique

Pancreatic islet transplantation is a promising treatment that could potentially reverse diabetes, but its clinical applicability is severely limited by a shortage of organ donors. Various cell loading approaches using polymeric porous microspheres (PMs) have been developed for tissue regeneration; however, PM-based multicellular artificial pancreatic islets' construction has been scarcely reported. In this study, MIN6 (a mouse insulinoma cell line) and MS1 (a mouse pancreatic islet endothelial cell line) cells were seeded into poly(lactic-co-glycolic acid) (PLGA) PMs via an upgraded centrifugation-based cell perfusion seeding technique invented and patented by our group. Cell morphology, distribution, viability, migration, and proliferation were all evaluated. Results from glucose-stimulated insulin secretion (GSIS) assay and RNA-seq analysis suggested that MIN6 and MS1-loaded PLGA PMs exhibited better glucose responsiveness, which is partly attributable to vascular formation during PM-dependent islet construction. The present study suggests that the PLGA PM-based artificial pancreatic islets may provide an alternative strategy for the potential treatment of diabetes in the future.

A cell adhesion-promoting multi-network 3D printing bio-ink based on natural polysaccharide hydrogel

Due to its high biosafety, gellan gum (GG) hydrogel, a naturally occurring polysaccharide released by microorganisms, is frequently utilized in food and pharmaceuticals. In recent years, like GG, natural polysaccharide-based hydrogels have become increasingly popular in 3D-printed biomedical engineering because of their simplicity of processing, considerable shear thinning characteristic, and minimal pH dependence. To mitigate the negative effects of the GG's high biological inertia, poor cell adhesion, single cross-linked network, and high brittleness. Mesoporous silica nanospheres (MMSN) and Aldehyde-based methacrylated hyaluronic acid (AHAMA) were combined to sulfhydrated GG (TGG) to create a multi-network AHAMA/TGG/MMSN hydrogel in this study. For this composite hydrogel system, the multi-component offers several crosslinking networks: the double bond in AHAMA can be photocrosslinked by activating the photoinitiator, aldehyde groups on its side chain can create Schiff base bonds with MMSN, while TGG can self-curing at room temperature. The AHAMA/TGG/MMSN hydrogel, with a mass ratio of 2:6:1, exhibits good cell adhesion, high strength and elasticity, and great printability. We believe that this innovative multi-network hydrogel has potential uses in tissue regeneration and biomedical engineering.

Hyaluronic acid-based clinical biomaterials derived for cell and molecule delivery in regenerative medicine

The development of injectable and biocompatible vehicles for delivery, retention, growth, and differentiation of stem cells is of paramount importance for regenerative medicine. For cell therapy and the development of clinical combination products, we created a hyaluronan (HA)-based synthetic extracellular matrix (sECM) that provides highly reproducible, manufacturable, approvable, and affordable biomaterials. The composition of the sECM can be customized for use with progenitor and mature cell populations obtained from skin, fat, liver, heart, muscle, bone, cartilage, nerves, and other tissues. This overview describes the design criteria for "living" HA derivatives, and the many uses of this in situ crosslinkable HA-based sECM hydrogel for three-dimensional (3-D) culture of cells in vitro and translational use in vivo. Recent advances allow rapid expansion and recovery of cells in 3-D, and the bioprinting of engineered tissue constructs. The uses of HA-derived sECMs for cell and molecule delivery in vivo will be reviewed, including applications in cancer biology and tumor imaging.

Hyaluronic acid hydrogels for biomedical applications

Hyaluronic acid (HA), an immunoneutral polysaccharide that is ubiquitous in the human body, is crucial for many cellular and tissue functions and has been in clinical use for over thirty years. When chemically modified, HA can be transformed into many physical forms-viscoelastic solutions, soft or stiff hydrogels, electrospun fibers, non-woven meshes, macroporous and fibrillar sponges, flexible sheets, and nanoparticulate fluids-for use in a range of preclinical and clinical settings. Many of these forms are derived from the chemical crosslinking of pendant reactive groups by addition/condensation chemistry or by radical polymerization. Clinical products for cell therapy and regenerative medicine require crosslinking chemistry that is compatible with the encapsulation of cells and injection into tissues. Moreover, an injectable clinical biomaterial must meet marketing, regulatory, and financial constraints to provide affordable products that can be approved, deployed to the clinic, and used by physicians. Many HA-derived hydrogels meet these criteria, and can deliver cells and therapeutic agents for tissue repair and regeneration. This progress report covers both basic concepts and recent advances in the development of HA-based hydrogels for biomedical applications.

Photocrosslinkable hyaluronan-gelatin hydrogels for two-step bioprinting

Bioprinting by the codeposition of cells and biomaterials is constrained by the availability of printable materials. Herein we describe a novel macromonomer, a new two-step photocrosslinking strategy, and the use of a simple rapid prototyping system to print a proof-of-concept tubular construct. First, we synthesized the methacrylated ethanolamide derivative of gelatin (GE-MA). Second, partial photochemical cocrosslinking of GE-MA with methacrylated hyaluronic acid (HA-MA) gave an extrudable gel-like fluid. Third, the new HA-MA:GE-MA hydrogels were biocompatible, supporting cell attachment and proliferation of HepG2 C3A, Int-407, and NIH 3T3 cells in vitro. Moreover, hydrogels injected subcutaneously in nude mice produced no inflammatory response. Fourth, using the Fab@Home printing system, we printed a tubular tissue construct. The partially crosslinked hydrogels were extruded from a syringe into a designed base layer, and irradiated again to create a firmer structure. The computer-driven protocol was iterated to complete a cellularized tubular construct with a cell-free core and a cell-free structural halo. Cells encapsulated within this printed construct were viable in culture, and gradually remodeled the synthetic extracellular matrix environment to a naturally secreted extracellular matrix. This two-step photocrosslinkable biomaterial addresses an unmet need for printable hydrogels useful in tissue engineering.

Bioprinting vessel-like constructs using hyaluronan hydrogels crosslinked with tetrahedral polyethylene glycol tetracrylates

Bioprinting enables deposition of cells and biomaterials into spatial orientations and complexities that mirror physiologically relevant geometries. To facilitate the development of bioartificial vessel-like grafts, two four-armed polyethylene glycol (PEG) derivatives with different PEG chain lengths, TetraPEG8 and TetraPEG13, were synthesized from tetrahedral pentaerythritol derivatives. The TetraPEGs are unique multi-armed PEGs with a compact and symmetrical core. The TetraPEGs were converted to tetra-acrylate derivatives (TetraPAcs) which were used in turn to co-crosslink thiolated hyaluronic acid and gelatin derivatives into extrudable hydrogels for printing tissue constructs. First, the hydrogels produced by TetraPAc crosslinking showed significantly higher shear storage moduli when compared to PEG diacrylate (PEGDA)-crosslinked synthetic extracellular matrices (sECMs) of similar composition. These stiffer hydrogels have rheological properties more suited to bioprinting high-density cell suspensions. Second, TetraPAc-crosslinked sECMs were equivalent or superior to PEGDA-crosslinked gels in supporting cell growth and proliferation. Third, the TetraPac sECMs were employed in a proof-of-concept experiment by encapsulation of NIH 3T3 cells in sausage-like hydrogel macrofilaments. These macrofilaments were then printed into tubular tissue constructs by layer-by-layer deposition using the Fab@Home printing system. LIVE/DEAD viability/cytotoxicity-stained cross-sectional images showed the bioprinted cell structures to be viable in culture for up to 4 weeks with little evidence of cell death. Thus, biofabrication of cell suspensions in TetraPAc sECMs demonstrates the feasibility of building bioartificial blood vessel-like constructs for research and potentially clinical uses.

The generation of 3-D tissue models based on hyaluronan hydrogel-coated microcarriers within a rotating wall vessel bioreactor

With the increasing necessity for functional tissue- and organ equivalents in the clinic, the optimization of techniques for the in vitro generation of organotypic structures that closely resemble the native tissue is of paramount importance. The engineering of a variety of highly differentiated tissues has been achieved using the rotating wall vessel (RWV) bioreactor technology, which is an optimized suspension culture allowing cells to grow in three-dimensions (3-D). However, certain cell types require the use of scaffolds, such as collagen-coated microcarrier beads, for optimal growth and differentiation in the RWV. Removal of the 3-D structures from the microcarriers involves enzymatic treatment, which disrupts the delicate 3-D architecture and makes it inapplicable for potential implantation. Therefore, we designed a microcarrier bead coated with a synthetic extracellular matrix (ECM) composed of a disulfide-crosslinked hyaluronan and gelatin hydrogel for 3-D tissue engineering, that allows for enzyme-free cell detachment under mild reductive conditions (i.e. by a thiol-disulfide exchange reaction). The ECM-coated beads (ECB) served as scaffold to culture human intestinal epithelial cells (Int-407) in the RWV, which formed viable multi-layered cell aggregates and expressed epithelial differentiation markers. The cell aggregates remained viable following dissociation from the microcarriers, and could be returned to the RWV bioreactor for further culturing into bead-free tissue assemblies. The developed ECBs thus offer the potential to generate scaffold-free 3-D tissue assemblies, which could further be explored for tissue replacement and remodeling.

Biofabrication: a 21st century manufacturing paradigm

Biofabrication can be defined as the production of complex living and non-living biological products from raw materials such as living cells, molecules, extracellular matrices, and biomaterials. Cell and developmental biology, biomaterials science, and mechanical engineering are the main disciplines contributing to the emergence of biofabrication technology. The industrial potential of biofabrication technology is far beyond the traditional medically oriented tissue engineering and organ printing and, in the short term, it is essential for developing potentially highly predictive human cell- and tissue-based technologies for drug discovery, drug toxicity, environmental toxicology assays, and complex in vitro models of human development and diseases. In the long term, biofabrication can also contribute to the development of novel biotechnologies for sustainable energy production in the future biofuel industry and dramatically transform traditional animal-based agriculture by inventing 'animal-free' food, leather, and fur products. Thus, the broad spectrum of potential applications and rapidly growing arsenal of biofabrication methods strongly suggests that biofabrication can become a dominant technological platform and new paradigm for 21st century manufacturing. The main objectives of this review are defining biofabrication, outlining the most essential disciplines critical for emergence of this field, analysis of the evolving arsenal of biofabrication technologies and their potential practical applications, as well as a discussion of the common challenges being faced by biofabrication technologies, and the necessary conditions for the development of a global biofabrication research community and commercially successful biofabrication industry.

Improved oxygen diffusion and mechanical aggregation of tumor colonies in a novel stirred mini-bioreactor

The feasibility of a novel stirred bioreactor, the rotating aerial disk (RAD) design, was tested in this study. The novelty lies in its method of medium recirculation by convective airflow using a non-contact planar disc, a variation on a physically defined theoretical model. Computational predictions of improved oxygenation were confirmed by increases in measured dissolved oxygen, even at Reynolds numbers (100-200) where flow is mostly laminar. EL-4 mouse lymphoma cells grown for the first time as suspension cultures in the RAD bioreactor, were mechanically re-organization into dense, circular three-dimensional colonies (diameter 3-5 mm, thickness 5-800 microm), more rapidly than we have observed previously. Cell proliferation in the RAD vessels was similar to static cultures, although lactate production from glucose was significantly lower, suggesting a shift toward aerobic glycolysis. This possible reversal of the 'Warburg effect' was accompanied by a decrease in mitochondrial activity, perhaps reflecting a more quiescent cytoplasmic state. The RAD device may be useful as scalable, three-dimensional solid tumor model under more physiological conditions then static culture.