Hydrogels, due to their unique biocompatibility, flexible methods of synthesis, range of constituents, and desirable physical characteristics, have been the material of choice for many applications in regenerative medicine. They can serve as scaffolds that provide structural integrity to tissue constructs, control drug and protein delivery to tissues and cultures, and serve as adhesives or barriers between tissue and material surfaces. In this work, the properties of hydrogels that are important for tissue engineering applications and the inherent material design constraints and challenges are discussed. Recent research involving several different hydrogels polymerized from a variety of synthetic and natural monomers using typical and novel synthetic methods are highlighted. Finally, special attention is given to the microfabrication techniques that are currently resulting in important advances in the field.

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Hydrogels in regenerative medicine

Methods for culturing mammalian cells ex vivo are increasingly needed to study cell and tissue physiology and to grow replacement tissue for regenerative medicine. Two-dimensional culture has been the paradigm for typical in vitro cell culture; however, it has been demonstrated that cells behave more natively when cultured in three-dimensional environments. Permissive, synthetic hydrogels and promoting, natural hydrogels have become popular as three-dimensional cell culture platforms; yet, both of these systems possess limitations. In this perspective, we discuss the use of both synthetic and natural hydrogels as scaffolds for three-dimensional cell culture as well as synthetic hydrogels that incorporate sophisticated biochemical and mechanical cues as mimics of the native extracellular matrix. Ultimately, advances in synthetic-biologic hydrogel hybrids are needed to provide robust platforms for investigating cell physiology and fabricating tissue outside of the organism.

/pdf/hydrogels-as-extracellular-matrix-mimics-for-3d-cell-culture-1i5i1jyboa.pdf

Hydrogels as Extracellular Matrix Mimics for 3D Cell Culture

Additive manufacturing processes such as 3D printing use time-consuming, stepwise layer-by-layer approaches to object fabrication. We demonstrate the continuous generation of monolithic polymeric parts up to tens of centimeters in size with feature resolution below 100 micrometers. Continuous liquid interface production is achieved with an oxygen-permeable window below the ultraviolet image projection plane, which creates a "dead zone" (persistent liquid interface) where photopolymerization is inhibited between the window and the polymerizing part. We delineate critical control parameters and show that complex solid parts can be drawn out of the resin at rates of hundreds of millimeters per hour. These print speeds allow parts to be produced in minutes instead of hours.

http://science.sciencemag.org/content/sci/347/6228/1349.full.pdf

Continuous liquid interface production of 3D objects

https://pure.hw.ac.uk/ws/files/10766137/Melchels2010_Biomaterials_31_24_6121.pdf

A review on stereolithography and its applications in biomedical engineering

Microscale technologies are emerging as powerful tools for tissue engineering and biological studies. In this review, we present an overview of these technologies in various tissue engineering applications, such as for fabricating 3D microfabricated scaffolds, as templates for cell aggregate formation, or for fabricating materials in a spatially regulated manner. In addition, we give examples of the use of microscale technologies for controlling the cellular microenvironment in vitro and for performing high-throughput assays. The use of microfluidics, surface patterning, and patterned cocultures in regulating various aspects of cellular microenvironment is discussed, as well as the application of these technologies in directing cell fate and elucidating the underlying biology. Throughout this review, we will use specific examples where available and will provide trends and future directions in the field.

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Microscale technologies for tissue engineering and biology

Biomaterials for stem cell differentiation

Our ability to create precise, pre-designed, spa- tially patterned biochemical and physical microenviron- ments inside polymer scaffolds could provide a powerful tool in studying progenitor cell behavior and differentiation under biomimetic, three-dimensional (3D) culture condi- tions. We have developed a simple and fast, layer-by-layer microstereolithography system consisting of an ultra-violet light source, a digital micro-mirror masking device, and a conventional computer projector, that allows fabrication of complex internal features along with precise spatial distri- bution of biological factors inside a single scaffold. Photo- crosslinkable poly(ethylene glycol) diacrylates were used as the scaffold material, and murine bone marrow-derived cells were successfully encapsulated or seeded on fibronectin- functionalized scaffolds. Fluorescently-labeled polystyrene microparticles were used to show the capability of this sys- tem to create scaffolds with complex internal architectures and spatial patterns. We demonstrate that precisely con- trolled pore size and shapes can be easily fabricated using a simple, computer-aided process. Our results further indicate that multi-layered scaffolds with spatially distributed fac- tors in the same layer or across different layers can be efficiently manufactured using this technique. These micro- fabricated scaffolds are conducive for osteogenic differenti- ation of marrow-derived stem cells, as indicated by efficient matrix mineralization. © 2006 Wiley Periodicals, Inc. J Biomed Mater Res 77A: 396 - 405, 2006

A digital micro-mirror device-based system for the microfabrication of complex, spatially patterned tissue engineering scaffolds.

Understanding cell behavior inside complex, three-dimensional (3D) microenvironments with controlled spatiotemporal patterning of physical and biochemical factors would provide significant insights into the basic biology of organ development and tissue functions. One of the fundamental limitations in studying such behavior has been the inability to create patterned microenvironments within 3D scaffold structures. Here a simple, layer-by-layer stereolithography (SL) method that can precisely pattern ligands, extracellular-matrix (ECM) components, and growth factors, as well as controlled release particles inside a single scaffold, has been developed. The process also allows fabrication of predesigned internal architectures and porosities. Photocrosslinkable poly(ethylene glycol) dimethacrylate (PEGDMA) was used as the basic structural component of these microfabricated scaffolds. PEG acrylates, covalently modified with the cell adhesive peptide arginine-glycine-aspartic acid (RGD) or the ECM component heparan sulfate, was incorporated within the scaffolds to facilitate cell attachment and to allow spatial sequestration of heparan-binding growth factors. Fluorescently labeled polymer microparticles and basic fibroblast growth factor (FGF-2) were chosen to illustrate the capability of SL to spatiotemporally pattern scaffolds. The results demonstrate that a precise, predesigned distribution of single or multiple factors within a single 3D structure can be created, and specific internal architectures can be fabricated. Functionalization of these scaffolds with RGD is demonstrated, and heparan sulfate allows efficient cell attachment and spatial localization of growth factors. Such patterned scaffolds might provide effective systems to study cell behavior in complex microenvironments and could eventually lead to engineering of complex, hybrid tissue structures through predesigned, multilineage differentiation of a single stem-cell population.

Laser-layered microfabrication of spatially patterned functionalized tissue-engineering scaffolds

This article presents a micromanufacturing method for direct projection printing of threedimensional scaffolds for applications in the field of tissue engineering by using a digital micromirror-array device (DMD) in a layer-by-layer process. Multilayered scaffolds are microfabricated using curable materials through an ultraviolet (UV) photopolymerization process. The prepatterned UV light is projected onto the photocurable polymer solution by creating the “photomask” design with a graphic software. Poly(ethylene glycol) diacrylate is mixed with a small amount of dye 0.3 wt % to enhance the fabrication resolution of the scaffold. The DMD fabrication system is equipped with a purging mechanism to prevent the accumulation of oligomer, which could interfere with the feature resolution of previously polymerized layers. The surfaces of the predesigned multilayered scaffold are covalently conjugated with fibronectin for efficient cellular attachment. Our results show that murine marrow-derived progenitor cells successfully attached to fibronectin-modified scaffolds. DOI: 10.1115/1.2823079

/pdf/projection-microfabrication-of-three-dimensional-scaffolds-41ybjdkvxi.pdf

Gazell Mapili

Papers

Biomaterials for stem cell differentiation

A digital micro-mirror device-based system for the microfabrication of complex, spatially patterned tissue engineering scaffolds.

Laser-layered microfabrication of spatially patterned functionalized tissue-engineering scaffolds

Projection Microfabrication of Three-Dimensional Scaffolds for Tissue Engineering