Microfabricated Porous Silk Scaffolds for Vascularizing Engineered Tissues

Abstract

There is critical clinical demand for tissue-engineered (TE), three-dimensional (3D) constructs for tissue repair and organ replacements. Current efforts toward this goal are prone to necrosis at the core of larger constructs because of limited oxygen and nutrient diffusion. Therefore, critically sized 3D TE constructs demand an immediate vascular system for sustained tissue function upon implantation. To address this challenge the goal of this project was to develop a strategy to incorporate microchannels into a porous silk TE scaffold that could be fabricated reproducibly using microfabrication and soft lithography. Silk is a suitable biopolymer material for this application because it is mechanically robust, biocompatible, slowly degrades in vivo, and has been used in a variety of TE constructs. We report the fabrication of a silk-based TE scaffold that contains an embedded network of porous microchannels. Enclosed porous microchannels support endothelial lumen formation, a critical step toward development of the vascular niche, while the porous scaffold surrounding the microchannels supports tissue formation, demonstrated using human mesenchymal stem cells. This approach for fabricating vascularized TE constructs is advantageous compared to previous systems, which lack porosity and biodegradability or degrade too rapidly to sustain tissue structure and function. The broader impact of this research will enable the systemic study and development of complex, critically-sized engineered tissues, from regenerative medicine to in vitro tissue models of disease states.

In vivo bioresponses to silk proteins

“…Even highly porous scaffolds limit cell infiltration as demonstrated by the difficulty of evenly seeding scaffolds in vitro . To combat this issue, scaffolds have been designed with large, 250–500 micron diameter, arrayed channels to allow cells to quickly infiltrate deep within the scaffold (64). In vivo , these scaffolds had increased vascularization by 14 days post implant as compared to scaffolds without channels (32).…”

A mild process to design silk scaffolds with reduced β-sheet structure and various topographies at the nanometer scale

“…Recent studies have revealed the feasibility of silk as a supporting matrix for cells, including fibroblasts, osteoblasts, hepatocytes, nerve and stem cells, as well as a scaffolding for tissue engineering of bone, ligaments, blood vessels, skin, cartilage and other tissues [7–11]. Following the increasing applications, there remains a demand to further improve the bioactivity of silk scaffolds.…”

The effect of scaffold macroporosity on angiogenesis and cell survival in tissue-engineered smooth muscle

“…By diffusion alone, thick scaffolds can only sustain cells within a few hundred μm of the liquid-scaffold interface in vitro , a limitation which is closely approximated in vivo [2,3]. Various techniques to improving vascular conduction have been studied [4], including use of growth factors [5–7], bioactive materials [8,9], microfabrication [10–12], angiogenic cells [13–15], decellularized organs with intact microvasculature [16,17], non-biological factors [18], and even oxygen-producing biomaterials [19]. Microfabrication is especially important since improved conduction pathways can accelerate or improve other efforts to increase angiogenesis in implants [20].…”