The natural healing process cannot restore form and function to critical size bone defects without the presence of a graft to support and guide tissue regeneration [1]. Critical size bone defects in humans are typically on the order of centimeters or larger [2]. Thus, a major limitation of synthetic grafts or bone tissue engineering constructs is the lack of vascularization to support cell viability after placement in vivo [3]. Cells that participate in bone regeneration, must reside within 150–200 microns of a blood supply in order to gain proper nutrients and to eliminate waste [4]. Consequently, a tissue engineering construct of a clinically relevant size cannot rely on diffusion for transport of nutrients and waste. Previous research has shown that blood vessels can infiltrate scaffolds, but the overall process is too slow to prevent death of cells located in the center of a construct [5].

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