 Tissue-engineering scaffold

A tissue-engineering scaffold is a material structure that supplies an initial architecture and local environment for cells in a tissue construct. It can provide attachment surfaces, mechanical support and paths for transport while cells form extracellular matrix. Some designs undergo scaffold degradation; others are intended to persist. A scaffold is therefore part of a developing cell–material system rather than merely a mould shaped like an organ.

Important design variables include material chemistry, biocompatibility, pore geometry, mechanical response and transport. Porosity measures the fraction of volume occupied by voids, but scaffold pore connectivity determines whether those spaces form communicating paths. For example, $30\mathrm{m}{\mathrm{m}}^3$ of solid in a $100\mathrm{m}{\mathrm{m}}^3$ envelope gives porosity $1-30/100=0.70$. The same porosity could describe connected channels or isolated cavities; the two structures need not admit the same fluid flow or access for cells.

Under an ideal Darcy law model for steady flow through a saturated scaffold slab,where $k$ is the intrinsic permeability, $A$ the area normal to flow, $L$ the thickness and $\mu$ the fluid’s dynamic viscosity. Taking illustrative $k=10^{-12}{\mathrm{m}}^2$, $A=10^{-4}{\mathrm{m}}^2$, $L=5\times 10^{-3}\mathrm{m}$, $\mu =10^{-3}\mathrm{Pa}\mathrm{s}$ and $\Delta p=100\mathrm{Pa}$ gives $Q=2\times 10^{-9}{\mathrm{m}}^3{\mathrm{s}}^{-1}=0.12\mathrm{mL}\mathrm{mi}{\mathrm{n}}^{-1}$. Doubling $L$ halves the predicted flow at fixed pressure difference. These chosen parameters are a transport example, not a measured scaffold or a biological operating limit.

The Darcy law calculation averages over the pore network. It does not reveal local shear stress, dead-end pores or gradients of oxygen within the construct. A useful experiment therefore combines measurements of geometry and flow with spatial observations of cell viability and matrix formation, instead of inferring all these properties from porosity alone.

The primary report of functional arteries grown in vitro provides a motivating example of evaluating a constructed tissue by both mechanical and biological criteria.^[ref] As the cells add matrix and the starting material changes, the mature construct can differ substantially from its initial scaffold. Scaffold design and bioreactor design must consequently be assessed together: geometry affects transport, and the culture conditions affect the tissue that develops.