= 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, $30\,\mathrm{mm^3}$ of solid in a $100\,\mathrm{mm^3}$ 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,
$$
Q=\frac{kA\Delta p}{\mu L},
$$
where $k$ is the intrinsic permeability, $A$ the area normal to flow, $L$ the thickness and $\mu$ the fluid’s <dynamic viscosity>. Taking illustrative $k=10^{-12}\,\mathrm{m^2}$, $A=10^{-4}\,\mathrm{m^2}$, $L=5\times10^{-3}\,\mathrm m$, $\mu=10^{-3}\,\mathrm{Pa\,s}$ and $\Delta p=100\,\mathrm{Pa}$ gives $Q=2\times10^{-9}\,\mathrm{m^3\,s^{-1}}=0.12\,\mathrm{mL\,min^{-1}}$. 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.<Functional arteries grown in vitro by L. E. Niklason and others>[Primary abstract, cultured vessel function and mechanical measurements]{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.
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