= Tissue engineering
<Tissue engineering> uses <biological cells>, engineered environments and material structures to build or restore tissue-like function. A construct must do more than resemble the shape of a <tissue>: its <cells> need suitable surroundings, and the resulting structure must perform the intended biological or mechanical task. The field lies within <biomedical engineering>, whose scope includes engineering methods applied to biological materials and systems.<Bioengineering in the National Library of Medicine Collection Development Guidelines>[Definition and Discussion]{ref}
One approach seeds <cells> onto a <tissue-engineering scaffold> that provides an initial geometry and attachment environment. The <cells> can deposit <extracellular matrix> while the construct is maintained under controlled culture conditions. A <bioreactor> can supply nutrients and impose a defined mechanical environment. These roles should be distinguished: a scaffold’s initial strength, the matrix produced by the <cells> and the mature construct’s function need not be identical.
A motivating primary experiment is the 1999 report of arteries grown from vascular <smooth muscle cells> and <endothelial cells>. Bovine vessels were cultured under pulsatile conditions, then assessed for rupture strength, suture retention, contraction and cellular differentiation. The study also reported implantation in miniature swine, with a finite follow-up observation. The combination of mechanical and biological measurements illustrates why a tissue-like shape alone is insufficient evidence of function.<Functional arteries grown in vitro by L. E. Niklason and others>[Primary abstract: cell sources, pulsatile culture, functional measurements and animal implantation]{ref}
A simple design calculation shows how size affects cell support. For <diffusion> with characteristic diffusivity $D$, the diffusion time over distance $L$ scales as
$$
t_D\sim\frac{L^2}{D}.
$$
For illustrative $D=10^{-9}\,\mathrm{m^2\,s^{-1}}$, a $0.1\,\mathrm{mm}$ distance gives $t_D\sim10\,\mathrm s$, whereas $1\,\mathrm{mm}$ gives $t_D\sim1000\,\mathrm s$. The tenfold increase in distance produces a hundredfold time scale. These chosen values do not establish a survival limit: consumption, binding, boundary concentrations and fluid transport also matter. They explain why scaling up a dense construct introduces a transport problem, rather than merely requiring more <cells>.
The engineering task is to coordinate transport, <cell adhesion>, matrix formation, <biocompatibility> and mechanical demands. An experiment can measure each separately: nutrient conditions, viable-cell distribution, matrix organization and force response. Together those observations test whether the designed environment has produced the intended tissue function.
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