Tissue Engineering, Regenerative Medicine and Advanced Therapeutics Engineering
This domain tests the engineering of living constructs: the cell–scaffold–signal triad, mass transport limits, manufacturing under GMP and the regulatory gap that advanced therapies occupy. It rewards an engineer's scepticism about scaling biology.
The triad
- Cells: autologous (no rejection, limited supply), allogeneic (off-the-shelf, immunogenicity risk), mesenchymal stromal cells, and induced pluripotent stem cells (high expansion, teratoma and genomic stability risk).
- Scaffolds: porosity and interconnectivity for cell migration and vascularisation; typical pore sizes around 100–500 µm favour bone ingrowth; degradation rate matched to tissue formation; mechanical properties matched to the host site.
- Signals: growth factors, mechanical loading and bioreactor conditions (perfusion, hypoxia, electrical or mechanical stimulation).
Mass transport is the binding constraint
- Oxygen diffuses only about 100–200 µm into tissue before depletion, so any construct thicker than roughly 1 mm needs a perfusion bioreactor or a pre-formed vascular network.
- Perfusion improves nutrient flux but introduces shear stress that can damage shear-sensitive cells; the design variable is flow rate per unit cell mass, not bulk flow.
Manufacturing and regulation
- Aseptic processing in an ISO 13485-compatible GMP facility is typical because terminally sterilising living cells is not possible; sterility assurance rests on process validation and environmental monitoring.
- Biological evaluation uses ISO 10993-5 (cytotoxicity), -6 (implantation) and -11 (systemic toxicity); device standards alone do not cover viable cell products.
- Advanced therapy products often fall outside conventional device listing schemes, so the classification question matters early in design.
Worked example
A 4 mm thick cartilage construct is cultured statically. Oxygen penetration is roughly 150 µm from each surface, so about 3.7 mm of the core is hypoxic and will necrose or dedifferentiate. Options: reduce thickness to under 300 µm, introduce perfusion, or add a channel network. Each changes the mechanical and manufacturing specification, which is the answer the examiner wants, not just the diffusion number.
Traps that cost marks
- Proposing terminal sterilisation for a living cell product.
- Assuming 2D culture data predicts 3D construct behaviour.
- Ignoring immunogenicity for allogeneic products.
- Specifying porosity without interconnectivity, which does not permit vascularisation.