Medical device biointerfaces

This page is about the interface: what tissue does to a device surface and what the surface does to tissue. Device architecture, structural materials and implant service life are covered separately under biomedical engineering of implants; here only the first few nanometres are in scope.

A cell never sees the material

Within seconds of contact with blood or interstitial fluid the surface is covered by an adsorbed protein layer, and every subsequent cellular recognition event happens on that layer rather than on titanium or polymer. Its composition changes with time: small, mobile, abundant proteins arrive first — albumin above all — and are then displaced by larger, more strongly binding ones, fibrinogen, then high-molecular-weight kininogen and fibronectin. The exchange is known as the Vroman sequence. The final set of adsorbed molecules, and more importantly their conformation after unfolding on the surface, determines which integrin-binding and receptor sites are exposed. A hydrophobic surface unfolds protein more and exposes cryptic epitopes, so a material that is chemically inert can be biologically very active.

The foreign-body response

What follows is stereotyped. Neutrophils bind adsorbed fibrinogen through the Mac-1 receptor; monocytes arrive and differentiate into macrophages. A macrophage cannot phagocytose a centimetre-scale object, so it enters frustrated phagocytosis, secreting reactive oxygen species and proteases directly onto the surface and fusing into multinucleated foreign-body giant cells. Their cytokines recruit fibroblasts, and an avascular collagen capsule tens to hundreds of micrometres thick forms around the device. For a mechanical implant that is usually tolerable. For a sensor, an electrode or a drug-eluting device the capsule is a diffusion barrier, and it is what explains sensitivity drift in glucose sensors and the upward drift of stimulation thresholds over time.

Blood and mechanics

Blood contact activates a different branch: adsorbed fibrinogen and factor XII drive contact activation of coagulation and platelet adhesion, which is why small-diameter vascular grafts and oxygenators need either systemic anticoagulation or a surface that substitutes for it — heparinised, endothelialised, or bearing polymers that mimic the phospholipid headgroups of a cell membrane. A separate and often underrated factor is modulus mismatch. Soft tissue has a stiffness measured in kilopascals; silicon and metal in tens to hundreds of gigapascals. Every movement produces micromotion at the boundary and sustains chronic inflammation, which is why reducing probe stiffness and size can matter as much as any chemical modification.

What works, and what is unsolved

What works: hydrophilic polymer brushes that suppress adsorption through steric repulsion and bound water; zwitterionic coatings; immobilised RGD peptides that selectively support adhesion of the cells you want. All of them attenuate the response; none abolishes it, and coating wear or hydrolytic degradation eventually returns the surface to its native behaviour. A long-term interface that is genuinely invisible to the immune system does not yet exist.