During the design and validation phase, medical devices undergo rigorous and extensive testing to establish both the biocompatibility and mechanical stability of the device before obtaining regulatory approval and being implanted into patients as treatments for disease and injury.
However, the human body is highly adaptive and cells and tissues can change their composition, structure and function in response to biophysical stimuli – in particular, those imposed by these medical devices when they are implanted into the body.
Indeed, it is well known that muscles get bigger and stronger when we exercise, but it is less widely understood that many other cells and tissues of the body, such as skin, vessels, bones, cartilage and heart tissue, also respond to changes in the mechanical forces they experience.
For example, when astronauts return from a long spaceflight, their bones are as weak as those of older people with osteoporosis. This is because of the weightlessness arising in space, which triggers the cells to think that the bone is not needed anymore and they then begin to eat away at it.
The success of medical devices is also dictated by such responses. For example, cardiovascular stents impose mechanical forces on the arterial wall when they are scaffolding open an atherosclerotic plaque. However, the mechanical stresses induced by the stent on the vessel wall actually stimulate biological responses in the cells, which activate the cells to start making new tissue (in an attempt to reduce the stress in their environment) but this ultimately leads to a re-blockage, known as restenosis.
Hip and knee replacements also activate unwanted responses in bone cells, due the fact that the metallic implant bears most of the applied stress and leaves the surrounding bone unloaded, which activates the cells to remove bone and leads to implant loosening (a process known clinically as stress-shielding) and the requirement for a revision surgery.
Although such responses are critical for the long-term performance of medical devices, they remain poorly understood and so the ability to control such reactions has not yet been fully harnessed. Our research in the Mechanobiology and Medical Device Research Group (MMDRG) at NUI Galway seeks to understand such responses so that they can ultimately be accounted for during the design of medical devices.