Tissue engineering is as much a mechanical as a biological problem. Although the field's focus has been largely on getting cells to grow in vitro in the right morphology — a question increasingly now concerned with eliciting the requisite modes of stem-cell differentiation through appropriate environmental cues — there are many applications in which an engineered tissue's dynamic properties are at least as important as the static ones. That's why tissues associated with moving parts of the body, such as the trachea, remain among the hardest to 'grow' in a medically viable form.

It is precisely because the mechanical performance of some biological tissues is so admirable that they present such a challenge. Of these, cartilage — the connective tissue at the ends of long bones — is among the most impressive. It can provide lubrication and joint articulation without compromising wear or damage for seven or more decades of constant use. Of course, the problem is that it does degenerate eventually, and the resulting osteoarthritis is a source of pain for millions of people. Although the static mechanical properties of native cartilage have been reasonably well matched in artificial tissues1, it is harder to replicate the lubricating function2.

An alternative to growing real cartilage with the right mechanics is to mimic its properties in a biocompatible material. Indeed, there is plenty worth emulating here in a synthetic material that might find tribological applications beyond medicine. It has been long recognized that cartilage, like so many biological tissues, has a hierarchical structure3; the question is how much of, or how closely, this structure need be copied to achieve acceptable mimicry of function.

Previous efforts to make wholly synthetic cartilage have tended to fixate on just one aspect of its behaviour4 — not because that is all that's needed, but because it was all that's feasible. But the tribological superiority of natural cartilage relies on a synergy that will only be attainable by approaching it as an integrated system.

Credit: PHILIP BALL

Greene et al. have now taken a step towards a more sophisticated cartilage mimic that combines two of the key features responsible for its lubrication5. One is that the tissue is a fluid-filled porous matrix in which most of the compressive load is carried by the fluid itself, as it is trapped and pressurized. This reduces the stress on the matrix, but it doesn't alone account for lubrication and wear resistance. Furthermore, highly charged macromolecules called proteoglycans bound to the surface both resist collapse of the pores through electrosteric repulsion and provide detachable 'boundary lubricants' under shear.

In the analogue material developed by Greene et al., cellulose with submicrometre porosity mimics the collagen fibril network of cartilage, while the boundary lubrication is supplied by copolymers of the polyelectrolyte carboxymethylcellulose with polyethylene glycol. The resulting material shows not only low friction (albeit higher than real cartilage) but also the time-dependent static frictional response diagnostic of the fluid-pressurization mechanism of lubrication. This composite isn't yet robust enough for medical applications, but it shows that the strategy is sound.