PERSUADING human bodies to accept foreign implants is a challenge. If those implants are living organs, the challenge can be met by tissue-matching and powerful drugs which suppress the immune system. But if the implant is a synthetic hip- or knee-replacement, it is a question of sticking it in and keeping one's fingers crossed. Such an artificial joint may integrate successfully with the bone to which it is attached, or it may not. So Thomas Webster of Purdue University, in Indiana, and Hicham Fenniri, of the University of Alberta, in Edmonton, Canada, hope to make the process more reliable by employing nanotechnology.

Though the term nanotechnology is bandied about promiscuously by grant-hungry researchers keen to jump on the latest bandwagon, it is, in this case, an appropriate one. That is because the osteoblast cells that secrete the mineral matrix of bone are most comfortable on surfaces whose bumps measure a few nanometres (billionths of a metre) across. And it is bumps of this size that Dr Webster and Dr Fenniri propose to provide.

The Webster-Fenniri nanobumps are stacks of triangular molecules arranged in layers. Each layer of a stack contains six triangles, like wedges of a Camembert cheese. And each of the triangles has a side made of guanine, a side made of cytosine, and a side made of one of a number of “wild-card” molecules that will fit into the available space.

Guanine and cytosine are two of the components of DNA, and it is their intimate love of bonding to one another that helps to hold the DNA double helix together. In this case, that love of bonding means the triangles fit together in a way that presents the wild-card molecule to the outside world. The tops and bottoms of the hexagonal rosettes are also attracted to one another. As a result, stacks of rosettes several millimetres long can form.

The trick is to get these stacks to stick to the surface of an implant, and thus form a suitable habitat for osteoblasts. Fortunately, implants are often made of titanium, a metal that is covered with a negatively charged layer of oxide. To get the stacks to bond to titanium, all that is necessary is to make the wild-card a molecule that has a positive charge on the outside. Two wild-cards that will do this are amino acids called lysine and arginine—molecules more familiar as two of the building blocks of proteins.

To see if they could create a conducive habitat for osteoblasts, Dr Webster and Dr Fenniri coated some titanium plates with stacks made in this way and put them in dishes containing suspensions of osteoblasts that had been coloured with a fluorescent dye for easy observation. Of the 2,500 osteoblasts in each dish, 2,300-2,400 adhered within a few hours to titanium coated with the stacks. Only 1,500 cells attached themselves to titanium with no coating. If that were replicated in a real replacement joint, it would greatly increase the chance of the joint integrating successfully with the bones it was attached to.

And that is only the beginning. Osteoblasts have receptors on their surfaces which recognise and stick to particular sequences of amino acids—for example lysine-arginine-serine-arginine, and arginine-glycine-aspartate. So, by adding serine, glycine and aspartate to their mixture, Dr Webster and Dr Fenniri hope to improve adhesion rates still further. They also hope to deploy their stacks for other purposes. By adding different mixtures of amino acids, they think they can encourage adhesion between other sorts of cells. That might, for example, promote wound healing. They could also attach molecules called growth factors to the stacks. These promote the proliferation of particular cells and could be used to treat, for example, anaemia.

Indeed, their rosette stacks may prove to be a perfect drug-delivery device. Suitably doctored, they could be made to carry a drug molecule, bind specifically to a protein found only on the surface of one particular sort of cell, and then penetrate the cell's membrane like a nano hypodermic needle, injecting the drug as they did so. That, if it came to pass, truly would be nanotechnology in action.