杏吧原创

SILKWORM cocoons boiled in soap solution could help mend badly damaged bones. A team of researchers from Massachusetts has found that bone cells grow well on silk sheets made from the treated cocoons. They now plan to build tough silk pads seeded with bone cells that would grow strong enough to patch up weight-bearing bones, such as femurs.

One way to fix cracks or holes in bones is to place bone cells into a 鈥渟caffold鈥� which is then implanted into the damaged area. Scaffolds can be made from biodegradable polymers such as polylactic acid, or natural materials such as coral. As the bone cells multiply, they fill the scaffold-or replace it altogether if it is degradable.

But these scaffolds are too weak to bear weight, so they can only be used to heal certain bones. 鈥淭his is one of the biggest problems-getting scaffolds that are strong enough,鈥� says Joost de Bruijn, who heads the bone programme at IsoTis, a human tissue engineering company in Bilthoven, the Netherlands. 鈥淐eramic ones are quite brittle and the polymer ones aren鈥檛 strong enough for load-bearing uses,鈥� he says.

David Kaplan and a team at Tufts University in Massachusetts reasoned that silk would be strong enough for the job, but didn鈥檛 know if bone cells would grow on it. To find out, they took silkworm cocoons and boiled them for an hour in soap solution to remove a protein called sericin, which triggers an immune response in the body. The team then coated sheets of the treated silk with specially chosen sequences of amino acids that bind to bone cells. Finally they spread human bone cells onto the silk.

After four weeks, the bone cells were producing messenger RNA for procollagen, a precursor of the collagen found in bones. They were also depositing calcium, just as they do in the body.

Kaplan says silk scaffolds seeded with bone cells would be sufficiently strong to be load-bearing until enough bone cells had formed to take the strain. 鈥淚t would be degradable, and so fully replaced by native tissue after repair and regrowth, but provide support in the interim,鈥� he says. 鈥淚n the long run, we would not use sheets but three-dimensional silk sponges or fibres.鈥�

One way to make such structures might be to weave a tube, or roll a mat up into a cylinder, suggests Christopher Viney, who studies the properties of silk at Heriot-Watt University in Edinburgh. 鈥淥r you could do it layer by layer,鈥� he says.

鈥淚t鈥檚 a very novel and interesting use of silk,鈥� he adds. 鈥淏ones can undergo lots of deformation during loading, but silk can sustain large deformations without breaking.鈥�

But Viney points out that further tests are needed to ensure that the silk degrades safely in the body. 鈥淚f you get molecular fragments breaking off, they could potentially cause an immunological reaction,鈥� he warns.

Tim Hardington, who is setting up a tissue engineering centre at the University of Manchester, agrees. 鈥淯ntil you know what kinds of fragments are produced as it breaks down, there鈥檒l be a chance they could induce a response,鈥� he says.