HKIE Online

PolyU develops biocomposite for bone repair

A researcher at the Hong Kong Polytechnic University (PolyU) has successfully extracted a substance from the cocoons of silkworm for development into a new kind of biocomposite for bone repair.

Although biocompatible metallic materials such as stainless steel and titanium have been widely used by surgeons as bone fixators due to their inert characteristics within the human body, the recovered bone becomes weaker after the metal plates are removed. Another common problem is the growth of excess bone cells around the metal plates and the porosis underneath; and the need for a second surgical operation to remove the metal plates.

In light of these problems, PolyU researchers began looking for less stiff materials to replace traditional metallic materials as bone plates. They started with investigating the use of a synthetic biodegradable polymer which shows desirable chemical and physical properties. Inspired by the properties of silk, they subsequently reduced the polymer's brittleness by blending it with silk fibre to develop a new kind of biocomposite material.

The silk-fibre-reinforced biocomposite shows extremely high strength and toughness combined with high elasticity. At the same time, it provides the necessary support for cell attachment and cell growth. Being biodegradable, the biocomposite will gradually decompose into lactic acid which is ultimately metabolised and excreted from the human body, which means another surgical operation to extract metal plates can be avoided.

Dr Karen Cheung Hoi-yan worked under the supervision of associate professor Ir Dr Alan Lau Kin-tak of the Department of Mechanical Engineering to develop the new biocomposite and was recently presented with the Young Scientist Award by the Hong Kong Institution of Science in recognition of her work.

Initial findings showed that the new biocomposite worked on the broken bones of animals. In the next phase of their study, Dr Cheung will apply for a postdoctoral fellowship at PolyU to continue this project in collaboration with material science and medical researchers from the University of Cambridge, University of California and University of Hong Kong, to test the biocompatibility and bioresorbability of the biocomposite on animal bodies.

Ir Dr Lau and Dr Cheung with samples of the biocomposite

Polymers developed to mop up nuclear waste

Nuclear power has rather nasty by-products: radioactive waste. Not only the disposal of the old core rods but also reactor operation results in a large amount of low-level waste, especially contaminated cooling water.

Now Sevilimendu Narasimhan of the Bhabha Atomic Research Centre in Kalpakkam, India, along with chemist Dr Börje Sellergren of the Institute of Environmental Research at Technische Universität Dortmund, Germany, have developed a new method to reduce the amount of radioactive waste considerably. Their approach: small beads consisting of a special polymer that "fishes" the radioactivity out of the water.

In pressurised-water reactors, the most common reactor, hot water circulates at high pressure through the steel pipes, dissolving metal ions from the walls of the pipes. When the water is pumped through the reactor's core, these ions are bombarded by neutrons.

Because the pipes are steel pipes, most of the ions are common iron-isotopes (56 Fe), which do not become radioactive when bombarded by neutrons. But the steel in the pipes is usually alloyed with cobalt, and when this cobalt absorbs neutrons, an unstable cobalt-isotope (60 Co) emerges which is radioactive with a half-life of more than five years.

Usually the water is cleaned with ion exchangers, but this technique has a crucial disadvantage because it does not differentiate between non-radioactive iron-ions and radioactive cobalt-ions.

To overcome this problem, Sellergren and Narasimhan looked for a material which would bind cobalt and not iron. They developed a special polymer which is made through a procedure called "molecular imprinting". The polymer is made in an environment containing cobalt, then the cobalt-ions are extracted with hydrochloric acid. The resulting cobalt-sized holes - the imprinting - are able to trap cobalt - and just cobalt - in other environments. The result: a small amount of this polymer can mop up a large amount of radioactive isotopes.

The team is now forming the polymer into small beads that can pass through the cooling system of a nuclear-power station. They expect that it would be more economical and environmentally-friendly to concentrate radioactivity into such beads than to dispose of large amounts of low-level radioactive waste. There is expected to be considerable demand for the polymer: some 40 new nuclear-power stations are being built around the world and the International Atomic Energy Agency estimates that a further 70 will be built in the next 15 years.