Softening the impact

Cracking the cartilage riddle, Deakin scientists synthetically mimic the body's most complex lubrication system.

Dr Wren Greene
Dr Wren Greene

Deakin researcher Dr George “Wren” Greene has achieved what many in his field had thought impossible.  Dr Greene has managed to synthetically mimic the most complex lubrication system within the human body – the cartilage lubrication system.

An Associate Research Fellow within the Institute for Frontier Materials, Dr Greene is hopeful that one day his research could help millions of people who suffer from osteoarthritis and other conditions related to worn out cartilage.

“No one has previously attempted to replicate the cartilage system because it just seemed too complicated,” he says. “I had been working on cartilage since 2003 and I always wondered if replicating the lubrication system was possible.”

Cartilage acts to provide both lubrication and joint movement, without compromising wear, for the constant use that we all require. The body has evolved a highly effective water-based lubrication system that provides low friction and protection from wear that is so complex that researchers have found it difficult to engineer tissues associated with it.

“The key to solving the puzzle was realising that we didn’t need to duplicate the entire system,” says Dr Greene.

“What we needed was a few key aspects where everything was optimised. So, relatively speaking, it was quite simple. The challenge has been to get the material right, with the right kind of porosity and permeability, and the right mechanical properties, so that it can deform when compressed, while also being able to retain and support very high fluid pressure.”

“It is much easier to lubricate things when they are in constant, uniform motion,” he adds, “but when movement goes back and forth, when it changes direction and when it stops, this is where you get high wear – and it is very hard to lubricate.”  

Dr Greene and his colleagues, Professor Roger Horn and Dr. Haijin Zhu in the IFM and Professor Monika Osterberg and Dr. Anna Olszewska at Aalto University in Finland, have been able to mimic the properties of the system with a biocompatible material that combines three of the key features involved in lubrication.

The first of these is a special type of fluid pressurisation lubrication mechanism that is unique to cartilage. This involves absorbing the weight bearing, compressive load, which is achieved through a network of interconnected fluid-filled, porous cellulose fibres. This network mimics the collagen network of cartilage.

The second part involves modifying the porous cellulose network with immobilized, highly charged polymers which function like the large proteoglycan complexes in cartilage. These polymers generate an osmotic pressure and electro-steric repulsive forces that allow the fluid pressurisation lubrication mechanism to be controlled and optimised.  

The third part of the system replicates the boundary lubrication of cartilage, provided by molecules such as hyaluronic acid and lubricin protein, using a specially synthesised copolymer.  This copolymer boundary lubricant further reduces the friction and wear between the shearing surfaces.

Dr Greene adds that there is still much work to be done and, while the proof of concept phase has been achieved, the material needs to be developed substantially before it can be used in the human body. He is exploring bacterial cellulose and other porous materials for their potential use as cartilage replacements.

“Currently artificial joints have a finite lifetime, of 15 to 20 years, with the second replacement having about half that. The goal is to produce materials that can last 40 to 50 years,” he says.

Dr Greene moved to Deakin two years ago from the University of California, where he worked in the areas of biolubrication, the biophysics of cell locomotion, and ‘pressure solution,’ a geological phenomena important to rock diagenesis. Apart from working on knee cartilage, he is currently undertaking a DECRA fellowship on bio-lubrication, looking at the protein lubricin and its anti-adhesive properties, for use in micro devices and anti-fouling coatings.

Dr. Greene will also be working closely with Dr Patrick Howlett and Professor Maria Forsyth on developing organic plastic crystal composite materials for electrodes in all-solid-state batteries.

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