Sea cucumbers are marine invertebrates which live on the sea floor and feed on debris that drift down. When threatened, they can harden their skin within seconds, so that they are less likely to be devoured by the approaching predator.
This behaviour is made possible by the structure of the sea cucumber’s skin, whose deeper layers contain a network of collagen nanofibres enveloped within a viscous and elastic matrix of connective tissue. The arrangement of the collagen can be transiently modified, in response to a protein secreted by nerve cells found in the skin, which alters the chemical bonds between the fibres.
Taking their inspiration from these properties of the sea cucumber skin, researchers from Case Western Reserve University in Cleveland, Ohio, have created a morphing nanocomposite material which they say could be used to develop advanced adaptive microelectrodes for brain implants.
The new material, created by Christoff Weder, a professor at Case Western’s Department of Macromolecular Science and Engineering, and his colleagues, consists of cellulose nanofibres embedded within a polymer. The fibres were obtained from sea squirts and are similar to, but longer than, the cellulose fibres found in cotton wool.
This polymer nanocomposite mimics the mutable mechanical properties of the sea cucumber dermis. In the “on” state, the material is rigid, because the cellulose fibres are connected to each other by strong hydrogen bonds. Addition of water breaks some of these bonds and gives the material a rubber-like consistency which is about 1,000 times softer.
Weder and his colleagues suggest that the material has many potential applications. It could, for example, be used to make “smart” bullet proof vests, smart castes for broken limbs, and mechanically adaptive microelectrodes for brain implants for conditions such as Parkinson’s Disease stroke and spinal cord injuries.
Currently, brain implants are being applied widely. For example, deep brain stimulation (DBS) have been used to treat Tourette’s Syndrome, depression and, more recently, obesity. (In the latter case, there were some unexpected results – the stimulation evoked long-lost memories in the patient.) The technique has also been used to improve brain function in a minimally conscious patient.
In DBS, the brain implant stimulates a specific region of the brain to bring about the desired effects. Other types of implants, called brain-computer interfaces (BCIs), can be used to monitor brain activity and transmit the data, via a computer, to some device. In recent years, BCIs have been used to enable a quadruplegic patient to control a prosthetic limb.
Although they have in most cases proved to be beneficial, brain implants have their limitations, because they are rigid devices which are implanted into the soft and extremely very fragile brain tissue. The quality of the signal degrades with time, because of the damage caused to the surrounding tissue.
Mechanically adaptive implants made from the new nanocomposite could overcome this problem. These new devices would initially be rigid, so that they could penetrate the tough membranes surrounding the brain during implantation. Once in place, the water in the cortical tissue would soften the microelectrodes, so that they could mould themselves appropriately, thus minimizing the damage to the brain tissue.
This flim clip shows how a small piece of the nanocomposite can soften within seconds of being dipped in water:
Capadona, J. R., et al (2008). Stimlui-responsive polymer nanocomposites inspired by the sea cucumber dermis. Science 319: 1370-1374. [Abstract]