Living learning, self-organising materials systems may sound like an engineer’s dream. But you only have to look down at your own body to see such a system at work. Millions of other systems are out there in the natural world. Mimicking their actions in the human body to aid the restoration of materials made by tissues - or to rehabilitate those who have suffered injuries - using smart prostheses, rapid communication systems and physical aids is a major part of the growing science of biomaterials.
Where are Biomaterials Going?
The Health and Life Science Panel of the OST’s Technology Foresight Programme identified this theme in its April 1995 report, and many issues are taken up in ‘Materials Technology Foresight in Biomaterials’, published by The Institute of Materials in 1995, which highlights applications of biomaterials in orthopaedics, dental/craniofacial and cardiovascular devices. One summary suggests the materials will move on from first generation solutions, based on bioinert materials, to a second generation of bioactive materials (including surface coatings) which encourage the regeneration of natural tissue, and finally on to a third generation of intelligent adaptive systems.
There is much overlap in the loM report with the Berry Report to the NHS Central Research and Development Committee which adds further clinical specialities, utilising biomaterials for urology, wound repair and ophthalmology. Generic themes for biomaterials research and development were given as chemical and biochemical sensors (including the detection of the early deterioration of body tissue function), drug release, hydrogels, membranes and artificial organs.
The Evolution of Biomaterials
Having identified the requirements, the materials scene is evolving to meet them. The article by Bonfield and Tanner (Materials World, January 1997) summarises the development of a bioactive material, HAPEX, a composite of polyethylene and hydroxyapatite. This is a valuable first step, the success of which was largely determined by the development of suitable processing techniques which will also determine the development of bioactive materials for load-bearing applications.
A biomimetic strategy acknowledges that in vivo processing of materials offers lessons in how problems should be approached. Bioactive components can be based on biopolymers, such as collagen, and various inorganic materials such as calcium phosphate and carbonate. We are coming to understand how nature builds calcium carbonate into complex, tough structures such as shells using biopolymers as fibres in the composite.
A high degree of ordering, as found in single crystals, is an important structural theme in the development of hybrid (bio)materials. That is the route to enhanced physical properties and is also the background to the exciting progress being made on self-assembly and self organisation.
There are two broad strategies for tissue engineering. A materials-based approach relies on the appropriate cells organising automatically and correctly to produce new tissue, such as attempting to create large cellular masses by growth of cells on a synthetic polymer substrate. The. biological approach looks to use cues or signals to direct the assembly of cells and its responses into a particular organisation.
The first strategy is effective for fabricating tissues where the microscopic structure is less critical, as in auricular cartilage or in structures such as tubes where the cell layers involved form natural, stable boundaries. The central feature of the biological approach is providing spatial, directional and rate-limiting signals or cues for cells developing in vitro. Regulatory cues can be chemical, mechanical, topographical features of the substrate and/or electromagnetic forces. This biological approach provides greater potential to produce complex, multilayered tissues, the possibility of synthesising complex architectures (for example, the minimum requirement in tendon and ligament replacements must be a parallel, axially aligned collagen fibre structure), and a control of the interface between different cell types to produce either adherence or gliding.
Self-Organisation and Ordered Structures
Self-organisation is a major theme for generating ordered structures. Molecules called cyclic octapeptides can self-organise to form regular nanotubes. These beautiful nanotubes can self-assemble inside lipid membranes to function as efficient ion transport channels - such channels may be pharmacologically very useful as potential vehicles for drug delivery into living cells. The sol-gel process for the synthesis of porous ceramics can provide xerogels containing oriented cylindrical pores, and a range of micro- and mesoporous oxide materials can be synthesised with well-defined structures and with pore size radii varying from 0.5-2.0nm or up to 20nm with alternative processing techniques. Ceramic or ceramic-coated membranes may well make their mark in clinical applications in the near future
Control of Order and Orientation
A final example of the ability to control order and orientation is given in a recent report by Belcher and others, which looks at in vitro studies of the crystallisation of calcium carbonate in the presence of soluble polyanionic proteins extracted from mollusc shells. The report concludes that ‘these proteins alone are sufficient to control the crystal phase, allowing us to switch abruptly and sequentially between aragonite and calcite without the need for deposition of an intervening sheet ... soluble organic components can exert greater hierarchical biomineral growth than hitherto suspected, offering the prospect of similar phase control in materials chemistry’.
The identification of sensors as an important theme coincides with the rapid development of sensor technologies. Early warning is a clinical requirement, but requires sensitive and reliable measurements of relatively small changes over a relatively long time scale. Medical applications of newly formulated acoustic sensors include ultrasonic Doppler calibration, foetal monitoring, diagnosis and long-term monitoring of osteoporosis, all polymer two dimensional microimaging arrays for diagnostic purposes, non-invasive surgery, shock wave lithotrophy treatment, and focused high energy ultrasound treatment for cancer.
Sensors and their integration for imaging purposes are a good illustration of the transfer of defence technologies and systems analysis into the civil sector. Rather like information technology, healthcare technologies will depend heavily on developments in materials and their processing.