One of the most important areas from which science and technology can gain knowledge is from nature. Over the past one hundred years, research and development has begun to focus on the lessons which can be learned from nature. This avenue of work is generally referred to as biomimetics. Biomimesis has been incorporated into the fabrication of compounds with enzyme-like catalytic action for a long time. During the last quarter of a century, Biomimesis has been used in the field of material research. The aim has been to improve the toughness of materials produced from ceramic and composites and achieve similar properties to shell, bone and tooth. The concept of producing exact replicas to biological materials did not sit kindly within the materials fraternity. Therefore biomimesis actively involved producing materials which replicated only some of the essential aspects of the natural biological material, rather than duplicating it (Calvert 2001).
Unfortunately, this research takes time to develop new, novel materials which are suitable for commercial production. For example, Kevlar high Tc super conductors, gallium arsenide and piezoelectric polymers did not reach commercialisation for twenty years after their initial development (Calvert 1994).
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History of biomimetics chemistry, mechanical and material
Biomimetics translated from Japanese means Imitation of living things . The use of the term, biomimetics was initially tagged by the German-American neurophysiologist, Otto Schmitt in the late 50s, who invented the Schmitt trigger . The Schmitt trigger was designed to simulate signal processing in the nervous system. It entailed an electric circuit which eliminates superimposed noise and converts it into a series of rectangular pulses (Schmitt 2011). Evidence in the form of a Velcro type material used in Japan and the light reflecting plate in the center of roads known as Cats Eye show that biomimetics was exercised before this time.
Figure 1 Schmitt trigger (Talkingelectronics.com 2011)
The 1970s brought about a resurgence in biomimetic research. Biomimetic chemistry began investigating emulation of enzymes and biomembranes at a molecular level. X-ray analysis displayed the capability of providing detail of enzymes and biocatalysts which enabled molecular level elucidation of biological reactions using chemistry. Through the convergence of biology and chemistry, many biological effects were now becoming clear for research in the field of engineering applications. The 1980s brought research in artificial photosynthesis. This laid the platform for actuator research and dye sensitive solar cells using gels as shown in Figure2, resulting in inventions such as synthetic muscles (Shimomura 2010).
Figure 2 Dye sensitive solar cells (qwikstep.eu 2011)
With major advancements being made in molecular biology during this period, research began to focus on the elucidation of life phenomena with the gene at its core. During the 1980s, biomimetic chemistry began converging with molecular electronics research. This resulted in the migration toward chemistry of molecular assemblies and supramolecules. The 1990s experienced a severance of ties with biology. Research now focused heavily on molecular nanotechnology and nanobiology. Biomimetic Chemistry became almost unheard of as an academic subject and was surpassed by the concept of bioinspired .
Mechanical biomimetics became a mainstream field of research during the 1970s. Mechanical engineering began to develop substantially during this period due to the convergence with biomimetics. This collaboration resulted in devices such as;
* Robots that replicated the movements of fish and insects,
* Radars and sonars which replicated echolocation abilities of bats,
* Hairs on insects which acted as sensors.
Mechanical biomimetics has grown throughout the decades which may be attributed to its affiliation to industries such as; the aeronautical, railway, ship and military industries. Micro-machines and micro electro mechanical systems have been greatly influenced by mechanical biomimetics, to the extent that in Japan biomimetics (as a term) is generally associated with research in the field of robotics (Shimomura 2010)
The turn of the century has seen the push to achieve actual results from all the biomimetic research undertaken. This has created a drive in material research to find new novel biomimetic materials (NationalAcademies 2008). Nanotechnology targets sizes ranging from nanometers to micrometers. These sizes must be analysed using electron microscopy, thus enabling a platform from which collaborations with biology and material science are possible due to this similar method of analysis. The hierarchial structures of living things have been identified at the nanoscale. With this knowledge material scientists have been able to produce artificial structures to mimic those from nature. These new advancements by material researchers have evolved through the convergence of natural history, biology and nanotechnology.
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Lessons learned from nature
Biological materials created by nature are what make up the body of all plants and animals. These materials enable the function of cells, eyes and the ability of animals to move and plants to reach towards the light. These characteristics have led to the study of nature to provide the solutions to a number of biological architectural, materials, aerodynamic and mechanical issues (Kemp 2004). Unfortunately very few of the elements used to synthesise minerals and polymers exist in natural materials. Nature has developed materials with extraordinary properties. This has been made possible by the hierarchial structuring of the materials which result from their growth from genes rather than a predetermined design (Currey 2005).
Figure 3 Biological and engineering fabrication (Fratzl 2007)
Figure3 details how biological and engineering materials have different base elements and fabrication methods. This highlights why a different approach from the one used by nature must be used in biomimetic material research. The available range of elements for the researcher is far greater than the range of elements available to nature. The majority of the materials used by nature for structural purposes are composed of polymers and composites. In the engineering world, the researcher has materials such as chromium and iron which can be used for structural and mechanical applications. Iron is used by nature in blood cells, but nature cannot use it in its metallic form for structural or mechanical applications. It uses iron to bind oxygen in the cell. The structural polymeric materials utilised by nature to build trees and skeletons would not be used by engineers for strong mechanical structures. The material fabrication also differs. The engineer shall chose a material and then use it to produce a required part according to the predetermined design. Whereas, nature grows the material and the part by self assembly, thus providing full control of the material at all hierarchial levels. This method enables nature to use these polymeric materials for structural purposes.
An example of how lessons may be learned from nature is in the analysis of the design of the femoral head. It is unclear which mechanical property has been optimised. Rather nature has evolved the structure of the head over time to meet a wide range of requirements. This evolution has solved nature s requirements for the femoral head. Unfortunately the engineering world has not been able to identify what the requirements are. Therefore, the biological system s structures and materials must be studied and understood. This ideal is crucial for biomimetic research (Fratzl 2007).
Hierarchial structuring enables nature to produce such astounding functions. This method of production/evolution enables the formation of large organs based on smaller building blocks. An example of such a material is collagen fibrils in bone. They have the capability of being assembled in different formations to create different types of bone. Further natural materials which consist of the hierarchial structuring are;
* Spider silk,
* Geckos super hydrophobic surface,
* Lotus effect,
* Skeleton of glass sponge.
Figure 4 hierarchy in the structure of the glass sponge Euplectella (Weaver, Aizenberg et al. 2007)
Figure4 displays the varying levels in the hierarchial structure of the Euplectella glass sponge (Weaver, Aizenberg et al. 2007).
a) Whole basket
b) Woven glass fibres
c) Fibre bundle joined by glass matrix
d) Laminated structure of single glass fibre
e) Protein layer gluing successive glass layers
f) Colloidal structure of glass
The ability of hierarchial structures to form materials with outstanding properties provides a platform from which bio inspired materials may be synthesised and modified for specific functional requirements (Tirrell, Aksay et al. 1994). Materials which are graded according to their function exhibit hierarchial structures. By structuring the material in this way may provide the required property. Such examples of materials in nature are composites with lamellar structures. These structures exist in seashells, fibrous structures, wood, glass spicules and bone. Manmade materials like fibre-glass and ceramic laminates have been made differently but resulted in similar materials. This leads to the understanding that the interfaces in the material s composition have a significant role to play in hierarchial composites (Weaver, Aizenberg et al. 2007).
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Biological materials have the ability to self repair. This is a remarkable characteristic which biomimetic studies have been trying to replicate. This may be possible with a clear understanding of nature s synthesis of these materials. Deformation of wood and bone occurs through the breaking of bonds which can reform automatically. This concept is very similar to plastic deformation, and can be implemented into a variety of materials. In other cases, the material in nature remodels itself. Bone for example, regenerates itself by having osteoclast cells which remove material while osteoblasts deposit new tissue. This regeneration enables the material to adapt its structure to suit external conditions, while also removing damaged material (Fratzl 2007).
Figure 5 Process of bone regeneration (AIST 2001)
Figure5 shows how bone regeneration occurs. The osteoclast cells dispose old bone tissue while the osteoblast cells grow new bone tissue (AIST 2001). Nature can also heal damaged tissue by forming an intermediate tissue, then scar tissue. The understanding of self healing materials is still in its early days, but this capability could have a major impact on biomimetic material research. One such area in which biomimetics is continually trying to strive in is the field of tissue engineering.
Biomimetic materials have a major part to play in tissue engineering. Such synthetic polymers created through biomimetics can extract specific cellular functions and direct cell interaction in cell free implants. This activity can generate a matrix which will permit tissue regeneration and support cell transplantation. Various studies focus on polymers with bio adhesive receptor binding peptides and mono and oligosaccharides. This area may assist the production of complex organisations of multiple cell types due to the materials containing a two and three dimension pattern to create multicellular tissue architecture models. Polymers have been produced which combine the characteristics of natural polymers with the beneficial properties of synthetic polymers. Such materials have been used to accelerate natural occurrences like tissue regeneration in wounds. This occurs by inducing cellular responses such as healing or the formation of a new vascular bed which can encapsulate a cell transplant, and can block reactions occurring as a result of rejection of the transplant caused by the immune system (Hubbell 1995).
Typically for tissue engineering, the main approach is to control tissue engineering in a three dimensional way. For this application it is critical to have a highly porous scaffold which will provide the synthetic temporary extra cellular matrix for;
* Cell attachment
* Cell regeneration
* Neo tissue genesis
Suitable materials for tissue engineering must have compatible chemical composition, structure and biological functions. The majority of metals and ceramics are not suitable for this application. While some of these materials may be suitable for various medical implants, their lack of degradability, brittleness and poor processability deem them unsuitable for tissue engineering. The main area of research for materials for tissue engineering has been polymers due to their composition and structure. They can easily be modified to achieve varying property requirements (Liu and Ma 2004).
Tissue engineering scaffold tissue requires a biodegradability rate which matches the neo tissue formation. Polymers with excellent biocompatibility and biodegradability such as polylactic acid (PLA) and polyglycolic acid (PGA) are often used for tissue scaffolding. Another polymer which is often used is polyethylene glycol (PEG). This material has poor biodegradability but has mechanical properties similar to soft tissues. It can be used as a hydrogel which enables it to be synthesised with PLA or PGA to overcome this problem. Another method used to induce biodegradability is through biomimetics. The PEG is synthesised with specific enzymes such as matrix metalloproteases (MMP). This allows the material to mimic the enzymatic biodegradability of collagen. An example of this is the telechelic BAB block copolymer consisting of PEG and oligopeptides. These oligopeptides act as cleavage sequences for targeted enzymes. The copolymer can be polymerised into a hydrogel by crosslinking and can be degraded by the cell secreted MMPS such as collagenase (West and Hubbell 1999).
Biomimetic mechanical properties
One of the focal objectives in engineering tissues for cardiac muscle, blood vessels and heart valves are to mimic their elastomeric properties. Polycaprolactone (PCL) is a semi-crystalline polymer with a low Tg. value (glass transition) which results in an elastic material at ambient temperature. However, this material degrades takes too long to degrade thus, rendering it less attractive for tissue engineering. Polyurethane (PU) is another polymer which seems suitable but unfortunately it consists of toxic precursors in its synthesis. Studies continue to develop a biodegradable PU using less toxic precursors (Skarja and Woodhouse 2001).
Biomimetics is also used to achieve these required elastomeric properties. Elastin is synthesised as tropoelastin which has a repeating structure consisting of crosslinking and hydrophobic regions. These can then self assemble to form insoluble aggregates at high temperatures. Further materials which utilise this method and are suitable for tissue engineering are elastin-like polypeptides (ELPs), and certain triblock elastin-mimetic materials which have been developed to self-assemble into thermo reversible hydrogels (Wright and Conticello 2002).
To engineer tissues for tendons and ligaments, tensile modulus and strength are critical. For centuries, silk from the silkworm has been used for surgical sutures due to its excellent mechanical properties. However there are concerns over the toxicity of this material. Biomimetic research has focused on the removal of sericin from the silk which has resulted in improved compatibility (Santin, Motta et al. 1999).
New Biomimetic materials
Biomimetic material research has studied the lessons which can be learned from nature. Through various mechanisms, new and novel materials are being developed which mimic the properties of nature s biological materials. Researchers at Cambridge University have developed three new materials, Al203-PMMA, Beta tricalcium phosphate and Nano-hydroxyapatite/polyamide, which they have made available on their CES database. The future looks bright with the potential discovery of many more biomimetic materials.
As previously discussed, biomimetic material research must focus on nature and try to mimic its architecture to achieve the required properties. Al203-PMMA is a hybrid material which can be used to mimic the architecture of shell which is a natural composite with an inorganic/organic layered structure. The hybrid material can model the shell s structure and build nacre-like composites by filling lamellar alumina scaffolds with a second polymer phase
Figure 6, Bio-inspired architecture, Courtesy of NOAA Coral Reef information Systems (CoRIS)
Figure 6 illustrates some examples of bioinspired architecture of an alumina based material. The first image displays the ceramic (light) and polymer (dark) phases in a lamellar structure. It details the distance (d) of the alumina layer and the distance between them (wavelength). The polymer has a brick and mortar architecture which gives it a load bearing function. The architecture has alumina contents with formed ceramic bridges between the blocks. This is formed by the pressing and sintering of the lamellar material. Stresses are relieved by the thin polymer films between the blocks which act as a lubricant. The lamellar architecture creates strength and toughness when the material is subjected to a load in bending. This is assisted by the Al203-PMMA hybrid having an inelastic deformation in opposition to alumina. The resulting properties from this are outstanding. The inelastic elongation is larger than natural nacre. The linear elastic fracture toughness is greater than that of PMMA. The nonlinear fracture toughness exceeds 30MPa and the bending strengths are comparable to ceramics.
This hybrid material is developed by building ceramic scaffolds and filling them with the polymer which crosslinks during polymerisation. Alumina powder is then mixed in water and cooled to create lamellar ice crystals. It is then freeze dried and sintered. Then it is mixed with the polymer which crosslinks through a chemical reaction during heat treatment. This method enabled the manipulation of the materials structure, thus, controlling the thickness and distance between the lamellae. These can be further reduced by increasing the cooling rate or inducing faster freezing which will improve the strength and fracture toughness of the material.
Figure 7 Propagation of composite and natural nacre, Courtesy of the dept. of Materials and Science, UC, Berkley
Figure7 displays a comparison between Al203-PMMA (left) and natural nacre (right). These images are electron micrographs which were taken during stable crack propagation. The similarities between the two can be clearly seen. This can be attributed to the thin organic phase acting as a lubricant thus, controlling the sliding of the load bearing ceramic blocks. This action relieves the stress in the structure. The presence of stiff ceramic bridges and the roughness of the ceramic interfaces between the grains contribute to the controlled sliding. This sliding increases the fracture resistance and toughness by dissipating the energy of the crack (CES 2011).
Beta tricalcium phosphate
Bioceramics have previously been found to have suitable properties for bone and graft regeneration. Due to their porous structure they can be modified to achieve the morphology of cancellous bone and can facilitate bone repair and tissue growth. One such modified material is Beta tricalcium phosphate (b-TCP).
Figure 8 Bioceramic sample (CES 2011)
The image in Figure8 details a b-TCP sample (left), inspired by natural bone (right). The optimal grain size was achieved by sintering at 1100?C. It has an outer structure which mimics the bone and an inner structure which mimics cancellous. This material has three polymorphs, alpha, alpha and beta phase. It is sintered below 1180?C which results in it avoiding cracking. The material has been designed with a dense outer shell surrounding the porous inner shell which has enabled it to overcome its poor mechanical properties. This material may have uses in bone replacements and possible regeneration in load bearing sites. The porous core may be capable of acting as a scaffold for bone tissue growth (CES 2011).
Nano-hydroxyapatite/polyamide (n-HA/PA) composites are materials with mechanical properties similar to bone. They can be used as a template for bone cells. Polyamide is similar to collagen with its active carboxyl and amide groups. The nano-HA particles are similar to bone apatite with reference to their size, phase composition and crystal structure. The resulting composite is biocompatible and bioactive.
Figure 9 PA66 and n-HA porous scaffolds. Courtesy of Research Center for Nano-Biomaterials, Sichuan University
Figure9 displays porous scaffolds produced from PA66 and n-HA. These have the ability to form bone bonding with living tissue and induce bone formation from surrounding tissue. The composite has a highly porous structure and is suitable for cell growth but has poor mechanical strength. The incorporation of a three dimension porous scaffold has the potential to increase the new bone formation and overcome the mechanical strength property. The composition and structure of the material can easily be controlled. This is due to the nano size of the hydroxyapatite (HA) powders. The bioactivity can be improved by increasing the HA content. The mechanical properties can also be modified by the n-HA particle size and surface activity. This enables its ability to join with polyamide. The reactions are similar to that of natural bone. This composite is ideal for applications in tissue engineering as scaffolds for bone repair, fixation devices and implants (CES 2011).
Biomimetic material research is beginning to make great strides in recent years. This has come about as a result of the converging of the biological and material science worlds. Biomimetics does not just consist of the observations of natural structures, but also involves the analysis and understanding of biological materials structure function relationship and the synthesis of the material. Nature has put forward a number of strategies to create materials with excellent properties such as healing and remodelling. Biomimetic materials research creates numerous opportunities for devising new strategies to create multifunctional materials by hierarchical assembly, for the clever use of interfaces and the development of active or self-healing materials (Fratzl 2007). Interdisciplinary teams must keep striving to develop a portfolio of bio-inspired processes for obtaining new function by structuring and assembling of known elements so the new formed list of biomimetic materials in CES can grow and grow.