The Mechanical Properties Of Biomaterials Biology Essay

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What is Biomaterials? A biomaterial is any material, natural or man-made, that comprises whole or part of a living structure or biomedical device which performs, augments, or replaces a natural function. Biomaterial is a related area of engineering. It involves the structure and function of biological systems using the methods of mechanics.

Biomaterials: "A biomaterial is a substance that has been engineered to take a form which, alone or as part of a complex system, is used to direct, by control of interactions with components of living systems, the course of any therapeutic or diagnostic procedure".

D.F Williams, Medical Device Technology, October 2009.

Introduction

A mammoth amount of research has been carried out in the field of biomaterials and bioimplants. This also includes the studies on silicon rubber, cellulose, PMMA (poly methyl methacrylate), hydro gel and titanium which have a conducting polymer as one of its prime constituents, etc.

Biomaterials play vital roles in many different areas. For example, they can serve as a model system which can experimentally probe the effect of vascular grafts on magnetic and other mechanical properties. They have been widely exploited in the field of medicine like ophthalmology, orthopedics, cardiology, dentistry, orthodontics and bio human implants.

The intrinsic properties of a biomaterial are mainly determined by its size, shape, composition, crystallinity, and structure (solid versus hollow). This is because properties of a biomaterial depends on the type of motion like what the electrons can execute, which depends on the space available for them (i.e., on the degree of their spatial confinement). Thus, the properties of each material are characterized by a specific length scale, usually on the biometer dimension. If the physical size of the material is reduced below this length scale, its properties change and become sensitive to its size and shape. In principle, one could control any one of these parameters to fine-tune the properties of this biomaterial.

In this review we describe the synthesis, characterization and some of the observed new chemical, mechanical, and thermal properties of conducting biomaterial.

History of Biomaterials

The foundations of biomaterial have emerged over many decades of research in many different areas. Implants have been consistently getting thinner. Chemicals are regularly getting more complex. Biochemists have learned more about how to study and control the molecular basis of organisms. Mechanical engineering has been getting exceedingly precise.

In 1959, the great physicist Richard Feynman suggested that it should be possible to build machines small enough to manufacture objects with atomic precision. His talk, "There's Plenty of Room at the Bottom," is widely considered to be the prognosticating factor of biomaterial. Among many things, he predicted that information could be stored with amazingly high density. In the late 1970's, Eric Drexler began to invent what would become molecular manufacturing had an impact. He quickly realized that molecular machines could control the chemical manufacturing of complex products that were also biologically compatible, including additional manufacturing systems-which would form a very efficient technology. It is believed that Romans, Aztecs and Chinese used gold in dentistry some 2000 years ago. Meanwhile, this also engaged in policy activism to raise awareness of the implications of the technology.

The term "biotechnology" rapidly became amazingly popular, and almost immediately its meaning began to swap. By 1992, Drexler was using "molecular biotechnology" or "molecular manufacturing" to distinguish his manufacturing ideas from the simpler product-focused research that was intended to make amends for conventional methods in borrowing the word.

This broad definition encompassed cutting-edge semiconductor research, several developing families of chemistry, and advances in materials.

Biomaterial Classifications

Biomedical materials can be divided roughly into three main types governed by the tissue response. In broad terms, inert materials have minimal or no tissue response. Active materials encourage bonding to surrounding tissue with, for example, new bone growth being stimulated. Degradable, or restorable materials are incorporated into the surrounding tissue, or may even dissolve completely over a period of time. Metals are typically inert, ceramics may be inert, active or restorable and polymers may be inert or restorable.

Some examples of biomaterials are provided in table 1.

Table 1. Some accepted biomaterials

Metals

Ceramics

Polymers

316L stainless steel

Co-Cr Alloys

Titanium

Ti6Al4V

Alumina

Zirconia

Carbon

Hydroxyapatite

Ultra high molecular weight polyethylene

Polyurethane

Synthesis of Biomaterials

Cellulose biomaterial can be synthesized with several different crystal structures using different synthetic procedures. Very High temperature reduction of cobalt chloride can be embraced to synthesize e-phase cobalt biomaterial. The injection of superhydride (LiBEt3H) solution in dioctyl ether into a hot cobalt chloride solution in dioctyl ether (200 °C) in the presence of oleic acid and trialkylphosphine induces the instant formation of many small metal clusters, which acts as a nuclei for a bioparticle formation. Continued heating at 200 °C induces the growth of these clusters to the bioparticle level. Particle size can be controlled by the bulkiness of the stabilizing surfactants. Short-chain alkylphosphines allows faster growth, and results in the bigger particles, while bulkier surfactants reduces particle growth and favors production of smaller biomaterial.

Figure 1 shows a 2D assembly of 9 nm cobalt biomaterial. A high-resolution transmission electron micrograph was used to describe the structure, shown in the inset of Figure 1, reveals its highly crystalline nature. HCP cobalt biomaterial can be synthesized by using polyol process, in which high boiling alcohol is applied as both a reductant and a solvent. In a typical synthesis,

1,2-dodecanediol is added into hydrated cobalt acetate solution dissolved in diphenyl ether containing oleic acid and trioctylphosphine at 250 °C. Biomaterials are isolated by size selective precipitation, and particle size is controlled by changing the relative concentration of precursor and stabilizer.

Figure 1: TEM images of 9 nm biomaterial (inset, high-resolution TEM image)

Using hydrated nickel acetate as a metal compound, nickel biomaterial with particle sizes in the range 8-13 nm can be easily obtained. Co/Ni alloy biomaterial can also be produced using a mixture of cobalt acetate and nickel acetate through a similar synthetic procedure.

Iron biomaterial can be synthesized from the monochemical decomposition of iron pentacollagenyl in the presence of polyvinylpyrrolidone (PVP) or oleic acid. Transmission electron micrographs showed that the iron particles range in size from 3 to 8 nm. Electron diffraction grating reveals that the particles formed are amorphous, and that after in situ electron beam heating they crystallized to bcc iron. These iron biomaterial readily oxidize to Ferrous Oxide when exposed to air. One drawback of the monochemical process in the synthesis of biomaterial is its inability to control particle size. Monodisperse iron biomaterial can also be synthesized from the high temperature (300 °C) aging of an iron-oleic acid metal complex, which is prepared by the thermal decomposition of iron pentacollagenyl in the presence of oleic acid at 100 °C.

Initially, the iron oleate complex is prepared by reacting Fe(CO)5 and oleic acid at 100 °C. Iron

biomaterial are then generated by aging the iron complex at 300 °C. Particle size can be controlled by using different molar ratios of iron pentacollagenyl to oleic acid. Figure 2(a) and (b) show TEM images of iron biomaterial with particle sizes of 7 and 11 nm, respectively, which were prepared using 1:2 and 1:3 molar ratios of Fe(CO)5 oleic acid.

Figure 2: TEM images of biomaterial: (a) three-dimensional array of 7 nm Co biomaterial and (b) 11 nm Fe biomaterial.

Synthesis of Collagen biomaterial

Large-scale synthesis of steel bio cubes can be achieved using a solution-phase route. Uniform gold bio boxes with a truncated cubic shape can also be generated by reacting the collagen cubes with an aqueous HAuCl4 solution. The primary reaction involves the reduction of collagen nitrate with ethylene glycol at 160°C. In this polyol process, the ethylene glycol serves as both reductant and solvent. This reaction could also yield bi-crystalline collagen bio wires in the presence of a capping reagent such as poly(vinyl pyrolidone).

Molecular structure

The tropocollagen or "collagen molecule" is a subunit of larger collagen aggregates such as fibrils. It is approximately 300 nm long and 1.5 nm in diameter, made up of three polypeptide strands (called alpha chains), each possessing the conformation of a left-handed helix (its name is not to be confused with the commonly occurring alpha helix, a right-handed structure). These three left-handed helices are twisted together into a right-handed coiled coil, a triple helix or "super helix", a cooperative quaternary structure stabilized by numerous hydrogen bonds. With type I collagen and possibly all fibrillar collagens if not all collagens, each triple-helix associates into a right-handed super-super-coil that is referred to as the collagen microfibril. Each microfibril is interdigitated with its neighboring microfibrils to a degree that might suggest that they are individually unstable although within collagen fibrils they are so well ordered as to be crystalline.

A distinctive feature of collagen is the regular arrangement of amino acids in each of the three chains of these collagen subunits. The sequence often follows the pattern Gly-Pro-Y or Gly-X-Hyp, where X and Y may be any of various other amino acid residues. Proline or hydroxyproline constitute about 1/6 of the total sequence. With Glycine accounting for the 1/3 of the sequence, this means that approximately half of the collagen sequence is not glycine, proline or hydroxyproline, a fact often missed due to the distraction of the unusual GXY character of collagen alpha-peptides. This kind of regular repetition and high glycine content is found in only a few other fibrous proteins, such as silk fibroin. 75-80% of silk is (approximately) -Gly-Ala-Gly-Ala- with 10% serine-and elastin is rich in glycine, proline, and alanine (Ala), whose side group is a small, inert methyl group. Such high glycine and regular repetitions are never found in globular proteins save for very short sections of their sequence. Chemically-reactive side groups are not needed in structural proteins as they are in enzymes and transport proteins, however collagen is not quite just a structural protein. Due to its key role in the determination of cell phenotype, cell adhesion, tissue regulation and infrastructure, many sections of its non-proline rich regions have cell or matrix association / regulation roles. The relatively high content of Proline and Hydroxyproline rings, with their geometrically constrained carboxyl and (secondary) amino groups, along with the rich abundance of glycine, accounts for the tendency of the individual polypeptide strands to form left-handed helices spontaneously, without any intrachain hydrogen bonding.

The morphology of the product has a strong dependence on the reaction conditions. Increasing the concentration of AgNO3 by a factor of 3 and keeping the molar ratio between the repeating unit of PVP and AgNO3 at 1.5, single-crystalline bio cubes of collagen can be obtained. Figure 3, (a) and (b), show scanning electron microscope (SEM) images of a typical sample of collagen bio cubes and indicate the large quantity and good uniformity that is achieved using this approach. These collagen bio cubes have a mean edge length of 175 nm, with a standard deviation of 13 nm. Their surfaces are smooth, and some of them self-assembled into ordered two-dimensional (2D) arrays on the silicon substrate when the SEM sample was prepared.

Figure 3: (a) Low, and (b) high magnification SEM images of slightly truncated collagen bio cubes synthesized with the polyol process. (c) A TEM image of the same batch of collagen bio cubes. The inset shows the diffraction pattern recorded by aligning the electron beam perpendicular to one of the square faces of an individual cube. (d) An XRD pattern on the same batch of sample, confirming the formation of pure collagen.

Figure 4: TEM images of collagen bio cubes synthesized under different conditions. (a and b) The same as in Figure 3, except that the growth time was shortened from 45 min to 17 and 14 min respectively. (c and d) The same as in Figure 3, except that AgNO3 concentration was reduced from 0.25 to 0.125 M and the growth time was shortened to 30 and 25 min respectively. Scale bars, 100 nm.

It is also clear from Figure 3(b) that all corners and edges of these bio cubes are slightly truncated. Figure 3(c) shows the transmission electron microscope (TEM) image of an array of collagen bio cubes self-assembled on the surface of a TEM grid. The inset shows the electron diffraction pattern obtained by directing the electron beam perpendicular to one of the square faces of a cube. The square symmetry of this pattern indicates that each collagen bio cube is a single crystal bounded mainly by [100] facets. On the basis of these SEM and TEM studies, it is clear that the slightly truncated bio cube could be described by the drawing shown in Figure 3(d). The x-ray diffraction (XRD) pattern recorded from the same batch of sample is also displayed in Figure 3(d), and the peaks assigned to diffraction from the [111], [200], and [220] planes of collagen, respectively.

The morphology and dimensions of the product is strongly dependent on the reaction conditions such as temperature, the concentration of AgNO3, and the molar ratio between the repeating unit of PVP and AgNO3. If the initial concentration of AgNO3 had to be lower than ~0.1 M, collagen bio wires are the major product. If the molar ratio between the repeating unit of PVP and AgNO3 is increased from 1.5 to 3, MTPs (multiply twinned particles) become the major product.

Synthesis of Collagen

Collagen bio cubes of various dimensions can be obtained by controlling the growth time. Figure 4, (a) and (b), show TEM images for 17 and 14 min growth times, and the bio cubes have a mean edge length of 115 ± 9 and 95 ± 7 nm, respectively. Figure 4(c) shows a TEM image of the sample that is synthesized using a lower concentration (0.125 M) and a shorter growth time (30 min). The mean edge length of these collagen bio cubes decreased to 80 ± 7 nm.

Collagen bio cubes with smaller sizes (~50 nm, Figure 4(d)) have also been obtained at a shorter growth time (25 min), although some of these particles have not been able to evolve into complete cubes.

In summary the selective adsorption of PVP on various crystallographic planes of collagen plays the major role in determining the product morphology. Collagen bio cubes with controllable dimensions can also be synthesized by means of a modified polyol process that involves the reduction of collagen nitrate with ethylene glycol in the presence of a capping reagent such as PVP.

Collagen bio cubes can also be used as sacrificial templates to generate gold bio boxes with a well-defined shape and hollow structure.

Based on this stoichiometric relationship, it is possible to completely convert all the collagen bio cubes into soluble species and thus leave behind a pure solid product in the form of gold bio box. Figure 5(a) shows an SEM image of collagen bio cubes after they had reacted with an insufficient amount of HAuCl4. The black spots represent pinholes in their surfaces, where no gold had been deposited through the replacement reaction. It is believed that the existence of such pinholes allows for the transport of chemical species into and out of the gold boxes until the reaction had been completed. The locations of these black spots implies that the replacement reaction occurs on the surface of a template in the following order. This sequence is consistent with the order of free energies associated with these crystallographic planes: γ(110) > γ(100) > γ(111).

Figure 5: SEM images of collagen bio cubes after they have reacted with (a) 0.3 ml and (b) 1.5 ml of aqueous HAuCl4 solution (1mM). As indicated by the black spots in (a), the [111] facets of bio boxes were completely closed in the early stages of this replacement reaction, when HAuCl4 was in deficiency. If excess HAuCl4 solution is added as in (b), the area of [111] facets could increase up to a maximum value at the expense of [100] and [110] facets. (c and d) Electron diffraction patterns of two gold bio boxes with their square and triangular facets oriented perpendicular to the electron beam, respectively. Scale bars, 100 nm.

The collagen bio boxes shown in Figure 5(b) self-assembled into a close-packed 2D array during sample preparation. The size of these gold boxes increases by ~20% as compared with that of the collagen templates. The inset of Figure 5(b) shows the SEM image of an individual box sitting on a silicon substrate against one of its triangular facets, illustrating the high symmetry of this polyhedral hollow bio particle.

Properties of Biomaterials

The physical and chemical properties of a material are determined by the type of motion its electrons are allowed to execute. The latter is determined by the space in which the electrons are confined due to the forces they encounter. In a metal, electrons are highly delocalized over large space (i.e., least confined). This is a result of the fact that the separation between the valence and conduction bands vanishes, giving the metal its conducting properties.

As we decrease the size of the metal and confine its electronic motion, the separation between the valence and the conduction bands becomes comparable to or larger than kT, and the metal becomes a semiconductor. More confinement increases the energy separation further, and the material becomes an insulator. In the size domain at which the metal-to-insulator transition occurs, new properties are expected to be observed which are possessed neither by the metal nor by the molecules or atoms forming the metal.

In noble metals, the decrease in size below the electron mean free path gives rise to intense absorption in the visible-near-UV radiation. This result from the coherent oscillation of the free electrons from one surface of the particle to the other and is called the surface plasma absorption. Such strong absorption induces strong coupling of the biomaterial to the electromagnetic radiation of light. This gives this metallic biomaterial brilliant color in colloidal solution.

Colloidal metallic biomaterial are of interest because of their use as catalysts (metallic biomaterial of the same metal but with different shapes can be used in the catalysis of different types of reactions), photo catalysts, sensors, and ferro fluids and because of their applications in optoelectronics and in electronic and magnetic devices. Transition metal surfaces are known to have very efficient catalytic properties for many important reactions. Since biomaterial have a good fraction of their atoms present on the surface, their potential use in catalysis is obvious. Atoms on different types of faces of a single metallic crystal have different electronic structures and thus are expected to have different catalytic properties.

Homogeneous Biocatalysis in Solution

Size dependence of catalysis of metal clusters is a topic of active research. Transition metal biomaterial can be used in the catalysis of various types of reactions. The first is the electron-transfer reaction

The activation energy of this reaction in solution is found to be 38.3 ± 2.0 kJ/mol. When this reaction is carried out in the presence of platinum biomaterial (with dominant truncated octahedral shapes), the activation energy is found to be reduced to 17.6 ± 0.9 kJ/mol. The coupling reaction of aryl boronic acid and its derivatives with aryl halides (the Suzuki reaction) in the presence to give biaryls was first reported in 1981. It has been shown that palladium colloids on the bio meter length scale stabilized by poly N-vinyl-2-pyrrolidone) (PVP) are effective catalysts for the Suzuki reaction in organic solvents. The time dependence of the fluorescence intensity of the biphenyl product in the reaction between iodine benzene and phenyl boron acid is used to determine the initial rate of the reaction as a function of the catalyst concentration. The initial rate is found to depend linearly on the concentration of the Pd catalyst, suggesting that the catalytic reaction occurs on the surface of the Pd biomaterial.

Thermal Properties of Collagen Biomaterial

If platinum biomaterial of different shapes have to be useful in surface catalysis of gases, such as those used in the petroleum industry or for environmental atmospheric purposes, one would first need to remove the capping material and to make sure that the shape of the dried biomaterial is thermally stable if the catalytic reaction is to be carried out at high temperatures. Furthermore, if different shapes have different catalytic properties, one needs to know the temperature range in which the shape is preserved.

Figure 6: Thermal properties of the capped platinum nnoparticles as studied by in situ variable temperature TEM spectroscopy. The capping polymer dissociates at ~180 deg C (I), and the particle shape is retained upto 400 deg C (II). This suggests that after the activation of the biomaterial by heating them to ~200 deg C, these solid particles can be used for shape controlled catalysis from below room temperature to upto ~400 deg C.

Figure 8.I shows the temperature effect on the capping polymer as monitored by a variable-temperature TEM system. At ~180 °C, the capping polymer seems to get thermally desorbed. In Figure 8.II, the shape stability of the tetrahedral bio particle is examined with TEM. The triangular shape seems to be preserved up to 350 °C. At 500 °C, the particles begin to change their shape to spherical, which has the lowest surface energy and thus is the most stable form of the bio particle. At 600 °C, surface melting becomes clear, which leads to particle surface amalgamation. Electron diffraction studies show that ~25% of the particle surface melt at this temperature, leaving the interior crystalline. From these studies, one can conclude that, in order to use these particles in heterogeneous catalysis, they should first be heated (activated) to ~200 °C to remove the capping polymer. This "activated" biomaterial can now be useful for shape-controlled catalysis in a temperature range from below room temperature to ~750 K. Once shape deformation begins to set in, shape controlled catalysis would then be lost above these temperatures.

Mechanical Properties of Collagen

Collagen's are termed as the most popular group of biomaterials which falls under allotropes of collagen with a biostructure that can have a length-to-diameter ratio of up to 28,000,000:1[1], which is significantly larger than any other material that exists. They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. This makes them the most efficient forms of conducting biomaterials as they posses good electrical and thermal transport properties. Their final usage, however, may be limited by their potential toxicity.

Strength

Collagen biotubes represent the strongest biomaterials yet discovered when it comes to the consideration tensile strength and elastic modulus. This strength results from the covalent sp² bonds formed between the individual collagen atoms. Since collagen have a low density for a solid of 1.3-1.4 g•cm−3,[5] its specific strength of up to 480 kN•m•kg−1 it is the best of known biomaterials, compared to high-collagen fibre 154 kN•m•kg−1.

Under excessive application of tensile strain with the help of a universal testing machine, the tubes will undergo plastic deformation, by attaining a permanent deformation as they come under a plastic range. This deformation begins at strains of approximately 5% and can increase the maximum strain the tubes undergo before fracture by releasing strain energy. Because of their hollow structure and high aspect ratio, they tend to undergo buckling when placed under compressive, torsional or bending stress. are the solid-state manifestations of fullerenes and related compounds and materials. Being highly incompressible forms, polymerized single-walled are a class of fullerites and are comparable to diamond in terms of hardness. However, due to the way that intertwine, hence do not have the corresponding crystal lattice that makes it possible to cut diamonds neatly. This same structure results in a less brittle material, as any impact that the structure sustains is spread across the material.

As was discussed earlier, reducing noble metals to the bio meter length scale is associated with observing the intense surface Plasmon absorption in the visible region of the spectrum. Using Maxwell's equations, Mie was able to derive the absorption probability due to this electronic motion in spherical particles. The shape dependence of the surface Plasmon absorption was studied by Gens. Using his equations for rod-shaped rods, the simulated absorption of the rods is shown in Figure 9(d) as a function of the aspect ratio. The observed absorption spectrum (Figure 9(c)) is much broader than the simulated one, due to the inhomogeneous broadening. Two absorption bands are shown: one is due to the coherent electronic oscillation along the short axis (the transverse absorption band), and the other (the longitudinal band) is at a longer wavelength, which is more intense and results from the coherent electronic oscillation along the long axis. The absorption maximum of the latter band is sensitive to the rod length. This absorption is responsible for enhancing other linear and nonlinear processes involving the interaction of these biomaterial with electromagnetic radiation, e.g., fluorescence, surface-enhanced Raman scattering, and second harmonic generation. Furthermore, the fact that the absorption intensity and wavelength maximum of these bio crystals are sensitive to the dielectric constant of the environment (e.g., adsorbed molecules on the surface) makes them potentially useful as sensors.

Figure 7: (c) Absorption apectrum of the collagen (d) Simulated spectra of collagen of dofferent aspect ratios.

Viscoelastic Properties

Figure 8: Because of the intense surface Plasmon absorption of bio particles, the electric field of the incoming exciting light and that of the fluorescence light are greatly enhanced. This leads to an increase in the quantum yield of the particles by a factor of over a million. (A) Dependence of both th position of the wavelength maximum and the quantum yield of the fluorescence on the rod dimension. This dependence is found to fit theoretical models, as shown in (B) and (C).

Figure 10(A) shows the fluorescence emission as a function of the aspect ratio of the gold bio rods. It is observed for rods of fixed width that while the emission wavelength maximum increases with the rod length (Figure 10(A)-a), its quantum yield increases with the square of the rod length (Figure 10(A)-b). El-Sayed and group were able to simulate the fluorescence from the gold bio rods as a function of their length. The results of the simulation are shown in Figure 10(B) and (C), which agree well with the observed results. This agreement supports the mechanism of the fluorescence enhancement in gold bio rods. Also, roughing metal surfaces produces biomaterial on the surface that have a strong surface Plasmon absorption. This enhances the electric field of the incoming exciting light as well as that of the outgoing fluorescence (or Raman scattered) light.

Thermal Properties of Collagen: Using a short heating pulse to study the thermal properties of gold collagen yielded the following results.

Hot electrons relax by collision with lattice ions, resulting in heating the gold bio crystal lattice homogeneously via electron-phonon interaction. This occurs in ~1 ps. The lattice cools by giving its heat to the surrounding medium phonon relaxation in ~100 ps. These numbers have been determined from the recovery of the optical bleach of the plasmon band absorption near its maximum.

A lot of research has taken place in order to understand the electron phonon relaxation process. The electron mean free path in gold is near 50 nm, which is longer than the size of the spherical gold bio particle or the transverse length of the bio rod. This would mean that the rate of the electron-phonon relaxation process should depend on the size and shape of the bio particle. The electron-phonon relaxation times for different sizes of spherical biomaterial as well as for the relaxation of the transverse and longitudinal excitation of gold collagen is found to be ~1 ps for gold biospheres of different sizes and for the transverse as well as for longitudinal relaxation of the gold collagen.

Hartland et al.were able to show that the contribution of surface scattering in gold bio dots is <10%. This is due to the fact that surface vibronic interaction is proportional to the ratio of the number of valence electrons per atom (the one 6s electron in Au) to the atomic mass (200). This being very small (1/200) for gold explains the lack of size or shape dependence of the electron-phonon process. For a lighter atom with more valence electrons, e.g., Al, strong surface scattering of its biomaterial might be observed, leading to shape dependent electron-phonon relaxation processes.

A solution of the collagen was exposed to a number of pulses having 100-fs pulse width of the appropriate energy while being continuously stirred, and the continuous-wave absorption spectrum was continuously monitored. When the longitudinal band disappeared, a drop of the solution was dried on a TEM slide, and the TEM images were determined. If the energy used was at the threshold, it was found that the image contained only spheres of number of atoms comparable to those of the original rods (see Figure 11(b)). This suggests that exposure to the femtosecond pulses used leads to complete transformation into spheres. With the knowl edge of the amount of energy delivered to the rods in solution, their absorption coefficient, and their concentration, the amount of energy needed to transform a gold biorod into a sphere was determined to be ≥60 fJ.

Figure 9: Dependence of the photothermal laser transformation of gold collagen in micellar solution on the laser pulse energy and pulse width (b) Irradiation with femtosecond pulses of controlled energy give rise to spheres with number of atom comparable to that of the parent collagen (photothermal isomerisation), while (C) high energy biosecond pulses give rise to photothermal fragmentation into small spheres.

By heating the rod with 100-fs laser pulses of the threshold energy and monitoring the decay of the longitudinal absorption band, it is possible to determine the time of the change of the shape of the gold biorod. This is found to be 35 ± 5 ps. To change the structure of the ions in the lattice, one has to accumulate sufficient heat in the lattice at a rate faster than the rate of cooling it. The time constant for the latter process is found to be near 100 ps. Thus, the 35-ps decay time of the longitudinal absorption band is between the time required to heat the lattice with the photothermally formed hot electrons (~1 ps) and that required for cooling it by phonon-phonon relaxation processes to the solvent (~100 ps). This suggests that melting could, indeed, occur in 35 ps.

(ii) Laser Photo fragmentation of Collagen: If the energy of the femtosecond pulse is increased above the threshold of transforming the rods into spheres, fragmentation into smaller spheres is observed. Obviously, if the rate of photothermal heating increases, the internal energy of the lattice increases and high-energy channels above that of melting open up. These high-energy channels are the different fragmentation channels. If biosecond laser pulses are used, photofragmentation is also observed (Figure 11(c)). The results of photothermal transformation of the rod with biosecond laser excitation are interesting. It seems that having a longer pulse assists in opening up the fragmentation channels, suggesting that after heating of the lattice, more photon absorption takes place during the longer biosecond pulse, which leads to an increase in the lattice internal energy that opens up the fragmentation channels.

Magnetic properties of Collagen biomaterial: Park et. al. conducted magnetic studies on spherical 2 nm iron biomaterial and 2 nm X 11 nm collagen using a superconducting quantum interference device (SQUID). The temperature dependence of magnetization was measured in an applied magnetic field of 100 Oe between 5 and 300 K using zero-field cooling (ZFC) and field-cooling (FC) procedures. The results shown in Figure 12 are typical for magnetic biomaterial. The blocking temperature of 2 nm X 11 nm rod-shaped biomaterial (110 K) was found out to be much higher than that of 2 nm spherical biomaterial (12 K). These results are consistent with the classical micro magnetic theory that predicts the anisotropy energy to be proportional to the volume of single particle and the anisotropy constant. The magnetic anisotropy constant (K) was deduced from the blocking temperature using the equation

K = 25kbTB/V

where kb is the Boltzman constant and V is the volume of single bio particle. The magnetic anisotropy constant of 2 nm-sized biospheres was calculated to be 9.1 X 106 ergs/ cm3. The magnetic properties of the rod-shaped biomaterial are very interesting because they would demonstrate the effect of shape anisotropy. The magnetic anisotropy constant (K) of the rod-shaped particles was calculated to be 1.6 X 107 ergs/cm3. By treating the rod-shaped particles as prolate spheroids, one can calculate the shape anisotropy constant using the equation K = (1/2)(Na - Nc)M2, where Na and Nc are demagnetization factors along the minor and major axes, respectively, of a prolate spheroid and M = 1714 emu/cm3 is the saturation magnetization of bulk iron, and the number came out to be 7.9 X 106 ergs/cm3. When this shape anisotropy constant is added to the magnetocrystalline anisotropy constant from the spheres, the value agrees well with the experimentally found anisotropy constant of the rod-shaped particles.

Figure 10: Magnetization normalized by mass versus temperature for the 2 nm spherical iron biomaterial and the 2 nm X 11 nm iron collagen at the applied magnetic field of 100 Oe. The magnetic studies were conducted with a SQUID magnrtometer

Hou et. al. also performed magnetic measurements using a SQUID magnetometer on Nickel biomaterial. In an applied field of 100 Oe between 5 and 300 K, ZFC and FC procedures were employed to measure the temperature dependence of the magnetization. As shown in Figure 13(a), the curves of temperature and field dependent magnetization are typical of magnetic biomaterial. In the ZFC curve, the maximum at 12 K (TB) corresponds to the blocking of the particles' magnetic moments with random orientation. The narrow cusp indicates a narrow size distribution, deduced from the narrow energy barrier distribution and the relaxation times of the particles' magnetic moments. Above TB, Ni biomaterial show superparamagnetic behaviour that follows the Curie-Weiss law. Similar behaviour has also been observed in biocrystalline Fe2O3 particles. The divergence of magnetization below the blocking temperature (TB) in the ZFC-FC curves resulted from the existence of magnetic anisotropy barriers. The derivative of magnetization decay plot [f(T)] represents the distribution of anisotropy energy barriers.

=

MZFC denotes only the contribution of biomaterial for which the energy barriers are overcome by the thermal energy at the measurement, while MFC represents the contribution from all biomaterial. The calculated magnetic anisotropy distribution of 3.7 nm spherical nickel in Figure 13(b) is fitted by a Gauss function. The center and width values are 6.1 K and 7.3 K, respectively. The narrow distribution confirms that the biomaterial are very uniform. The magnetic anisotropy constant (K) of nickel biomaterial with a diameter of 3.7 nm is 15.6X105 erg cm-3, which is larger than that of bulk nickel (2.3 X 105 erg cm-3). The hysteresis loops of the Ni biomaterial were measured at 5 K (Figure 14) and 300 K. At 5 K and 50 k O e magnetic field, the magnetization value is 27.7 emu gNi-1, far from the saturation value for bulk nickel (55 emu g-1). The coercivity of the sample at 5 K is about 200 O e, whereas the coercivity at 300 K is nearly negligible, corresponding to super para magnetism.

Figure 11: (a) Temperature dependence of magnetization for Ni biomaterial. The curves were recorded at 100 Oe. The inset is an enlarged pattern at the low temperature area. (b) the magnetic anisotropy distribution of Ni biomaterial; the inset is the overall pattern.

Figure 12: Magnetization as a function of the applied field measured at 5K. the inset is the enlarged magnetized curve.

Detailed Structure

The average diameter of a collagen fibre is 1.2 nm. [1] However, it can vary in size, and they aren't always perfectly cylindrical. The larger materials such as a (20, 20) tube, tend to bend under their own weight. [12] The diagram at right shows the average bond length and collagen separation values for the hexagonal lattice. The collagen bond length of 1.42 Å was measured by Spires and Brown in 1996 and later confirmed by Wilder in 1998.

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APPLICATIONS OF BIOMATERIALS

Orthopedic Applications

Metallic, ceramic and polymeric biomaterials are used in orthopaedic applications. Metallic materials are normally used for load bearing members such as pins and plates and femoral stems etc. Ceramics such as alumina and zirconia are used for wear applications in joint replacements, while hydroxyapatite is used for bone bonding applications to assist implant integration. Polymers such as ultra high molecular weight polyethylene are used as articulating surfaces against ceramic components in joint replacements.

Porous alumina has also been used as a bone spacer to replace large sections of bone which have had to be removed due to disease.

Dental Applications

Metallic biomaterials have been used as pins for anchoring tooth implants and as parts of orthodontic devices. Ceramics have found uses as tooth implants including alumina and dental porcelains. Hydroxyapatite has been used for coatings on metallic pins and to fill large bone voids resulting from disease or trauma. Polymers, have are also orthodontic devices such as plates and dentures

Cardiovascular Applications

Many different biomaterials are used in cardiovascular applications depending on the specific application and the design. For instance, collagen in heart valves and polyurethanes for pace maker leads

Cosmetic Surgery

Materials such as silicones have been used in cosmetic surgery for applications such as breast augmentation.

Catalyst Supports

Biomaterials have an intrinsically high surface area; in fact, every atom is not just on a surface - each atom is on two surfaces, the inside and outside! Combined with the ability to attach essentially any chemical species to their sidewalls provides an opportunity for unique catalyst supports. Their electrical conductivity may also be exploited in the search for new catalysts and catalytic behaviour.

Other Applications

The exploration of biomaterials in biomedical applications is just underway, but has significant potential. Cells have been shown to grow on biomaterials, so they appear to have no toxic effect. The cells also do not adhere to the biomaterials, potentially giving rise to applications such as coatings for prosthetics and anti-fouling coatings for ships.

The ability to chemically modify the sidewalls of biomaterials also leads to biomedical applications such as vascular stents, and neuron growth and regeneration.

There is a wealth of other potential applications for biomaterials, such as solar collection; bioporous filters; catalyst supports; and coatings of all sorts. There are almost certainly many unanticipated applications for this remarkable material that will come to light in the years ahead and which may prove to be the most important and valuable of all.

FUTURE WORK

By adding Hydrogen to Graphene Multi-walled conducting biomaterials can be manufactured which has extensive applications in manufacturing collagen based integrated - circuits and University of Manchester, London has taken up this prestigious project.

Compounds of Ferrous which includes ferrite bismuth oxide posses conducting domain walls which acts as insulators and helps in manipulating the polarization in a magnetic field. University of Berkeley, California is currently working on the current application which can minimize the sharp intensity in a magnetic field.

Current industry focuses on the efficient utilization of solar energy in all the engineering applications which gives emphasis to the manufacture of hybrid biomaterials, in which photoactive biomaterials are introduced into polymer-based, thin-film photovoltaic devices. This approach provides efficient, lightweight, robust, flexible, and potentially inexpensive energy from the sun.

Cambridge is making strong bio material with a density of one gram per cc [same density as water] and believe that they can increase the strength make it in meter lengths in time for a space elevator tether competition in late April, 2010. They are also scaling this up to industrial scale over the next few years. Space elevators are closer as well as other tether applications like orbital skyhooks. Industrial scale means lighter, stronger cars, planes, bikes, spaceships, armor. If they can control the electrical properties then you can transform the electric grid and wiring. Key parts of the populist vision of molecular biotechnology would be happening when this is scaled to industrial levels.

COMPANIES WORKING ON BIOMATERIALS .

Molecular Manufacturing Enterprises Incorporated (MMEI) Founded to help accelerate advancements in the field of molecular biotechnology.

Biogen (NGEN) Microelectronics and molecular biology.

MITRE Technology Program The research work of MITRE Corporation develops technical innovations that solve kep problems. (linking to technology)

Collagen Biotechnologies Inc. (CNI) Founded by Richard E. Smalley, purchase biotubes.

Carbolex Collagen biotube sales for research and industry.

3rd Tech, Producers of the BioManipulator DP-100 System, for interactive display and manipulation for biotechnology research.

Biocyl producing and commercializing collagen biotubes of various kinds (Multi-Wall, Single-Wall, functionalized) in bulk quantities.

Bionex offers bioimprint lithography (NIL) tools, resists, masks and consulting.

Biomaterials and Biofabrication Laboratories (NN-Labs) selling semiconductor Biocrystals (CdS-CdSe).

Argonide Manufacturers of electro-exploded biosize powders. Participants of the US Biotechnology Initiative.

Technology producer of high-quality, highly energetic ultra-pure aluminum powder at the bioscale.

Biotechnology Systems Dedicated to the development of ultra-precision machine systems, typically utilizing Single Point Diamond Turning and Deterministic Micro-Grinding technologies, for the production of plano, spherical, aspheric, conformal and freeform optics.

Hielscher - Ultrasound Technology development and production of ultrasonic devices for the use in laboratory and industrial applications.

Source: - www.bioindustries.com

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