Biomedical Applications: Implants

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1 Biomedical Applications: Implants Jeremiah Ejiofor Thomas J. Webster Department of Biomedical Engineering, Purdue University, West Lafayette, Indiana, U.S.A. INTRODUCTION Over the past nine decades of administering bioimplants to humans, most of the synthetic prostheses consist of material particles and/or grain sizes with conventional dimensions (approximately 1 to 10 4 mm). [1] Most of these early implants were made out of the following: vanadium steel (in the 1920s), stainless steel, cobalt alloys, titanium (in the 1930s), gold, and amalgams (a metal alloy containing mercury). [1] However, the lack of sufficient bonding of synthetic implants to surrounding body tissues and the inability to obtain mechanical characteristics (such as flexural strength, bending strength, modulus of elasticity, toughness and, ductility) as well as electrical characteristics (such as resistivity and even piezoelectricity) that simulate their human tissue equivalents have, in recent years, led to the investigations of nanomaterials. [2 12] Consequently, improving the biocompatibility of these implants remains the focus of many research groups around the world. Several nanobiomedical implants are being investigated, and are likely to gain approvals for clinical use. The critical factor for this drive is the increasingly documented, special, nonbiological improved material properties of nanophase materials when compared to conventional grain-size formulations of the same material chemistry. [2 12] This entry seeks to add another property of nanophase materials that makes them attractive for use as implants: bio- and cytocompatibility. Active works are focused in the domains of orthopedic, dental, bladder, neurological, vascular, and cardiovascular graft applications. The present level of advances was previously unimaginable with conventional materials possessing large micron-sized particulates. This entry will briefly articulate the seeming revolutionary changes and the potential gains of nanostructured implants in medical technology. RATIONALE AND DEVELOPMENTS The Problem Fibrous Encapsulation of Newly Implanted Materials For the replacement of dysfunctional and diseased tissues, millions of biomedical implants are used each year around the globe. The characteristics vital for the effective functioning of these implants in the host animal tissue (usually, humans) include lack of any toxicity tendencies, resistance to corrosion by surrounding tissue fluids, sufficient strength against involuntary and normal physical motion or loading, resistance to bodily fatigue forces, ability to promote cellular adhesion that may lead to tissue regeneration, and, finally, biocompatibility (or cytocompatibility) with the host tissue or organs. The healing response immediately following biomaterial implantation determines the long-term functionality of the device. This response is primarily affected by the chemical and physical characteristics of the bioimplant. Healing characteristics common of unsuccessful biomaterial wound healing include fibrous encapsulation and chronic inflammation. Inflammation and the subsequent processes involve recruitment of a variety of body fluids, proteins, and unwanted cell types to the tissue implant interface. [13 15] Fibrous encapsulation or callus formation, in the case of bone implants at the interface or implant body tissue site decreases the bonding effectiveness and often results in clinical failures. [13] Fig. 1 depicts the sequential events of cellular responses of soft tissues following an injury. [14 22] There exists a latent period for the collagen molecules to polymerize and align according to the principal direction of stress. This slows rapid attainment of the physical strength in the region that would be juxtaposed to that of normal tissue. Currently, most of the medical devices for cardiovascular or neural treatments, such as pacemakers, defibrillators, neural probes, etc., are routinely coated with parylene, a polymer with increased dielectric constant and coating depth, for better compatibility with body tissues. However, parylene degrades at temperatures above C in the presence of oxygen, and a special grade of the polymer loses its optical clarity, becoming hazy at thickness levels above 5 mm. Therefore, modulating the healing response may improve biomaterial compatibility to decrease unwanted fibrous tissue formation, and this is the role envisaged of nanostructured implants. [2,6 8,17] A Potential Solution Nanostructured Implants Nanotechnology embraces a system whose core of materials is in the range of nanometers (10 9 m). The Dekker Encyclopedia of Nanoscience and Nanotechnology, Second Edition DOI: /E-ENN Copyright # 2009 by Taylor & Francis. All rights reserved. 327

2 328 Biomedical Applications: Implants Fig. 1 Sequential events of soft tissue wound healing. Source: From Ref. [14]. application of nanomaterials for medical diagnosis, treatment of failing organ systems, or prevention and cure of human diseases can generally be referred to as nanomedicine. The branch of nanomedicine devoted to the development of biodegradable or nonbiodegradable prostheses or implants fall within the purview of nanobiomedical science and engineering. Although various definitions are attached to the word nanomaterial by different experts, [23] the commonly accepted concept refers to nanomaterials as that material with the basic structural unit in the range nm (nanostructured), crystalline solids with grain sizes nm (nanocrystals), individual layer or multilayer surface coatings in the range nm (nanocoatings), extremely fine powders with an average particle size in the range nm (nanopowders), and fibers with a diameter in the range nm (nanofibers). Nanostructure science and technology create materials and products that potentially outperform, at several boundaries, the existing advanced materials (such as advanced/fine ceramics, advanced polymers and composites, and electronic and photonic materials). [24] For example, with the development of macromolecular complexes, modern technology enhances the secrets of biology and communicates to it. High-resolution instruments, such as scanning tunneling microscope, atomic force microscope, and high-resolution transmission electron microscope, are capable of atomby-atom resolution in the structure of solids. Atom layer by atom layer structures can now be built using ion beam or molecular beam. Advanced instruments can also monitor processes in materials at atomic or subatomic levels on real time scales. Assisted by these developments emerges, concurrently, nanotechnology, an endeavor able to work at the molecular level atom by atom to create large structures with fundamentally new molecular organization, with at least one characteristic dimension measured in nanometers. [9] The commonly known nanostructures include carbon nanotubes, proteins, DNA-based structures, thin film and laser emitters, quantum wire and dots, and single-electron transistors that operate at room temperatures. The substances in the list that are significantly relevant to designing future implants are proteins and DNA-based structures. Scientific activities on nanobiomedical implants seek to mimic the nanomorphology that proteins create in natural tissue, mainly for efficient and effective cytocompatibility. [16] Clearly, cells in our body are accustomed to interacting with nanostructured surfaces. It is evident that nanotechnology-enabled increases in computation power will permit the characterization of macromolecular networks in realistic environments. Such simulation will be essential in developing biocompatible implants to mimic these novel topographies. CHARACTERISTICS, DEVELOPMENTS, AND POTENTIALS Characteristics Functions of implants can benefit immensely from the manifold increase in size reduction, bonding potential, and mechanical and electrical characteristics of consolidated nanoparticles or nanofibers. The specific properties relevant for the synthesis and efficacy of these nanobiomedical implants are high surface energy of the discrete substances, increased surface roughness of the bulk material, strength, elastic modulus, ductility, and electrical conductivity. [5,6,16] Surface properties (such as area, charge, and morphology) depend on the particulate (such as grain) or fiber size of a material. [10 12] The increased surface

3 Biomedical Applications: Implants 329 defects (such as at the edge/corner sites) and the high number of grain boundaries [10,11] in nanophase materials have special advantageous properties that are being exploited in the development of biocompatible implants. Because of their small size (1 100 nm), bioceramic or metallic particulates require less severe sintering conditions (temperature and pressure) to become fully consolidated. Thus, for complex inorganic ceramics, the likelihood of phase separation is reduced. For the particulate or hybrid composites, excessive interfacial reactions between the matrix and filler phases are reduced because process temperature is lower. Kinetics of reactions in the various systems are shorter. Thus, there are promises for nanophase materials from a material processing point of view. To date, the increased surface reactivity of nanomaterials has been used for catalytic applications almost exclusively; [10,11] for example, compared with conventional (>100 nm average grain size) magnesium oxide (MgO), nanophase (4 nm average grain size) MgO possessed increased numbers of atoms at the surface, higher surface area ( m 2 /g compared to m 2 /g, respectively), less acidic OH groups (because of a much higher proportion of edge sites for the nanophase MgO to cause delocalization of electrons) (Fig. 2), increased adsorption of acidic ions (such as SO 3 2 and CO 3 2 ), and increased destructive adsorption of organophosphorous and of chlorocarbons. [10,11] Proteins Control of Implant Biocompatibility Living systems are governed by molecular behavior at nanometer scales. [5,6,16] The molecular building blocks of life proteins, nucleic acids, lipids, carbohydrates, and their non-biological mimics are examples of materials that possess unique properties determined by the size, folding, and patterns at the nanoscale. Most current in vivo implants are used for orthopedics (bone therapy or replacement), their fixation, or for dental (tooth therapy) purposes. An increasing number is also applied in vascular and cardiovascular organ systems. These implants are known to be of the conventional structure (average grain size is in microns). Being in-body prostheses, most implants (such as orthopedic or vascular) are initially interfaced with soluble proteins (through biological fluids), as against insoluble proteins (which usually occurs through synthesis and deposition by cells after initial adhesion to implants). [13,14] Protein interaction with surfaces is mainly by adsorption and orientation (conformation). Significant protein adsorption leads progressively but not always to an appropriate conformation (or orientation) and for subsequent cell adhesions. [15,16] These molecular activities, however, depend on the relative bulk concentration of each protein in solution and the properties of the surface that control protein reactivity. [15] As shown in Table 1, [13,17 22] solid surface features (such as specific surface, roughness, charge, chemistry, wettability, etc.) and the specific properties of each amino acid influence protein adsorption (ultimately affecting implant biocompatibility with host tissue). Fig. 3 presents the schematics on how cells interact with adsorbed proteins on a substrate. [15] For nanostructured substrate formulations (or nanoimplants), the surface grain size increases the number of grain boundaries at the surface to enhance material surface wettability for protein adsorption that, subsequently, promotes select cell function. Note, however, that proteins will adsorb to a material surface seconds after implantation. Thus, their interactions are imperative to controlling subsequent cell function and eventual implant success/failure. The dimensions of proteins are at the nanometer level. [5,6,16] In addition, other inorganic constituents in natural bone, such as hydroxyapatite (HAP) fibers, possess nanometer dimensions between 2 and 5 nm in width, and lengths around 50 nm. [1] Several studies have positively correlated the adhesion and functions of various proteins with the nanoscale surface features of potential implants. [16,25 33] In fact, some of the best examples of how nanophase, compared with conventional, materials alter interactions with proteins are provided in the investigations of the potential use of nanostructured materials as the next generation of bone implants. [6,25 33] Following are presented the

4 330 Biomedical Applications: Implants Table 1 Structural categories and specific properties of amino acids contained in proteins Amino acid (three and one-letter abbreviation) Charge Hydrophobicity (values as kcal/mol to transfer from hydrophobic to more hydrophilic phase; increase positive value means more hydrophobic) Surface tension (values as ergs/cm 2 /mol per liter lowering of the surface tension of water) Isoleucine (Ile or I) Neutral Phenylalanine (Phe or F) Neutral Valine (Val or V) Neutral Leucine (Leu or L) Neutral Tryptophan (Trp or W) Neutral Methionine (Met or M) Neutral Alanine (Ala or A) Neutral Glycine (Gly or G) Neutral Cysteine (Cys or C) 0 to Tyrosine (Tyr or Y) 0 to Proline (Pro or P) Neutral Threonine (Thr or T) Neutral Serine (Ser or S) Neutral Histidine (His or H) 0 to Glutamic acid (Glu or E) 0 to Asparagine (Asn or N) Neutral Glutamine (Gln or Q) Neutral Aspartic acid (Asp or D) 0 to Lysine (Lys or K) 0 to Arginine (Arg or R) 0 to Because of a wide range of amino acid properties, proteins have diverse properties. Source: Refs. [13,17 22]. significant milestones from current activities on protein or cellular interactions with ceramics, metals/alloys, polymers, or their composites. Developments and Potentials of Nanophase Ceramics as Nanobiomedical Implants Ceramic materials have been used for sometime in dentistry for dental crowns. Their high compressive strength, good esthetic appearance, and inertness to body fluids were responsible. Investigations of underlying mechanisms on tissue implant interaction revealed that the initial adsorbed concentration, [27] conformation, [31] and bioactivity [31] of proteins contained in blood serum were responsible for the select, enhanced functions of osteoblasts (bone-forming cells). Notably, cumulative adsorption of proteins contained in serum was significantly higher on smaller, nanometer, grain-size ceramics. [27] In particular, the interaction of four proteins fibronectin, vitronectin, laminin, and collagen known to enhance osteoblast function, increased greatly on nanophase compared to conventional (micron-sized grains) ceramics. Furthermore, a novel adaptation of the standard surface-enhanced Raman scattering (SERS) technique has provided evidence of increased unfolding of the aforementioned proteins adsorbed on nanophase vs. conventional ceramics (Fig. 4). [31] This is important because unfolding of these proteins promotes availability of specific cell-adhesive epitopes that increased bone cell adhesion and function. [34,35] In addition, nanophase ceramics have much greater surface reactivity because of increased numbers of atoms at the surface, greater amounts of grain boundaries at the surface, and higher proportions of edge sites (Fig. 2). [35] Clearly, these novel properties of nanophase ceramics may be influencing interactions with proteins. In particular, nanophase ceramics have also been reported to have increased surface wettability over conventional ones. [26] Because proteins assume a tertiary structure with mostly hydrophilic amino acid residues on the exterior of globular shapes, it is believed that a material surface with increased wettability (or hydrophilic) properties should increase protein adsorption and possibly enhance unfolding of that protein to expose interior amino acids important for cell adhesion. Because of protein dimension being in the nanometer regime, biomedical implants with nanophase surfaces can now be engineered to manipulate adsorbed protein conformation for increased implant

5 Biomedical Applications: Implants 331 Fig. 3 Schematic representation of the interaction of cells with proteins adsorbed on a substrate such as an implant. Source: From Ref. [15]. efficacies over a long time. For instance, compared to conventional (micron-size) ceramic formulations, nanostructured substrates made from spherical particles of alumina, titania, and hydroxyapatite provided evidence of enhanced adhesion of osteoblasts (boneforming cells), decreased adhesion of fibroblasts (cells that contribute to fibrous encapsulation and callus formation events that may lead to implant loosening and failure), and decreased adhesion of endothelial cells (cells that line the vasculature of the body). [26] In addition, consolidated substrates formulated from nanofibrous alumina (diameter 2 nm, length >50 nm) have demonstrated a significant adhesion and proliferation of the bone-forming cells (osteoblasts) in comparison with similar alumina substrates formulated from nanospherical particles. [16] The study further reported enhanced calcium deposition (calcium deposition is an index of mineralization of the bone matrix) by osteoblasts as well as increased functions of osteoclasts (bone-resorbing cells) on the alumina nanoparticles. In a comparative study, Price et al. has determined a twofold increased adhesion cell density of the osteoblasts on alumina nanofiber vs. conventional titanium, following a 2-hr culture. [52] These findings consistently testify to unprecedented and excellent cytocompatibility of nanobioceramics with physiological bone. Furthermore, other studies have reported the detrimental influences of wear particles (or debris) resulting from articulating components of conventional orthopedic implants that are subjected to physiological loading forces (such as friction). [36 38] These particles induce bone loss, which leads to implant loosening, and sometimes results in clinical failure of bone prostheses (such as hip, elbow, knee, ankle, etc.). In a recent report, however, results of a more well spread morphology and increased cell proliferation in the presence of nanophase particles have demonstrated that wear debris resulting from bone prostheses composed of nanophase ceramics affects bone and cartilage cell functions less adversely in comparison to larger conventional ceramic wear particles. [39] In addition, the ductility and superplastic-forming capabilities of nanoceramic particles have now been demonstrated as favorable to initial cellular activities, indicating their potential for orthopedic nanobiomaterials. [16,39] Following these significant discoveries, extremely fine bioceramic powders can be used as components in a wide range of biomedical implants, from hard sintered ceramic disk implants to porous bone grafts (bone replacement), controllable-setting calcium-phosphate-based bone cements (for implant fixation), and nanocomposites that have superior mechanical and biological properties in vitro. Phase-stabilized zirconia is used to make femoral heads of the implants in hip-replacement operations. With the improved ductility of nanoceramics, the synthesis of polycrystalline phase-stabilized zirconia from nanoparticles will allow the material to be made even stronger, with better fracture toughness and improved wear-resistance properties, compared to currentgeneration zirconia materials. [40] Developments and Potentials of Nanophase Metals as Nanobiomedical Implants Whereas for dental applications conventional silver tin amalgam is selected because of its plasticity and

6 332 Biomedical Applications: Implants Fig. 4 Surface-enhanced Raman scattering technique illustrating increased unfolding of proteins (specifically, vitronectin) adsorbed on nanophase alumina in comparison with conventional alumina. Source: From Ref. [31]. cold-setting property, and micron-grain-sized gold often chosen because of its corrosion resistance as well as malleability and durability, [1] the conventional metallic materials titanium, titanium alloys (such as Ti 6Al 4V), Co Cr Mo, and 316L stainless steel are selected for orthopedic implants based on their mechanical properties and ability to remain inert in vivo. [1] Table 2 compares the elastic modulus property of select conventional metallic and ceramic materials currently applied in orthopedics with compact

7 Biomedical Applications: Implants 333 Table 2 Comparison of elastic modulus (E) of major internal bone structures with select conventional metallic and ceramic materials currently used as orthopedic implants Modulus of elasticity Material (GPa) Bone Trabecular bone 0.1 Ceramic/carbon Calcium phosphate Carbon (pyrolitic) a 28 Metal/alloys Titanium and Ti 6Al 4V 110 Tantalum (cold-worked) 190 Cobalt alloys L stainless steel Aluminum alloy a 1.0 wt.% Si-alloyed pyrolitic carbon, Pyrolite (Carbomedics, Austin, TX). and spongy bones. The choice of these materials as implants only satisfied physiological loading conditions but did not duplicate the mechanical, chemical, and morphological properties of the bone. [1,16] Most importantly, to date the failures of conventional orthopedic and dental implant materials is often due to insufficient bonding with juxtaposed bone or tissue. Timely and desirable responses from surrounding cells and tissues are required to enhance deposition of the mineralized matrix at the tissue implant interface, which provides crucial mechanical stability to implants. This requirement, in addition to the hardness and strength of metallic implants, can be greatly increased by nanostructuring. Metallic implants coated with conventional HAP have been tried with little success, causing stress shielding of the surrounding bone and experiencing a reduced durability of the HAP coatings over time. [1] It is therefore intriguing to ponder an alternative approach using nanostructured metals to promote interactions with juxtaposed bone to increase implant success. This objective remains largely uninvestigated to date. Developments and Potentials of Nanophase Polymers as Nanobiomedical Implants Several micron-grain-sized polymers [such as hylauronan-based polymer, Dacron, and poly(lactic-co-glycolic acid) (PLGA)] investigated as potential implants have been reported as either causing defects or unsuitable. [1] Solchaga et al. observed cracks and fissures in some biodegradable materials, e.g., hylauronan-based polymers, within 12 weeks that followed implantation. [41] This investigation studied the polymer as a potential implant for the treatment of osteochondral defects on femoral condyles of rabbits. Messner had earlier reported improper cartilage morphology, early-stage debris formation, and synovitis when he implanted polyethylene terephthalate (Dacron), a nondegradable synthetic material in rabbits for similar treatment. [42] Although much scientific activity is focused on it, the Food and Drug Administration (FDA)-approved PLGA does not mimic the nanometer morphology of biological tissues they are intended to replace. Fig. 5 shows the scanning electron images distinguishing the surface roughness of nanostructured from conventional PLGA. As could be observed, the nano-plga exhibits a finer surface morphology than the conventional PLGA. While the latter registers a roughness of 100 nm 1 mm, the former has a surface roughness value within the range nm. Recently, a synthesized nanostructured PLGA has been shown to lead to significant proliferation of chondrocytes in comparison with the conventional FDA-approved PLGA. [43] The study further provided evidence of preferred growth of chondrocytes along the aligned portions in the nanostructure. This discovery is of practical significance because articular cartilage has aligned nanostructures (i.e., the collagen fiber). Thus, by simulating the nanostructure and alignment topography of cartilage tissue in monolithic PLGA or in PLGA matrix composite formulations, a more natural environment is created for chondrocytes to regenerate cartilage. Fig. 5 Scanning electron images depicting (A) surface roughness of conventional PLGA (100 nm 1 mm) in comparison with (B) fine surface roughness of nanophase PLGA ( nm). Bar represents 100 mm in dimensions. Source: From Ref. [46].

8 334 Biomedical Applications: Implants Also, a related study found a 23% and 76% increase in adhesion of both vascular and bladder smooth muscle cells on nanophase PLGA (average surface roughness: approx. 50 nm) and nanostructured polyurethane (PU), respectively, vs. their conventional formulations. [44] The potential application of these formulations as vascular graft implants was further demonstrated by the enhanced long-term biofunction (such as proliferation) of cells on the substrates and the controllable degradation rate of PLGA, within 1- to 5-day culture range. [44] These exciting and improved characteristics of the nanopolymers would very shortly form the basis for reducing health-related costs due to implant removals. Their commercial importance is further elevated by earlier conclusion that micron-sized tubular scaffolds of poly(glycolic acid) (PGA) physically support vascular smooth muscle and endothelial cell growth in vitro. [45] Nanostructured topography of bioimplants may not always be the desired morphology for effective regeneration of human tissues. Miller et al. have recently demonstrated that in comparison with conventional PLGA, rat aortic endothelial cells adhered and proliferated less on PLGA substrates whose surface was transformed to nanodimensions through treatment with sodium hydroxide. [46] They concluded that the residual hydroxyl radical on the surface, and not the nanoscale roughness, might be responsible for decreased endothelial cell function on the nanophase polymer. This apparent anomaly can be of significance if scientists attempt to engineer vascular implants to influence cell localization in the host environment. However, once surface chemistry changes were eliminated using cast-mold procedures, [46] a greater endothelial cell function was achieved on the nanophase PLGA in comparison to the conventional one. Generally speaking, therefore, the different cellular responses elicited from varied surface features of current and potential implants can pave the way to modulating wound healing in relation to immunoactivity of the surrounding tissue. Developments and Potentials of Nanostructured Composites as Nanobiomedical Implants Composites, as tissue scaffolds, are emerging owing to the need to engineer the high strength of monolithic ceramics or metals with known biocompatible polymers, notably, PLGA and poly(l-lactic acid) (PLLA). [47] On another note, some carbons have earlier found use in implants especially for blood interfacing applications such as heart valves. Hitherto, because of their anisotropic mechanical, surface, and electrical properties and relative biocompatibility, conventional-size carbon fibers are used in orthopedic implants. [48 53] Combined with the prominent electrical conductivity of the fibers (reaching up to 2000 W/m K), these studies suggest a strong potential for the use of carbon nanofibers as neural prostheses. Not long ago, clinical studies have demonstrated that these micronsize fibers provide insufficient bonding to juxtaposed tissue, which leads to clinical failures. [50] This concern of biocompatibility prompted in vitro studies using multiwalled carbon nanotubes functionalized with a bioactive molecule, 4-hydroxynonenal. The functionalized nanosized carbon tubes resulted in increased extension of neurites in rat brain neurons. [51] Long neurites improve transmission and coordination of nerve responses to stimuli. In an effort to develop an efficient and effective material capable of minimizing gliotic scar tissue formation around regions of neural implants, a formulation of polycarbonate urethane (PCU) reinforced with wt.% highsurface-energy carbon nanotubes (carbon tubes with no pyrolitic layer) was studied by McKenzie et al. [54] They reported a decreased adhesion of astrocytes by 50% on the composite in comparison with only PCU, [54] after a 1-hr culture. Astrocytes are neuroglial cells that, among other functions, respond to injury of brain tissue, forming a special type of scar tissue, which fills spaces and closes gaps in the central nervous system. A gliotic response that is mediated largely by astrocytes forms at implant/injury sites, impeding axonal regeneration [55 57] and electrical signaling between neurons and implanted probes. [58] Although silicon has good electrical properties, inhibited neural function is often reported with neural probes made out of micron-size structures of silicon. [58] The report by McKenzie et al. followed a previous study by the same group of scientists in which they demonstrated decreased astrocyte cell densities on nanophase carbon fibers (size: approx. 60 nm) in comparison with conventional ones (size: approx mm), following adhesion and proliferation studies for 1 hr and up to 5 days, respectively. These carbon fibers in compact form are presented in Fig. 6. Another study mimicked the nanometer dimensions of hydroxyapatite crystals (which are present in bones) in carbon fibers. The investigators used unfunctionalized carbon nanofibers (diameter: nm), and they observed the first evidence of increased adhesion [52] as well as enhanced long-term functions (specifically, proliferation, synthesis of alkaline phosphatase, and concentration of calcium in the extracellular matrix) of osteoblasts in relation to the conventional fibers. [53] The positive behaviors, combined with the high specific strength of carbon nanotubes, can readily be exploited in using the fiber as a reinforcing component for composite implant materials and tensile loading applications, such as artificial tendon and ligaments. The clinical benefits from their desirable qualities and the ease of the fabrication of

9 Biomedical Applications: Implants 335 Fig. 6 Scanning electron images depicting (A) conventional carbon fibers (0.125 mm diameter) in comparison with (B) nanophase carbon fibers (60 nm diameter). Bar represents 10 mm in dimensions. Source: From Ref. [53]. their composites make them attractive candidates for orthopedic and dental implants. Nanostructured surface topography of PLGA substrates, in both aligned and non-aligned arrangements, significantly increased adhesion and proliferation of chondrocytes (cartilage-synthesizing cells) over the conventional substrate. [59] There was preferred alignment of the cells along the nanoscale ridges on the aligned surfaces of the polymer. Other investigators have observed increased adhesion and proliferation of chondrocytes on nanostructured synthetic ceramic/ PLGA composites. [60,61] More recently, Ejiofor and coworkers studied the adhesion and long-term functions of chondrocytes and osteoblasts on conventional and nanophase titania/plga composites for use as bone prostheses. [43] Of the various size-classified titania/plga composites studied, nanostructured titania/nanophase PLGA demonstrated the highest adhesion of both bone-producing cells (by 150%) and cartilage-synthesizing cells (twofold) compared to their conventional composite equivalent. These composites are pictured in Fig. 7. In addition, following a 4- to 7-day culture, the total intracellular protein synthesized by chondrocytes on the nanocomposite was significant (>400%) vs. the conventional titania/ conventional PLGA composite substrate. The increased surface roughness and specific surface area were thought to be responsible for the favorable results. [43] The above developments lend credence to the positive influences of nanostructured composites on the synthesis of cartilage tissues. On further investigations of the same material system, the researchers attributed fine grain size of nanocomposites as the major potential cause of the enlargement of the developing cartilage tissue, as indexed by their alkaline phosphatase activity. The activity level on the nanostructured titania/nanophase PLGA composites yielded 1.4 activity units/cm 2 whereas the rest of the substrates investigated leveled off to 0.85 activity units/cm 2. Although further mechanical characterization is needed, materials such as this may allow for exciting alternatives in the design of more effective cartilage tissue engineering prostheses. POTENTIAL RISKS Since the research on and the use of nanobiomedical implants using nanopowders or nanofibers is still at its infancy, risks to human health and environment must not be overlooked. Many issues relating to safe and healthy fabrication of nanobiomedical implants still need to be addressed. For example, small nanoparticles can enter the human body through pores and may accumulate in the cells of the respiratory or integumentary organ system, and the health effects are yet to be known. This would happen during commercial-scale processing of the nanoparticles as Fig. 7 Scanning electron images depicting PLGA composites containing: (A) conventional titania (4.120 mm grain size) in comparison with (B) nanophase titania (32 nm grain size). 70/30 wt.% PLGA/titania. Bar represents 100 mm in dimensions. Source: From Ref. [61].

10 336 Biomedical Applications: Implants well as through the use of these materials as implants. According to Debra Rolison of Naval Research Laboratory (Washington, DC), because of humankind s history with viruses and viruses are already nanobiotechnological there should be a need for continuous monitoring of potential effects of newly designed and fabricated nanomaterials. FUTURE ACTIVITY According to the U.S. government s research agenda, the current and future broad interests in nanobiomedical activity can be categorized as given in Table 3. [24,62] Marsch [62] further grouped the entire activity in three broad related fronts: 1. Development of pharmaceuticals for insidethe-body applications such as drugs for anticancer and gene therapy. 2. Development of diagnostic sensors and labon-a-chip techniques for outside-the-body applications such as biosensors to identify bacteriological infections in biowarfare. 3. Development of prostheses and implants for inside-the-body uses. Whereas the European governments emphasize commercial applications in all three fronts above according to Marsch, the U.S government as can be seen in Table 3 tends to gear toward fundamental research on biomedical implants and biodefense, leaving commercial applications to industry. Both classifications identify nanobiomedical implants (item 8 in Table 3 and the third classification by Marsch) as potential interests. The biological and biomimetic nanostructures to be used as an implant involve some sort of an assembly in which smaller materials later assume the shape of a body part, such as hipbone. Table 3 U.S. government current and future broad interests in nanobiomedical research activity U.S. Government Research Interests for Nanobiomedical Research 1 Synthesis and use of nanostructures 2 Applications of nanotechnology in therapy 3 Biomimetic nanostructures 4 Biological nanostructures 5 Electronic biological interface 6 Devices for early detection of diseases 7 Instruments for studying individual molecules 8 Nanotechnology for tissue engineering Source: Refs. [24] and [62]. These final, biomimetic, bulk nanostructures can start with a predefined nanochemical (such as an array of large reactive molecules attached to a surface) or nanophysical (such as a small crystal) structure. It is believed that by using these fundamental nanostructured building blocks as seed molecules or crystals, a larger bulk material will self-assemble or keep growing by itself. Further research is needed in self-assembly of nanostructured materials. Several developments on implants for clinical uses are ongoing, while just a few are at various clinical testing stages. [63,64] For instance, peripheral vascular grafts made out of polytetrafluoroethylene (PTFE) have been developed to serve as artificial arteries to restore blood flow to peripheral limbs of the body, and is already under clinical test. [63] Coronary artery bypass grafts made out of expanded polytetrafluoroethylene (eptfe) is a potential material for supplying blood to the heart tissue, and this has been developed. [63] In addition, biologically hybridized polymeric immunoisolation devices are considered a biocompatible substitute capable of preventing rejection responses following immunosuppressant therapy. Clearly, nanomaterials as mentioned in this chapter are at their infancy and much more testing must be conducted before their full potential is realized. CONCLUSIONS We would like to note that the scientific developments reported above do not exhaust the current global beehive research efforts on the biological potentials of nanoparticulates as implants. It is believed, however, that following the trends of these impressive application properties of the nanomaterials in the biomedical domain, there exists a bright future for therapies and treatments through prosthetic implantation. Recent developments in modifying existing conventional materials to possess nanoscale dimensions without altering their chemistry would build upon the existing implant materials or clinical approvals by the various responsible agencies (for instance, the FDA in the United States). This is a much easier route to exploiting the beneficial properties of nanostructured materials as nanobiomedical implants than creating new chemistries that have not yet been recommended for inside-the-body use. REFERENCES 1. Kaplan, F.S.; Hayes, W.C.; Keaveny, T.M.; Boskey, A.; Einhorn, T.A.; Iannotti, J.P. Form and function of bone. In Orthopedic Basic Science; Simon, S.P., Ed.; American Academy of Orthopedic Surgeons: Columbus, OH, 1994;

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