Introduction

Osteoblasts and osteoclasts are located in bone, a natural nanostructured-mineralized organic matrix. While osteoblasts make bone, osteoclasts decompose bone by releasing acid that degrades calcium phosphate-based apatite minerals into an aqueous environment. The synthesis, deposition, and mineralization of this organic matrix, in which osteoblasts proliferate and mineralize (that is, deposit calcium), require the ordered expression of a number of osteoblast genes. Bone has the ability to self-repair or remodel routinely. However, osteoporosis (unbalanced bone remodeling) and other joint diseases (such as osteoarthritis, rheumatoid arthritis, or traumatic arthritis) can lead to bone fractures. These disabilities associated with bone lead to difficulties in performing common activities and may require an orthopedic implant. However, the average functional lifetime of, for example, a hip implant (usually composed of titanium) is only 10-15 years. A lack of fixation into surrounding bone eventually loosens the implant and is the most common cause of hip replacement failure.

Approximately, 200,000 total and 100,000 partial hip replacements as well as 36,000 revision hip replacement surgeries were performed in the United States in 2003 (Zhan et al. 2007). Unfortunately, only limited techniques, such as insensitive imaging (through X-rays or bone scans), exist to determine if sufficient bone growth is occurring next to a titanium implant after surgery. Thus, techniques used in orthopedic diagnostics need to more accurately identify musculoskeletal injuries and conditions after implantation. Although there have been improvements in implants to increase bone formation, the clinical diagnosis of new bone growth surrounding implants remains problematic, sometimes significantly increasing hospital stays and decreasing the ability of doctors to quickly prescribe a change in action if new bone growth is insufficient.

Currently, a physical examination (e.g., palpation, or laboratory testing) might be completed before imaging techniques are used to inform a clinician about a patient's health. Although advanced imaging techniques, such as bone scans, computer tomography scans, and radiographs (X-rays) are important in medical diagnosis, each has its own limitations and difficulties. A bone scan is used to identify areas of abnormal active bone formation, such as in arthritis, infection, or bone cancer. However, bone scans require an injection of a radioactive substance (e.g., techne-tium) and a prolonged delay for absorbance before performing the scan. Computer tomography combines X-rays with computer technology to produce a two dimensional cross-sectional image of a body on the computer screen. Although this technique produces more details than an X-ray, in some cases (e.g., severe trauma to the chest, abdomen, pelvis or spinal cord), a dye (e.g., barium sulfate) must be injected in order to improve the clarity of the image. This often causes pain to the patient. Another technique, called electromyography, has been used to analyze/diagnose nerve functions inside body conditions. Thin electrodes are placed in soft tissues to help analyze and record electrical activity in the muscles. However, this electrode technique leads to pain and discomfort for the patient. When the needles are removed, soreness and bruising can occur.

Electrochemical biosensors on the implant itself show promise for medical diagnostics in situ to possibly determine new bone growth surrounding the implant. Yet, this technology has not been fully explored. Incorporation of such electrochemical biosensors into current bone implants may be possible through nanotech-nology; different types of nanoscale biomaterials have varying abilities to enhance in vitro/vivo bone formation. Carbon nanotubes (CNTs) are macromolecules of carbon, classified either as single-walled carbon nanotubes (SWCNTs) with diameters of 0.4-2 nm or multiwalled carbon nanotubes (MWCNTs) with diameters of 2-100 nm (Iijima 1991; Lin et al. 2004). Due to their unique electrical, mechanical, chemical, and biological properties (Webster et al. 2004; Smart et al. 2006; Zanello 2006; Zanello et al. 2006; Balani et al. 2007; Chen et al. 2007; Wei et al. 2007; Harrison and Atala 2007), CNTs have shown promise for bone implantation. For example, nanocomposites of polylactic acid and MWCNTs increased osteoblast proliferation by 46% and calcium production by greater than 300% when an alternating current was applied in vitro (Supronowicz et al. 2002; Ciombor and Aaron 2005). In addition, combining MWCNTs with precursor powders improves the mechanical properties of as-aligned hydroxyapatite (HA) composite coatings (Chen et al. 2007). MWCNTs, reinforced with HA coatings, promoted human osteoblast proliferation in vitro, as observed in the appearance of cells near MWCNT regions (Balani et al. 2007). Osteoblasts extended in all directions within the CNT scaffold that formed on polycarbonate membranes (Aoki et al. 2005). Osteoblasts significantly enhanced their adhesion on vertically aligned MWCNT arrays, according to Giannona et al. by recognizing nanoscaled features (Giannona et al. 2007). In addition, Zanello et al. (2006) showed that CNTs are suitable for proliferation of osteosarcoma ROS 17/2.8 cells on SWCNT and MWCNT scaffolds (Zanello et al. 2006). Human osteoblast-like (Saos2) cells grown on the MWCNT scaffolds showed a higher cell density and transforming growth factor-b1 concentration compared with those grown on polystyrene and polycarbonate scaffolds (Tsuchiya et al. 2007).

Our previous studies have shown greater osteoblast differentiation on MWCNT-Ti than on Ti alone (Sirivisoot et al. 2007). Moreover, MWCNTs are a promising material for electrochemical biosensors because they also possess relatively well-characterized behavior in terms of electron transport (Padigi et al. 2007; Roy et al. 2006; Tang et al. 2004; Kurusu et al. 2006). Coating CNTs on Ti electrodes also increases the active surface area and enhances direct-electron-transfer (Liu et al. 2005, 2007). These studies encourage the use of MWCNTs to modify currently used Ti for orthopedic implants. CNTs can be produced by the laser furnace arc and chemical vapor deposition (CVD) methods. Unlike the laser furnace arc, CVD is a scalable and controllable method of obtaining a high purity of CNTs (Robertson 2004). A strong mechanical connection between CNTs and the metal is needed for nanosensor applications (Talapatra et al. 2006), and the reduced resistance between the Ti and CNTs (Ngo et al. 2004) leads to increased current passage. For example,

Sato et al. showed great potential for using CNTs in electrochemical biosensors by using CVD to grow MWCNTs with Ti bimetallic particles and cobalt (Co) as a catalyst. They revealed that Co has the ability to combine with Ti (Sato et al. 2005), and since CNTs were grown by using a cobalt catalyst in that study, a strong electrical contact between metallic Ti and MWCNTs was possible.

As mentioned, in order to form a more robust interconnection, CNTs have been anchored in the pores of anodized nanotubular Ti. MWCNTs have then been grown using CVD techniques out of the anodized Ti nanotubes (with diameters of 50-60 nm and depths of 200 nm) as a template. In vivo, many cell processes important during new bone growth rely on the redox reactions of various biomolecules and ions. The mechanisms of electron-transfer reaction and the role of proteins in aiding the electron transfer of redox processes can be examined by electrochemical analysis. In electronic theory, when two different materials come in contact with each other, electron transfer will occur in an attempt to balance Fermi levels, causing the formation of a double layer of electrical charge at the interface. Because the formation of electric contact between the redox proteins and the electrode surface is the fundamental challenge of electrochemical biosensor devices, in one study, the redox reactions of iron (II/III) and the osteoblast extracellular components at the surface of Ti, anodized Ti, and MWCNT-Ti have been investigated. How to create such novel orthopedic sensors along with the latest data concerning their use is presented below.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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