Fundamentals of the Interface Between Sensors and Bone

3.2.1 Events at the Sensor-Bone Tissue Interface

Bone is a very complicated biological system that consists of both hierarchical structures and living bone remodeling units. The architecture of bone is composed of nanofibrous collagen matrices, noncollagenous proteins, and nanocrystalline calcium phosphate (mainly hydroxyapatite). Bone remodeling units involve three major types of bone cells: osteoblasts (bone forming cells), osteocytes (bone-maintaining cells), and osteoclasts (bone-resorbing cells). Therefore, one can expect the interactions at the interface of bone tissue and implant materials are very intricate events, as illustrated in Fig. 2.

Basically, all of these events can be categorized as host responses toward the implant and, conversely, material responses to the host (Puleo and Nanci 1999). In the present context, it is important to realize that an implanted wireless sensor will interact in all of the events in Fig. 2. It is also important to mention that for this application, an optimal sensor will not only sense new bone growth (and whether it is occurring), but it will also improve new bone growth. Though all of these events described in Fig. 1 are of great importance when designing a wireless medical device, there are several body responses that are of particular interest since such information can be used to determine bone health next to the sensor. (1) Protein adsorption is the immediate event once a material is implanted. Also, the type and density of the adsorbed proteins can regulate cell adhesion and subsequent cellular activities. (2) Adhesion of osteogenic cells, subsequent bone deposition, and bone remodeling are crucial factors at a successful orthopedic sensor-bone interface. (3) Sensor surface properties are also important events closely related to biomaterial cytocompatibility, immune or inflammatory responses that may ultimately cause implant failure. Immune and inflammatory cell responses are not introduced bone bone

Fig. 2 Events at the bone-implant interface. (a) Protein adsorption from blood and tissue fluids, (b) protein desorption, (c) surface changes and material release, (d) inflammatory and connective tissue cells approach the implant, (e) possible targeted release of matrix proteins and selected adsorption of proteins, (f ) formation of lamina limitans and adhesion of osteogenic cells, (g) bone deposition on both the exposed bone and implant surfaces, and (h) remodeling of newly formed bone (adapted from Puleo and Nanci (1999))

Fig. 2 Events at the bone-implant interface. (a) Protein adsorption from blood and tissue fluids, (b) protein desorption, (c) surface changes and material release, (d) inflammatory and connective tissue cells approach the implant, (e) possible targeted release of matrix proteins and selected adsorption of proteins, (f ) formation of lamina limitans and adhesion of osteogenic cells, (g) bone deposition on both the exposed bone and implant surfaces, and (h) remodeling of newly formed bone (adapted from Puleo and Nanci (1999))

in this chapter; however, the reader is cautioned to bear in mind that they are of exceptional importance at the bone-implant interface because if the sensor is encapsulated in scar tissue, it will not be able to "sense."

Understandably, the bone-sensor interface is difficult to characterize because of the complexities of the various biological responses and in vivo environment. Therefore, in vitro bone cell culture models become practical and effective tools to initially investigate biological responses and cell functions on sensor surfaces. Most of the experimental results discussed in this chapter are, thus, based on bone cell culture models. Importantly, nanotechnology (or more specifically, nanomaterials) has been shown to control such events and, thus, should be an integral part of an implantable wireless sensor.

3.2.2 Novel Properties of Nanomaterials/Nanotechnology

Nanoscale materials are defined as materials (e.g., particles, fibers, tubes, etc.) with size scales within 1-100 nm in at least one dimension. The research and development aimed at understanding and working with (e.g., detecting, measuring and manipulating) these kinds of materials has been called nanotechnology (Balasundaram 2007). Nanotechnology emerged in the last century and has shown extraordinary potential in biomedical research applications. This potential originates from unique properties of nanomaterials compared to bulk conventional (e.g., micron grain size) materials, such as: (1) more surface reactivity as a result of much larger surface areas; (2) greatly enhanced mechanical properties (such as high ductility and high yield strength) due to various mechanisms such as increased grain boundary sliding and short-range diffusion-healing; (3) exceptional magnetic, optical, and electrical properties because of stacking, alignment, and orientation of nanoscale building blocks (grains, supermolecules, etc.); and (4) homogeneity and high purity in composition or structure thanks to reacting or mixing at the molecular or atomic level.

Furthermore, nanoscale materials or structures provide the bone community with not only novel properties to utilize but also a wide landscape to understand the biological responses on a material surface. One important reason behind this is that the comparable size of nanoscale materials enables researchers to really detect, interact, and analyze biomolecules or bio-microstructures. The other reason is that nanostructured materials can be readily tailored to reveal extraordinary variations in surface properties, which leads to accurate observations of their effects on cellular or tissue responses.

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