Bone tissue hierarchical structure

Bone is an extremely dense connective tissue that, in various shapes, constitutes the skeleton. Although it is one of the hardest structures in the body, bone maintains a degree of elasticity owing to its structure and composition. It possesses a hierarchical structure and, as most of the tissues, is nanostructured in fact, it is a nanoscaled composite of collagen (organic extracellular matrix) and hydroxycarbonate apatite, (HCA, bone mineral). This nanostructure is in intimate contact with the bone cells (several microns in size), which result (at the macroscopic level) in the bone tissue. Figure 12.2 shows the bone hierarchical ordering from the bone to the crystalline structure of HCA. 

Techniques to produce multiscale biomaterial scaffolds with designer geometries are the need of the hour to provide improved biomimetic properties for functional tissue replacements. While micrometer fibers generate an open pore stnicture, nanofibers support cell adhesion and facilitate cell-cell interactions. This was further proven by cell penetration studies, which showed superior ingrowth of cells into hierarchical structures. Mixed bimodal scaffolds of two different polymers are another promising approach, because they exhibit hierarchical pore/ surface systems and combine the beneficial properties of both polymers at two different scales. Vaiious 3D micro- and nanoscale multiscale scaffolds have been fabricated through various techniques and were found to have the potential to essentially recreate natural bone, cardiac, neural, and vascular tissues. 

In the musculoskeletal system, bone is the primary tissue/organ interacting with prosthetic implants/biomaterials and their interface is a crucial region where the interactions pertinent to new tissue formation and implant efficacy occur. Bone is a complex biological system that comprises both hierarchical structures and living boneremodeling components. The architecture of bone is composed of nanoscale fibrous... 

The enormous scale of the challenge to build a truly biomimetic muscle can be appreciated by considering what is known about natural muscle structure. The contractions of these motors involve a highly complex and coordinated sequence of electrical, chemical, and physical phenomena within a composite, gel-like polymeric material that is known to possess a detailed hierarchical structure, stretching from the nano-scale assembly of proteins through the cellular fibrillar textures to the macroscopic tissue. The exact function of many of these features is still the subject of on-going research. In general, the skeletal muscles consist of tendons (non-active) and muscle belly (active). While tendons mainly provide muscle connectivity to hard bones,... 

The knowledge about bone tissue structure and morphology described in detail in [2, 13, 20-25] proves to be very complex because bone is arranged in several hierarchical structures. Rational design of artificial implant materials should take into account the tissue s characteristics - the more similarities possessed by the fabricated implant to the bone tissue (biomimetism), the greater the chance of acceptance of the alien system by the human organism. 

In many cases, biomimetic strategies do not set out to copy directly the structures of biological materials, but aim to abstract the key concepts from biological systems, such that they can be adapted within a synthetic context. Thus, biomimetic materials are invariably less complex than their biological counterparts and, to date, hierarchical architectures (such as those observed in bone) remain outside the current technologies. Currently, the simplest biomimetic approach involves the design of single-component systems that mimic the chemistry of bone tissue. 

Although much of the interest in biological nanostructures has focused on relatively complex functionality, cells and organisms themselves can be considered as a collection of self-assembled materials lipid bilayers, the extracellular matrix, tendon and connective tissue, skin, spider silk, cotton fiber, wood, and bone are all self-assembled biological materials, with an internal structure hierarchically ordered from the molecular to the macroscopic scale. 

It is important to emphasize that many natural tissues are essentially composed of nanoscale biopolymers or biocomposites with hierarchical architectures. Therefore, by mimicking the structure and property of their natural counterparts, synthetic nanopoiymers and nanocomposites are very likely to enhance/regulate the functions of specific cells or tissues. This principle has been demonstrated by the success of bioinspired polymers and composites in both clinical practice and in laboratory research. In particular, bone is the hierarchical tissue that has inspired a myriad of biomimetic materials, devices, and systems for decades. This chapter focuses on this well-developed area of biomimetic or bioinspired nanopoiymers and nanocomposites for bone substitution and regeneration, especially those with high potentials for clinical applications in the near future. 

From a biological viewpoint, almost all of the human tissues and organs are deposited in nanofibrous forms or structures. Examples include bone, dentin, collagen, cartilage, and skin. All of them are characterized by well-organized hierarchical fibrous stmctures realigning in nanometer scale. 

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