David L. Christiansen

Role of Storage on Changes in the Mechanical Properties of Tendon and Self-Assembled Collagen Fibers

Fibrous collagen networks are the major elements that provide mechanical integrity to tissues; they are composed of fiber forming collagens in combination with proteoglycans (PGs). Using uniaxial tensile tests we have studied the viscoelastic mechanical properties of rat tail tendon (RTT) fibers and self-assembled collagen fibers that were stored at 22°C and 1 atm of pressure. Our results indicate that storage of RTT and self-assembled type I collagen fibers results in increased elastic and viscous components of the stress-strain behavior consistent with the hypothesis that storage causes the introduction of crosslinks. Analysis of the elastic and viscous mechanical data suggests that the elastic constant of the collagen molecule in RTT is about 7.7 GPa. Measurement of the viscous component of the stress-strain curves for RTTs and self-assembled collagen fibers suggests that PGs may increase the viscous component and effectively increase the collagen fibril length.

Transition from viscous to elastic-based dependency of mechanical properties of self-assembled type I collagen fibers

Fibrous collagen networks are the major elements that provide mechanical integrity to tissues; they are composed of fiber forming collagens in combination with proteoglycans and elastic fibers. Using uniaxial incremental tensile stress–strain tests we have studied the viscoelastic mechanical properties of self-assembled collagen fibers formed at pHs between 5.5 and 8.5 and temperatures of 25 and 37°C. Fibers formed at pH 7.5 and 37°C and crosslinked by aging at 22°C and 1 atmosphere pressure were also tested. Analysis of the mechanical tests showed that the ultimate tensile strength (UTS), and slopes of the total, elastic and viscous stress–strain curves were related directly to the volume fraction of polymer. Further analysis suggested that the UTS, and slopes of the total, elastic, and viscous stress–strain curves showed the highest correlation coefficient with the calculated effective fibril length and axial ratio. The mechanical data suggested that at low levels of crosslinking the mechanical properties were dominated by the viscous sliding of collagen molecules and fibrils by each other, which appears to be dependent on the collagen fibril length and axial ratio, while at higher levels of crosslinking the mechanical behavior is dominated by elastic stretching of the nonhelical ends, crosslinks, and collagen triple helix. The latter behavior appears to be dependent on the presence of crosslinks that stabilize fibrillar units. These results lead to the hypothesis that early in development viscous sliding of fibrils plays an important role in the mechanical response of animal tissues to forces experienced in utero, while later in development when locomotion is required, mechanical stability is primarily a result of elastic deformation of the different parts of the collagen molecule within crosslinked fibrils.

Mechanical Properties of Septai Cartilage Homografts

The compressive mechanical properties of untreated and chemically and physically treated nasal septum homografts were determined. Mechanical properties of control, saline-, thimerosal (Merthiolate)- and Alcide-treated specimens were similar. At high strains, the stiffness of treated cartilage ranged from 12.8 to 22.5 MPa and was unaffected by storage time. In comparison, irradiated and freeze-dried nasal septum exhibited stiffnesses of 35 and 37.5 MPa, respectively, after approximately 1 month of storage. These values of stiffness were significantly different from controls at a 0.95 confidence level. On the basis of these results, it was concluded that Alcide and Merthiolate treatment did not alter the compressive mechanical properties of cartilage and that a combination of these treatments may adequately sterilize and preserve nasal septum homografts.

Mechanical Properties of Tissues

Tissues are composed of macromolecules, water, ions, and minerals, and therefore their mechanical properties fall somewhere between that of random chain polymers and that of ceramics. Table 7.1 gives the physical properties of cells, soft and hard tissues, metals, polymers, ceramics, and composites. The properties of biological tissues are wide ranging, from cell membranes with a modulus of about 10−4 MPa to bone with ultimate tensile strength (UTS) and modulus of about 200 MPa and 15 to 20 GPa, respectively. This range of properties is achieved largely by use of protein building blocks in association with mineral. As discussed in Chapter 2, the mechanical properties of macromolecules are intimately related to the 3-D structures of the polypeptide chains. Because most proteins are composed of α helices, β structures, and collagen triple helixes, the mechanical properties of these structures are of interest. In this chapter, we introduce the terminology used to describe and measure mechanical properties, followed by a discussion of the mechanical properties of α helices, β structures, and collagen triple helixes.

Biomimetic Mineralization of an Aligned, Self-Assembled Collagenous Matrix.

An in-vitro method of mineralizing an aligned, self-assembled collagenous matrix is presented. Reconstituted collagen fibers were mineralized by exposure to saturated solutions of calcium and phosphate of varying pH in a double diffusion chamber for seven days at room temperature. Microscopic investigation of the mineral precipitate within the fibers indicate the formation of hydroxyapatite crystals with features comparable to mineral observed in bone and avian tendon. Mechanical test results indicate that tensile strength and tangent modulus increase after mineralization in comparison to unmineralized control fibers. These results suggest that mineralization of collagen fiber in-vitro may parallel some of the events seen in mineralization of bone and turkey tendon. In addition, mineralized collagen fibers may be useful in the design of composites for the replacement or augmentation of hard tissue

Introduction to Biomaterials Science and Biocompatibility

The field of biomaterials science dates back centuries to the ancient Greeks and Chinese, who used natural materials to ameliorate the effects of diseases. However, not until late in the twentieth century did the design and use of medical devices using synthetic and natural materials advance rapidly. The largely empirical problem-solving strategies, such as trial-and-error materials selection procedures, have evolved into a multidisciplinary field that requires in-depth knowledge of biochemistry, anatomy, structural biology, immunology, histology, pathobiology, engineering, and materials science.

Assembly of Biological Macromolecules

The study of animal tissues is complex because water, ions, cells, macro-molecules, tissues, and organs exist in equilibrium. From a structural point of view, biological tissues contain highly ordered arrays of macromolecules. One might wonder why biological structures need to be made up of highly ordered arrays of proteins, polysaccharides, and lipids. The reason is that individual polymer molecules cannot sustain the weight of gravity without rearranging. For example, if your skin were made of just collagen molecules without being cross-linked into crystalline fibers, it would sag. This is because individual molecules, in a similar manner to water molecules, can move around or diffuse. In the case of water molecules, a container is needed to shape them. In the case of tissues, the molecules need to assemble into ordered structures and be cross-linked for the shape of the tissue to be maintained. In some cases, assemblies of macro-molecules are purposely not cross-linked so that shape can be changed quickly. For example, cytoskeletal actin filaments are rapidly assembled and disassembled to allow for changes in cell shape. In this example, cross-links prevent rapid shape changes; however, actin filaments by themselves are rigid enough to maintain cell shape at any one instant. In contrast, collagen fibers in the skin must be cross-linked to form force-bearing units to prevent tearing when skin is stretched. This comparison is used to underscore the complexity of biological tissue structure and its relationship to physical properties. In some cases, rigidity is sacrificed for structural flexibility. In other cases, structural flexibility is sacrificed for permanence.

Pathobiological Responses to Implants

The introduction of implants in the 1950s revolutionized the field of medicine. Vascular grafts, heart valves, orthopedic implants, contact lenses, wound dressings, and other implants have extended the life spans and improved quality of life. However, the introduction of this new technology is also associated with pathobiological responses that in some cases can be life-threatening. Many of the adverse responses to implants are associated with interactions between components of biological pathways and inflammatory mediators with the implant surface or materials emanating from implants. These responses include complement depletion and increased predisposition to infection, local inflammation and foreign body response to implant materials, symptoms of systemic connective-tissue-like disorders with silicone implants, phagocytosis of wear debris by macrophages and migration to local lymph nodes, immediate hypersensitivity to implant materials, nonphysiological calcification leading to implant failure, extrusion of plastic implants from beneath the surface of tissues, and intimal hyperplasia associated with vascular stents.

Pathobiology and Response to Tissue Injury

The biomaterials scientist must understand the normal cell functions in tissues that are injured or diseased. To do this, one must understand gross and microscopic anatomical structures, which were introduced in Chapter 4, and cellular and tissue changes that are observed in injured or diseased tissue. Once the anatomical and microscopic structure and biochemistry of normal tissues is well understood, it is then possible to understand how injury or disease affects cells and tissues.

Introduction to Structure and Properties of Polymers, Metals, and Ceramics

Polymers, metals, and ceramics are everyday kitchen items, from aluminum, stainless steel, and nonstick cookware to ceramic plates and mugs. These materials found their way into medicine because no other materials could simulate the properties of mineralized and nonmineralized tissues. In addition, many of these kitchen materials are stable; they do not break down into low molecular weight materials that are harmful to the bodys homeostasis.