This paper presents the results of several AFM case studies of water, tumor cells, pit cells, and type-I collagen samples. The AFM imaging procedures, surface structural characterization capabilities such as contact mode, tapping mode, and friction mode are discussed. The difference in surface morphology between the pit cell submerged in liquid and the cell in air was observed. The AFM tapping mode phase imaging technique was used to study the crosslinks in the type-I collagen. The calibration method for accurately measuring the AFM cantilever spring constant with the help of a nanoindenter is also presented.

Dental tissues have been reported to show a considerable decrease in both their mineral content and mechanical properties in carious lesions. The changed properties of dentine and enamel have been shown to be dependent on crystal size and not only mineral content [1], although the connectivity between the mineral crystals has been overlooked. Teeth with carious lesions were sectioned, embedded in polymethylmethacrylate (PMMA) and diamond polished. Nanoindentation and quantitative backscattered electron imaging were used to determine relationships between the elastic modulus and mineral density of sound and carious regions within dentine and enamel. The changes in elastic modulus with decreased mineralization for dentine and enamel could not be explained by simple composite mechanics expressions relating elastic modulus and mineral volume fraction. Finite element modeling of dentine and enamel as a two-phase composite material at the ultrastructure level were used to demonstrate how changes in the mineral phase connectivity can produce changes in the elastic modulus. Tissue models for enamel, in which the mineral phase is both the major component of the structure (∼ 85% by volume) and highly interconnected, were consistent with the modulus of sound enamel. The drastic change in enamel modulus with a relatively small change in mineral volume fraction could be modeled as a decrease in mineral phase connectivity at nearly constant volume fraction. The more gradual trend in the dentine data was also consistent with a structure that is initially highly connected in the mineral phase, consistent with the known structure of dentine, and for which the change in modulus is more directly related to changes in mineral content than mineral connectivity.

Enamel is made up of enamel prisms separated by thin layer of organic sheaths. The mechanical properties of the prisms and the organic sheaths are obviously different from each other due to different compositions and microstructures. However, quantitative measurements of such differences have been a challenge in the past. The objective of this study is to accurately study the mechanical properties in the isolated domains within single enamel prism. The technique of nanoindentation combined with Atomic Force Microscopy (AFM) was employed to test the enamel specimens from mature human maxillary third molar. It was revealed that the nanohardness and elastic modulus of the sheaths were about 73.6% and 52.7% lower than those of the prisms. AFM topographies of the residual indent impressions also visually confirmed the differences. In addition to nanoindentation tests, the microstructures of enamel were carefully investigated in terms of hierarchical levels of organization to understand the structural reasons of the mechanical differences. We found a close relation between the variations of mechanical properties of enamel and its hierarchical structure. The analysis of the mechanical properties within enamel upon hierarchy is not only helpful to understand its unique property, but may also inspire ideas for the design of novel synthetic materials.

The mechanical properties of healthy and diseased bone tissue are extensively studied in mechanical tests. Most of this research is motivated by the immense costs of health care and social impacts due to osteoporosis in post-menopausal women and the aged. Osteoporosis results in bone loss and change of trabecular architecture, causing a decrease in bone strength. To address the problem of assessing local failure behavior of bone, we combined mechanical compression testing of trabecular bone samples with high-speed photography. In this exploratory study, we investigated healthy, osteoarthritic, and osteoporotic human vertebral trabecular bone compressed at high strain rates simulating conditions experienced in individuals during falls. Apparent strains were found to translate to a broad range of local strains. Moreover, strained trabeculae were seen to whiten with increasing strain. We hypothesize that the effect seen is due to microcrack formation in these areas, similar to stress whitening seen in synthetic polymers. From the results of a motion energy filter applied to the recorded movies, we saw that the whitened areas are, presumably, also of high deformation. We believe that this method will allow further insights into bone failure mechanisms, and help toward a better understanding of the processes involved in bone failure.

There are evidences that in ancient cultures in Mexico, copal was probably used for gluing precious stones in teeth cavities and for dental restorations as well. This is an important reason for making experiments using copal and copal-based composites in order to have more information for the potential applications of this material in modern dentistry. In our experiments concerning with dental incrustations of turquoise, we have practiced round cavities (about 1 mm depth) in the middle of incisive teeth by using a low-speed air turbine. Turquoise was cut in such a way that fits exactly on the tooth cavity. Copal and powdered apatite were used to glue the stone into the cavity. With Scanning electron microscopy we have observed that the composite copal-powdered apatite, penetrates the dentin tubules in the tooth tissue (depth of penetration from 5.6 to 41 micrometers), suggesting the existence of a micromechanical adherence. Concerning to the characterization of the adhesive properties, we have applied the ASTM D2095-72 test considering the adhesion of two cylinders of bone, and the copal composite as the adhesive. A maximum cohesive tensile strength around 0.1 MPa for elastic response was measured for hydroxyapatite compositions of 0, 25 and 75 wt%. We will discuss these results considering the reports of dental incrustations in ancient Mexico.

The mechanical behavior of the membrane of the red blood cell is governed by two primary microstructural features: the lipid bilayer and the underlying spectrin network. The lipid bilayer is analogous to a 2D fluid in that it resists changes to its planar area, yet poses little resistance to planar shear. A skeletal network of spectrin molecules is crosslinked to the lipid bilayer and provides the shear stiffness of the membrane. Here, a continuum level constitutive model of the large stretch behavior of the red blood cell membrane that directly incorporates the microstructure of the spectrin network is developed. The resemblance of the spectrin network to a triangulated network is used to identify a representative volume element (RVE) for the model. A strain energy density function in terms of an arbitrary planar deformation field is proposed using the RVE. Differentiation of the strain energy density function provides expressions for the general multiaxial stress-stretch behavior of the material. The stress-strain behavior of the membrane when subjected to uniaxial loading conditions in different directions is given, showing the capabilities of the proposed microstructurally-detailed constitutive modeling approach in capturing the evolving anisotropic nature of the mechanical behavior.

Mammography is the primary method for screening and detecting breast cancers. However, it frequently fails to detect small tumors and is not quite specific in terms of tumor benignity and malignancy. The objective of this paper is to develop a new imaging modality called elastomammography that generates the modulus elastograms based conventional mammographs. A new elastic reconstruction method is described based on elastography and mammography for breast tissues. Elastic distribution can be reconstructed through the measurement of displacement provided by mammographic projection. It is shown that the proposed elasto-mammography provides higher sensitivity and specificity than the conventional mammography on its own for breast cancer diagnosis.

The review of our approach to the specific rheological properties modeling of myocardium has been given. We are working on the level of ‘fascicule’ as a tissue element, which consists of several cardiomyocytes surrounded by a connective tissue shell. Actually, these properties are characteristic of large majority of living soft tissues. In order to describe essentially nonlinear static ‘force-deformation’ curves and quasi-static hysteresis loops together with non-exponential stress relaxation and creep time courses we suggest a set of 2D graphs with different topology composed of classical linear Hook's springs and Newtonian damps. Each spring and dashpot represents a group of 3D tissue structural elements. The stress response functions of these models are found for the uniaxial step-wise, or pulse (column-like) external stretching. The inverse response functions of the longitudinal displacement for the external stress loading of the pulse shape time dependencies were also found. The values of elastic modules and viscous coefficients are estimated by comparison theoretical curves of relaxation, creep and recovery with the experimental data. The latter are obtained on rather different objects, passive muscle preparations (the stress relaxation response) and endothelium cells (the creep response). It has been stated that the proposed 2D models appear to be quite general to describe nonlinear relaxation and creep properties, which are lacking in the traditionally used uniaxial 1D models.

Connective tissues such as cartilage, tendon, skin, bone, and arteries are composite bio-materials that contain predominantly water, collagen, proteoglycans and hyaluronic acid. Like any composite material, the components themselves and their interactions dictate the properties of the material. Fibrillar collagens are the principal structural molecules of the connective tissues and require regulated assembly and growth. Previous work from our lab indicates that the amino propeptide (Npp) domain of collagen type XI α1 chain regulates fibril diameter growth. Npp is a globular domain that is thought to sterically hinder the dense packing assembly of collagen molecules in fibrils. This mechanism of regulating collagen fibril assembly may be more complex than steric hindrance. We hypothesize that the Npp domain has a more dynamic role in establishing the structure/function relationship of collagen fibrils in connective tissues. In this study, the molecular structure of Npp was predicted by modeling. The model predicted putative binding sites for heparan sulfate and divalent cations. These predicted binding sites were evaluated empirically by fluorescence spectroscopy and surface plasmon resonance.

Ultra-high molecular weight polyethylene (UHMWPE) is commonly used as a bearing surface in total joint replacements. This study examined the effects of cross-linking and oxidation on the mechanical properties of UHMWPE at the microstructural level, rather than at the bulk level. Nanoindentation tests were performed on the articulating surfaces of two freshly machined UHMWPE acetabular inserts, one of which was cross-linked, and also performed along a freshly cut cross-section of a UHMWPE tibial insert that had been sealed in its original packaging and shelf-aged for at least five years. The reduced modulus of cross-linked UHMWPE was significantly lower than native UHMWPE. Tests on the cross-section of the shelf-aged specimen revealed a reduced modulus profile with a subsurface peak that was similar to the oxidation profiles of aged UHMWPE observed by others. Future work should examine the microstructural mechanical properties of UHMWPE using methods that more accurately account for its viscoelastic nature, and should directly correlate microstructural properties with oxidation.

This paper presents the results of contact mode atomic force microscopy (AFM) study on the nanoscale Young's modulus and work of adhesion of ultra high molecular weight polyethylene (UHMWPE). Cryoultramicrotomed surfaces of UHMWPE were scanned using the contact mode of AFM. Fibril regions are commonly found on the sample, however, a non-fibril particulate region was also found. AFM force displacement curves were obtained for the sample. The JKR theory and Maugis Dugdale model were used for the analysis. A good fitting between the theories and experimental data was found. The nanoscale Young's modulus and work of adhesion of UHMWPE extracted from the experimental data were in reasonably good agreement with the values reported in other literatures.