Atomic force microscopy and indentation force measurement of bone

Applications of atomic force microscopy for the assessment of nanoscale morphological and mechanical properties of bone

Scanning probe microscopy (SPM) has been in use for 30 years, and the form of SPM known as atomic force microscopy (AFM) has been around for 25 of those years. AFM has been used to produce high resolution images of a variety of samples ranging from DNA to carbon nanotubes. Type I collagen and many collagen-based tissues (including dentin, tendon, cartilage, skin, fascia, vocal cords, and cornea) have been studied with AFM, but comparatively few studies of bone have been undertaken. The purpose of this review is to introduce the general principles of AFM operation, demonstrate what AFM has been used for in bone research, and discuss the new directions that this technique can take the study of bone at the nanoscale.

In situ force mapping of mammary gland transformation

Tumor progression is characterized by an incremental stiffening of the tissue. The importance of tissue rigidity to cancer is appreciated, yet the contribution of specific tissue elements to tumor stiffening and their physiological significance remains unclear. We performed high-resolution atomic force microscopy indentation in live and snap-frozen fluorescently labeled mammary tissues to explore the origin of the tissue stiffening associated with mammary tumor development in PyMT mice. The tumor epithelium, the tumor-associated vasculature and the extracellular matrix all contributed to mammary gland stiffening as it transitioned from normal to invasive carcinoma. Consistent with the concept that extracellular matrix stiffness modifies cell tension, we found that isolated transformed mammary epithelial cells were intrinsically stiffer than their normal counterparts but that the malignant epithelium in situ was far stiffer than isolated breast tumor cells. Moreover, using an in situ vitrification approach, we determined that the extracellular matrix adjacent to the epithelium progressively stiffened as tissue evolved from normal through benign to an invasive state. Importantly, we also noted that there was significant mechanical heterogeneity within the transformed tissue both in the epithelium and the tumor-associated neovasculature. The vascular bed within the tumor core was substantially stiffer than the large patent vessels at the invasive front that are surrounded by the stiffest extracellular matrix. These findings clarify the contribution of individual mammary gland tissue elements to the altered biomechanical landscape of cancerous tissues and emphasize the importance of studying cancer cell evolution under conditions that preserve native interactions.

Experimental research on a modular miniaturization nanoindentation device

Nanoindentation technology is developing toward the in situ test which requires miniaturization of indentation instruments. This paper presents a miniaturization nanoindentation device based on the modular idea. It mainly consists of macro-adjusting mechanism, x-y precise positioning platform, z axis precise driving unit, and the load-depth measuring unit. The device can be assembled with different forms and has minimum dimensions of 200 mm × 135 mm × 200 mm. The load resolution is about 0.1 mN and the displacement resolution is about 10 nm. A new calibration method named the reference-mapping method is proposed to calibrate the developed device. Output performance tests and indentation experiments indicate the feasibility of the developed device and calibration method. This paper gives an example that combining piezoelectric actuators with flexure hinge to realize nanoindentation tests. Integrating a smaller displacement sensor, a more compact nanoindentation device can be designed in the future.

Nanostructure and mineral composition of trabecular bone in the lateral femoral neck: implications for bone fragility in elderly women

Despite interest in investigating age-related hip fractures, the determinants of decreased bone strength in advanced age are not clear enough. Hitherto it has been obscure how the aging process affects the femoral neck nanostructure and composition, particularly in the lateral subregion of the femoral neck, which is considered as a fracture-initiating site. The femoral bone samples used in this study were obtained at autopsy in 10 women without skeletal disease (five younger: aged 20-40 years, and five elderly: aged 73-94 years). Atomic force microscopy (AFM) was applied to explore the mineral grain size in situ in young vs. old trabecular bone samples from the lateral femoral neck. The chemical compositions of the samples were determined using inductively coupled plasma optical emission spectroscopy and direct current argon arc plasma optical emission spectrometry. Our AFM study revealed differences in trabecular bone nanostructure between young and elderly women. The mineral grain size in the trabeculae of the old women was larger than that in the young (median: 95 vs. 59nm), with a particular bimodal distribution: 45% were small grains (similar to the young) and the rest were larger. Since chemical analyses showed that levels of calcium and phosphorus were unchanged with age, our study suggests that during aging the existing bone mineral is reorganized and forms larger aggregates. Given the mechanical disadvantage of large-grained structures (decreased material strength), the observed nanostructural differences contribute to our understanding of the increased fragility of the lateral femoral neck in aged females. Moreover, increasing data on mineral grains in natural bone is essential for advancing calcium-phosphate ceramics for bone tissue replacement.