Nanobiomechanics of living materials

Nanobiomechanics has been emerging as a powerful technology in characterizing mechanical properties of biological materials and structures, such as proteins, cells and soft tissues, as well as for monitoring their physiological and pathological processes. These soft living materials exhibit compliant deformation behaviours and their Young's modulus normally ranges from 1 MPa to 1 GPa, which is a 1000-fold lower than those of artificial/engineering materials [1]. Owing to such low elastic moduli, the biomechanical measurement of these materials normally requires force and displacement at nanoscale resolutions. This challenge can be resolved only due to the recent developments in nanobiomechanical instruments, including atomic force microscopy (AFM), optical tweezers/stretchers, cell traction force microscopy and nanoindentation. In this issue, we will focus on such state-of-the-art advanced instruments, materials and analyses for mechanical characterization of biological materials, including their potential applications for the future of biomedicine. As research in this cross-disciplinary topic has spread to journals of various fields—from physical sciences and engineering to biology and medicine—it is thus rendered more difficult for scientists working within this field to access comprehensive updated information. Subsequently, this issue aims to bring such research together so that researchers can benefit from an overview of the recent advances in nanobiomechanics, with a particular focus on the measurement and application of biomechanical properties of nanoscale biological matter. More importantly, biomechanics, nanotechnology, instrumentation, and cell and tissue biology will be synergized for tackling the physiological and pathological processes of biological materials such as proteins, cells and tissues.

Nanobiomechanics has recently been regarded as within the top 10 most significant technologies by a review published by MIT (http://www.technologyreview.com/article/16475/). Breakthroughs in the measurement for highly complex and dynamic living materials has not only opened a new horizon in scientific research but also generated a significant societal impact. For example, measuring nanoscale forces exerted by proteins on cells can potentially afford new exploration of various diseases including cancer [2], osteoarthritis [3] and diabetes [4], because these diseases alter the elasticity and adhesion of cells. Hence, such single-cell-based biomechanical markers will potentially provide low-cost, less-invasive and high-throughput diagnostic techniques for disease detection [5]. More importantly, nanomechanical force of cell–cell or cell–material interaction has a significant influence on mechanotransduction for cell differentiation and tissue regeneration, making it of great importance for the development of tissue engineering and regenerative medicine [6]. Hence, the research topic presented in the focus issue has a significant impact for twenty-first century society in meeting the biomedical needs of an increasingly aging population.

Characterizing the nanomechanics of smooth muscle cells (SMCs) is critical in the engineering of a three-dimensional tissue construct, e.g. blood vessel equivalents. Zhang et al. [7] have recently developed novel microfabricated arrays of discontinuous microwalls coated with fluorescence micro-particles to measure the mechanotransduction of the SMC layer by extending conventional single-cell traction force assay for the measurement of three-dimensional cell aggregates. In parallel, they applied AFM to measure the elastic modulus of a polyacrylamide gel layer coated on the microwall arrays. Their results demonstrated that the physical/mechanical constraints of microwalls contribute to both the formation of low stress zones and leads to the significant reduction in the expression of focal adhesion for SMCs. The results were cross-validated by performing differential proteomic analysis to examine the alternation of three major cytoskeleton-associated proteins between SMC on the microwall substrate and control SMCs.

Understanding the fundamental behaviour of cells and how such cells mechanically interact with their surrounding structures, as well as with other cells, is of critical importance, particularly when trying to develop new tissue engineering and regenerative medicine strategies. In this important topic, Ahearne [8] has provided a compressive overview of the cell–hydrogel nanomechanical relationship which is critical to regenerative medicine. First, the mechanical mechanisms that cells apply when remodelling and rebuilding their surrounding matrix thus allows for new tissue formations to be exposed. These include cellular processes such as adhesion, migration, contraction, degradation and extracellular matrix deposition. Following this, the roles that the mechanical properties of the hydrogels have on cells activities and phenotypes are discussed. More specifically, mechanical variations stemming from different degrees of stiffness, density and the viscoelastic characteristics of hydrogels have all been shown to influence cell behaviour. Finally, how mechanical forces affect cells in hydrogels and the different mechanisms by which force may be applied are investigated. By comprehensively examining the reciprocal mechanical relationship between cells and hydrogels, this article should be of interest to those who are involved in developing the next generation of tissue engineering and regenerative medicine therapies. Moreover, cell–matrix interactions are of great importance for angiogenesis. For example, endothelial filopodia play an important role in directing the tubular sprouting for the formation of new blood vessels. Xue et al. [9] report the use of such near-field electrospinning as a means of fabricating one-dimensional gelatin fibril patterns for investigating how the key morphological parameters define the filopodial guiding process. Their results have demonstrated that the behaviours of endothelial filopodia underpin the cellular contractility.

Apart from the aforementioned nanomechanical tools, recent advancement in microscopic imaging techniques enable high-resolution visualization of the structures of living materials which alternatively provide useful information of their mechanical properties. For example, as reviewed by Green et al [10], two photon fluorescence microscopy has provided a more sensitive detection technique than classical histological methods, while multi-photon microscopy has also proved a powerful tool in informing multi-scale models to assess the nanomechanical contribution of elastin networks to overall tissue mechanics. The distribution and structure of elastin fibres in tissues where the classical light and electron microscopy were inefficient have been evidenced via these new techniques, demonstrating that elastin in the form of fine fibre networks is a more widely distributed component than previously argued. This review offers insight into the application of multi-photon microscopy to investigate the mechanical function of elastic fibres, particularly in blood vessels, cartilage and intervertebral disc under-load. Moreover, the paper presents a timely discussion that the mechanical behaviours of these networks and the effects of elastin structure are associated with ageing as well as diseases such as diabetes and atherosclerosis. Further efforts are suggested by the authors in order to gain a better understanding of the molecular basis of processes, such as calcium and lipid binding and glycation, as they influence the mechanical properties of elastin in these pathological processes.

A nanoindention technique has been widely used as a powerful tool by which to characterize mechanical properties of living cells, contributing to the better understanding of a range of biomechanical and biophysical processes, such as disease progression and cell–material interactions. Chen [11] has reviewed the most cutting-edge nanoindentation methods, combined with suitable mechanical models for quantitative analysis of elastic properties and time-dependent behaviours of single biological cells. A comprehensive appraisal has also been given to the merits and demerits of the various mechanical models including the tensegrity model, percolation model, elastic model, viscoelastic model, power-law rheology model, poroelastic (biphasic) and triphasic model. More importantly, this review presents a strategic guide in selecting appropriate indenter shapes for specific types of cell indention experiments. It has also provided insightful discussions of cell mechanics regarding the relative deformation of the cell size during nanoindentation and presented a guide to assist in choosing suitable nanoindentation models.

Boccaccio et al. [12] report on the application of inverse finite-element analyses for fitting the experimental force–displacement curve of a zona pellucida (ZP) membrane under AFM indentation to quantitatively determine its visco-elasticity. This is an important work for mechanical characterization of ZP as the study presents the first visco-hyperelastic model ever developed to describe the viscous response of ZP. The technique provides not only a more accurate estimation of the membrane viscoelasticity but also a better understanding of the constitutive behaviours of ZP hardening and time-dependent effects. Chan et al. [13] introduce an improved microfluidic optical stretcher integrated with heating components which allows for the investigation of dynamically mechanical properties of single biological cells in response to a temperature change. Their work demonstrates the role of temperature-sensitive transient receptor potential vanilloid 2 ion channel in regulating the thermo-mechanical response of cells. The paper also sheds light on how cortical tension and osmotic pressure dictate cell mechanical behaviours and regulate cell shape changes when subjected to heat and mechanical stress.