Mechanics of Biological Systems and Materials, Volume 7

In living tissue, there exists a reciprocal relationship between mechanical properties and function. That is, properties affect function, and function can affect properties by means of adaptation. Thus, knowledge of mechanical properties leads directly to knowledge of function. Dynamic instrumented indentation provides a way to measure the mechanical properties of soft biological tissue that is relevant, localized, and accurate.

In this work, we measured the complex shear modulus of bovine muscle tissue, submerged in saline at bovine body temperature (38 °C). The muscle tissue was significantly stiffer in the direction of the grain (G′ = 24.4 ± 13.9 kPa) than perpendicular to it (G′ = 11.4 ± 2.9 kPa). This was expected, because the muscle tissue naturally acts in the direction of the grain to alternately exert and relax force. The loss factor was highly consistent and independent of testing direction: tan δ = 0.34 ± 0.035. These results were consistent with what others have measured for muscle tissue using dynamic mechanical analysis (DMA).

Tendons have an important structural function in biological systems, their mechanical proprieties are therefore of great interest in biomechanics engineering and reconstructive medicine. Their physiological characteristics require the study of specific experimental methods able to determine the mechanical properties.

In this work the authors propose a non-contact experimental method aimed to the characterization of the mechanical proprieties of rabbit patellar tendons based on the use of a single camera and a customized gripping system. The tensile test setup makes use of a fixed lens camera and a customized algorithm, providing the measurement of the local sample strain on different part along the tendon and of the cross-sectional area. The tensile stress is estimated by the value of the applied load and of the cross-section value of the sample; tensile stress values are calculated at a frequency of 8 Hz. Moreover a special design of the clamps and the use of the camera allow to protect the experimental tests from the well-known problem of peak force concentrations on the sample and slipping at its extremities, which indeed are typical problems in tensile testing of tendons.

The acousto-mechanical-transformer behavior of the Tympanic Membrane (TM) is defined by its shape, 3D displacements, and mechanical properties. In this paper, we report the quantification of these characteristics by full-field-of-view optoelectronic techniques. Due to geometrical constraints imposed by the ear canal, however, 3D displacement measurements with multiple sensitivity vectors in holographic interferometry or 3D Laser Doppler Vibrometry (LDV) have limited applications for testing in vivo. Therefore, we seek alternative methods to perform 3D measurements. In our work, we hypothesize that the TM behaves as a thin-shell, so that the principal components of vibration are parallel to the TM’s shape normal vectors, which allows the estimation of the 3D components of displacement with only 1D component of displacements and shape information. Full-field-of-view measurements of the TM are obtained with our digital holographic system, with shape measured in two-wavelength mode and 1D displacements measured in single-wavelength mode. The theoretically-estimated 3D components of displacement are then compared with those measured by methods of multiple sensitivity vectors. Preliminary data suggest that the thin-shell hypothesis is applicable for estimation of the 3D acoustically-induced vibrations of the TM excited at low and mid frequency ranges.

Biomimetics is a growing field, and abalone shells have become a recently studied topic because of their amazing strength properties. Their shell is composed of calcium carbonate, and it is formed by the mollusk in a unique structure of interlocking tablets called nacre, also known as mother of pearl. There has been much interest in the interactions between tablets, but there are also structural growth lines that occur between layers of nacre that have been unnoticed by other researchers. With these additional non-nacre layers, the material’s strength is increased. We show how abalones can be cultured with changes in temperature to adjust shell and nacre growth. Then, they are tested using nanoindentation for qualitative mechanical data. In all, we aim to utilize the abalone nacre architecture based on these composite layers for improved strength applications such as protective armor.

This work investigated the time dependent mechanical properties of cervine enamel. Three point bending tests were used to investigate mechanical properties of cervine enamel. The specimens of enamel were prepared from cervine incisors and the ramp-hold three point bending tests were performed to acquire the force-time and displacement-time relation of enamel specimens under three point bending load. The finite element analysis of specimen under three point bending load was performed by applying the three point bending ramp-hold displacement-time boundary condition to the plane strain finite element model of specimen and the reaction force of the loading tip was recorded. In this study, cervine enamel was considered as linear viscoelatic material. The long-term elastic modulus of enamel was evaluated from the stress and strain relations at the end of ramp-hold tests and the relaxation behaviour of enamel was estimated from the force-time and displacement time curves of three point bending ramp-hold tests. An inverse iterative finite element analysis procedure was developed to obtain the long-term elastic modulus, relaxation coefficients and characteristic times of linear viscoelastic model of cervine enamel. The results show that a linear viscoelastic model with two relaxation time constants can well describe the time dependent behaviour of cervine enamel.

This paper focuses on the development of full-field experimental methods for validating computational models of needle insertion, and specifically the development of suitable tissue surrogate materials. Gelatine also known as “ballistic gel” is commonly used as a tissue surrogate since the modulus of elasticity matches that of tissue. Its birefringent properties also allow the visualisation of strains in polarised light. However, other characteristics of tissue are not well emulated by gelatine, for example the fibrous network of cells of tissue is not well represented by the granular microstructure of gelatine, which tears easily. A range of birefringent flexible materials were developed and calibrated for photoelastic analysis. The most suitable were then used to explore quantitatively the different strain distributions in tissue when subjected to a range of needles with different tip profiles.

A polymer gel is a physically or chemically cross-linked polymer that is highly swollen by solvent. The gel properties can be tuned by varying the polymer chemistry, solvent type, polymer-solvent architecture and molecular weight, and solvent loading. In addition, small molecule additives and fillers can be incorporated into the gel formulation to enhance the properties further. This tunability offers the potential for gel implementation in an array of Army related technologies ranging from combat casualty care and tissue surrogates, to robotics and electronics devices. Several obstacles hinder widespread deployment of gel-based technologies including: (1) limited operational temperature windows and material lifetimes; (2) poor toughness and durability; (3) unstable performance in extreme mechanical, electrical, chemical, and radiation environments; and (4) limited multi-functional capability. We anticipate that a fundamental understanding of how to tailor the gel properties will have a broad impact on various technologies including robotics, smart clothing, armor, tissue simulants, sensors, energy storage, battlefield medicine, etc.

Mechanotransduction studies aim to understand the process of cells converting mechanical stimuli into a cellular response. As it can be difficult to study the impact of isolated factors using in vivo studies, in vitro studies are used as they offer more precisely controlled loads for experiments and allow cell culture on a variety of surfaces. Here, we developed a microloading platform for in vitro mechanotransduction studies, stretching the substrate by tenting it with a centrally contacting platen. This platform works through the use of a load cell and microactuator, which was characterized by comparing the reported and measured displacements. In addition, an alignment block was designed for the microloading platform to increase reproducibility between studies, and a cell culture handling system was designed to hold samples before experimentation and reduce preloads, allowing the study of only the controlled loading. A polydimethylsiloxane (PDMS) scaffold was also designed for cell loading, complete with a positional reference grid for observing the response of individual cells to strain. Initial work with this microloading platform includes studying osteocyte-like MLO-Y4 cells and changes in viability in response to mechanical load in vitro. These initial studies have demonstrated the ability to induce cell death in response to microdamage.

Mechanosensitive cells, such as osteocytes in bone, are capable of translating mechanical stimuli into cellular responses. This phenomenon can be widely found in cells throughout the body, and yet little is known about the mechanisms and pathways by which this occurs. Research in this field has focused on creating in vitro models that better reflect the in vivo environment in order to study these mechanisms and pathways. Where many variations on these systems exists, one major goal in improving these models is to use fewer cells in order to observe the response of specific cells and possibly more meaningful data. Using an uniaxial loading device, a substrate with cells seeded onto it can be mechanically strained and the response of these fewer cells can be quantified. In this study, two substrates of varying geometry are proposed that allow for a gradient of mechanical strains to be applied to cultured cells. These designs are characterized and compared using both physical and simulated testing. Utilizing designs, such as the ones used for these substrates, enables the effects of a wide range of mechanical strains on cells to be observed and studied under identical culture and loading environments.

The surface adhesion between C. elegans and the agar plates on which they are commonly grown has yet to be accurately quantified. C. elegans are a scientifically important species of nematode whose simple structure allowed the first mapping of the complete nervous system in a multicellular organism. One of the current topics of research in the C. elegans community is the investigation of neuronal function in locomotion. Locomotion models derived from mathematical analysis of nematode motion and surface interaction are frequently employed to investigate the influence of neuronal function in locomotion. Surface interactions, such as adhesion energy, play a critical role in the development of these models, but measurements of these parameters have not yet been performed. This paper presents the measurement of surface adhesion energy of nematodes on agar surfaces using a direct pull-off experiment. Adhesion energy was found to be W = 4.94 ± 1.19 mJ/m² for the wild type.

In recent years, electro-chemotherapy and gene electro-transfer have emerged as promising cancer therapies that use locally applied electric fields to facilitate the transport of chemotherapeutic drugs into tumor cells or genes into target cells based on the cell membrane electroporation. It is well known that the local electric field in the tissue depends on the applied voltage on the electrodes, the geometry and position of the electrodes, and on the heterogeneity and geometry of the tissue. So far, the local electric field distribution in tissues was found by solving the classic Laplace equation. However, tissues and tumors have evolving microstructures which affect the distribution of the applied electric field. Inspired by the successful application of fractional order constitutive models of tissues, in our exploratory study we propose a fractional calculus based approach to model the electric field and potential distribution in tissues. The resulting fractional differential equation of Laplace type is solved analytically. Our preliminary results on the local electric field distribution might help to find electrode configurations for optimal treatment outcome.

Arterial tissue failure leads to a number of potentially life-threatening clinical conditions such as atherosclerotic plaque rupture and aortic dissection, which often occur suddenly and unpredictably in vivo. Atherosclerotic plaque rupture is responsible for roughly 75 % of all newly developed and recurring myocardial infarctions. Mouse models of atherosclerosis are often used in research studies because plaque characteristics can be manipulated experimentally in a reproducible fashion. To simulate atherosclerotic plaque delamination in mouse abdominal aorta, we adopt the Holzapfel model for the bulk material behavior and the cohesive zone model (CZM) for the delamination behavior along the plaque-media interface. In the Holzapfel model, each artery layer is treated as a fiber-reinforced material with the fibers symmetrically disposed with respect to the axial direction of the aorta. In the CZM, delamination is governed by a traction-separation law. A proper set of Holzapfel parameter values and CZM parameter values is determined based on values suggested in the literature and through matching the simulation predictions of the load vs. load-point displacement curve with experimental measurements for one plaque delamination cycle. With the same set of Holzapfel parameter values and CZM parameter values, two more simulation predictions of the load vs. load-point displacement curve were obtained, which match well with experimental measurements, thus validating the CZM approach. Our approach can be readily modified to understand tissue failure processes in human pathologies, e.g. aortic dissection.