Mechanics of Biological Systems and Materials & Micro-and Nanomechanics, Volume 4

Understanding the way in which proteins vibrate in their folded state is pivotal for a broad comprehension of their biological activity. In particular, vibrations in the terahertz range are indicated in the current literature as being involved in protein conformational changes. Nowadays, frequencies around or below 1 THz can be detected for example by Raman spectroscopy using proper ultra-low frequency filters. In previous studies, some of the authors performed modal analysis of all-atom lattice models to investigate the expansion-contraction mode shapes associated to low-frequency Raman peaks detected experimentally on lysozyme and Na⁺/K⁺-ATPase powder samples. In this contribution, all-atom calculations are compared to new ones derived from a simplified coarse-grained mechanical model; the latter was built-up considering only C_α atoms, i.e., the protein backbone. The efficacy of the coarse-grained model in describing delocalized and global expansion-contraction protein vibrations as well as its limitations are discussed.

The study of protein vibration and dynamics is receiving increasing attention among researchers, both from a numerical and experimental perspective. By using terahertz spectroscopy techniques, it has been shown that conformational changes, crucial for protein biological function, are strictly related to low-frequency vibrational modes. These motions generally occur in the terahertz range (~0.1–2 THz) involving large portions of the protein. The present contribution aims at investigating the role of terahertz (expansion-contraction) vibrational modes to protein conformational change from a numerical viewpoint. Modal analysis is performed by using C_α-only coarse-grained mechanical models: the obtained mode shapes are compared, by means of three similarity indexes, to the displacement field of protein conformational change. In particular, lysine-arginine-ornithine (LAO) binding protein is selected as a case study.

Examples of methods for determining residual stresses and strains (RSS) in biological materials are reviewed and postulated roles of RSS in biomechanical behavior described. Residual strains are thought to exert a particularly important influence on the behavior of arteries. For several decades, determination of those strains has relied on the opening angle method, in which a ring removed from an artery is slit radially, causing the ring to spring open. The change in geometry of the ring provides input to analytical relations for estimating circumferential residual strains. The wall of an artery, which has three layers, contains a mixture of elastin, collagen fibrils and muscle cells. An attempt to use small angle X-ray scattering (SAXS) to characterize residual strains using collagen fibrils as internal “micro strain sensors” is presented. First, the results of SAXS experiments to investigate the response of collagen fibrils to strains applied to arterial tissue are presented. Strains as measured in fibrils are compared to those applied to the tissue. Then, SAXS experiments to explore residual strains in collagen fibrils within rings of arterial tissue are described. Results are compared to tissue-level residual strains estimated from the opening angle method.

Change in papillary muscle motion as a result of left ventricular (LV) remodeling after posterolateral myocardial infarction is thought to contribute to ischemic mitral regurgitation. A finite element (FE) model of the LV was created from magnetic resonance images acquired immediately before myocardial infarction and 8 weeks later in a cohort of 12 sheep. Severity of mitral regurgitation was rated by two-dimensional echocardiography and regurgitant volume was estimated using MRI. Of the cohort, six animals (DC) received hydrogel injection therapy shown to limit ventricular remodeling after myocardial infarction (Rodell, Christopher B., Circ. Cardiovasc. Interv. 9:e004058 2016) while the control group (MI) received a similar pattern of saline injections. LV pressure was determined by direct invasive measurement and volume was estimated from MRI. FE models of the LV for each animal included both healthy and infarct tissue regions as well as a simulated hydrogel injection pattern for the DC group. Constitutive model material parameters for each region in the FE model were assigned based on results from previous research. Invasive LV pressure measurements at end diastole and end systole were used as boundary conditions to drive model simulations for each animal. Passive stiffness (C) and active material parameter (T_max) were adjusted to match MRI estimations of LV volume at end systole and end diastole. Nodal positions of the chordae tendineae (CT) were determined by measurements obtained from the excised heart of each animal at the terminal time point. Changes in CT nodal displacements between end systole and end diastole at 0- and 8-week time points were used to investigate the potential contribution of changes in papillary muscle motion to the progression of ischemic mitral regurgitation after myocardial infarction. Nodal displacements were broken down into radial, circumferential, and longitudinal components relative to the anatomy of the individual animal model. Model results highlighted an outward radial movement in the infarct region after 8 weeks in untreated animals, while radial direction of motion observed in the treated animal group was preserved relative to baseline. Circumferential displacement decreased in the remote region in the untreated animal group after 8 weeks but was preserved relative to baseline in the treated animal group. MRI estimates of regurgitant volume increased significantly in the untreated animal group after 8 weeks but did not increase in the treated group. The results of this analysis suggest that hydrogel injection treatment may serve to limit changes in papillary muscle motion and severity of mitral regurgitation after posterolateral myocardial infarction.

Cellulose nanofibrils (CNFs) are a naturally abundant polymer with exceptional mechanical properties for their low density. Neat CNF materials have been reported with moduli ranging from 4 to 86 GPa, where the variation in moduli results from several preparation parameters, one of which is the fiber alignment. Because of their high aspect ratio (>100), CNFs form an entangled network in the absence of mechanisms for fiber alignment. In this study, the alignment of CNF fibers in films is achieved via control of printing and drying processes used to manufacture neat CNF films from aqueous suspensions containing low volume fractions of CNFs. The alignment of the CNFs is determined both globally and locally within printed CNF thin films and the effect of orientation on mechanical properties is characterized. Polarized light microscopy is used to characterize the orientation of CNFs through the bulk of the material (i.e., over areas >4 mm²) and shows that propagation of drying fronts can significantly impact alignment. The alignment of CNFs at the surface of the materials is imaged and quantified via atomic force microscopy (AFM). Both topographic and phase imaging, as well as different image processing techniques were evaluated for alignment characterization via AFM.

The mechanical properties of hydrogels suitable for applications in the field of bioprinting, which tries to develop three-dimensional tissue equivalents, are crucial for the proper fulfilment of their functions in the human body. This aspect is especially important regarding types of tissues which have to withstand applied mechanical forces. Due to their high water content similar to the human body and their tunable mechanical properties, hydrogels based on biopolymers are ideally suited for such applications. In this work, the first results of a novel method for the indirect measurement of the mechanical properties of hydrogels using laser-Doppler vibrometry and 3D-printed test structures are presented. Thanks to the experimental design hydrogels can be cast directly over such beam-like test structures without any leakage. First results show that the resonance frequencies of the beam structure are modulated by the material properties of the different hydrogels placed on it, enabling future applications and further experiments. For comparing the measurement data with the mechanical properties of the samples used, indentation-based measurements have been carried out. This approach can be integrated into existing bioprinting workflows and enables the non-destructive monitoring of biopolymer-based hydrogels in their mechanical properties.

Studying the mechanics of aortic tissue is a crucial component in understanding its behavior under healthy as well as diseased conditions. Wall shear stress and circumferential stress have been largely accepted as significant factors in arterial growth and remodeling as a response to changes in flow and pressure. But, experimental studies on aortic tissues have largely focused on uniaxial and biaxial tests which are more suited for investigating circumferential stress. This may be explained by the inherent convenience of gripping tissue and then applying deformations in the uniaxial and biaxial modes. As such, the behavior of aortic tissue under shear has been left relatively unstudied. We propose to study the response of porcine aortic wall tissue under cyclic constant torsional shear strain rate for high amplitude (50%) and at different shear strain rates (4%/s and 40%/s). Three to four 12.5 mm diameter samples are obtained from the descending porcine aorta. Initial results clearly indicate a non linear response for the moment as a function of the angle of twist while many popular models predict a linear response for the arterial wall even under large shear strain.

High-intensity focused ultrasound is a technology currently used to treat bodily tissue for various medical purposes. For example, it has been applied for tissue ablation as a treatment for prostate cancer. However, the effective targeting of tissue deeper inside the body remains challenging because bones obstruct and scatter ultrasonic waves, which reduces the energy transmission to the desired location. Thus, understanding wave-scattering effects on focal point location and intensity is crucial for the expansion of the technology. Previous ultrasound visualization studies have not examined these effects in detail, especially for curved bone geometries.

Hence, in this work, the effects of bones on the transmission of focused ultrasound is investigated for hollow and solid cylindrical bone geometries. In laboratory experiments, images of wave fields are captured using shadowgraph techniques. The method uses a pulsed laser synced with a CMOS camera. Ultrasonic waves cause periodic local changes in density of the water, producing bright and dark patterns of laser transmission corresponding to the wave peaks and troughs. Improvements in the experimental setup and image processing compared to previous work allow for an expansion of the field of view with higher contrast. The bone replicas scatter the ultrasonic wave field and the effect of obstruction on ultrasound focal point intensities is quantified using pixel intensity measurements. In addition to visualization, differences in the pressure fields are recorded using a hydrophone and compared to the results obtained from the shadowgraph images. This experimental work provides a reference for future research in medical ultrasound and has the potential to lead to the development of methods that optimize the targeting of tissue deep in the human body.

An adult skeleton has over 200 bones. Each bone has a specific function. These different functions bring different types of mechanical loadings. To ensure a healthy behavior of the bone, a throughout life process occurs on the micro-architecture of the cortical bone. This process highly depends on the stress applied on the bone. The micro-architecture is able to supply blood and nutrients into the bone matrix and acts on the bone remodeling process. The architecture is formed by vascular canals where the orientation depends on the main axis of the mechanical loading. Vascular canals of the cortical bone located in long bone diaphysis are mainly oriented along the longitudinal axis of the diaphysis. Moreover, the canal network is tortuous and several geometrical features should affect the macroscopic behavior of the bone. The novelty of this work is to use a method which accurately describes the vascular architecture of the cortical bone based on micro-computed tomography and to correlate these results with the macroscopic mechanical behavior of the cortical bone. This experimental study is based on the extraction of cortical bone samples from four cadaveric human subjects. For each human subject, left and right humeri and femurs are studied. Dumbbell-shaped bone specimens are prepared from each bone. Care was taken to preserve bone properties after extraction: a maximum of 15 days was fixed between the extraction and the mechanical testing. Samples were constantly kept hydrated and stored at 4 °C in order to avoid frost/defrost cycles. Specimens are scanned and subsequently mechanically loaded to failure. Results will show the impact of the laterality of the bone on the architecture, the impact of loading on the bone on the architecture by comparing humeral and femoral samples for each human subject and finally the impact of the architecture on the mechanical behavior of the bone.

High-intensity impulsive sounds (e.g., explosions, firearms, etc.) are known to damage the human eardrum (Tympanic Membrane, TM), and produce conductive hearing loss. However, the eardrum failure mechanism under high-pressure loading has not been well studied. Our previous investigations have shown the feasibility of using full-field-of-view 3D High-Speed Digital Image Correlation (3D-HSDIC) with a custom-made loading apparatus to study acoustically induced damage on artificial membranes. In this paper, we present new measurements on multiple human post-mortem eardrums. The dynamic 3D displacements and transient shape changes of the entire eardrum surface during the rupture are simultaneously quantified at high frame rates (i.e., >100,000 Hz) using the 3D-HSDIC method over the rapid time-course of the TM response. The results describe the high strain-rate and large displacements of the eardrum from the initial stages of rapid pressurization up to complete eardrum failure. The high spatio-temporal resolution measurements allow the determination of eardrum mechanical properties under high-pressure loading. This study indicates the potential utility of high-speed DIC to study high pressure induced TM failure mechanisms, which has impact on developing new hearing protection devices. Future measurements will be performed with a miniaturized optical system and an updated high-pressure loading apparatus designed using advanced thermo-acousto-fluidic numerical modeling and high-speed Schlieren imaging techniques.

We are developing a High-speed Digital Holographic (HDH) system to measure acoustically induced transient displacements and shapes of live mammalian Tympanic Membrane (TM) for research and clinical applications. Currently, the HDH system measures the shape of the entire TM with a resolution of about 120 μm, and sound-induced displacements with magnitude resolutions of <10 nm and temporal resolutions of <20 μs in full-field of view. In this paper, we apply Experimental Modal Analysis (EMA) to the HDH results to extract modal parameters of the TM and quantitatively compare these parameters among different TMs of normal and pathological middle ears. Transient displacements in response to broadband acoustic clicks and the shape of cadaveric human TMs are measured before and after different simulated middle-ear pathologies including various levels of fluid in the middle ear cavity, stapes fixation, and incudo-stapedial (IS) joint interruption. The transient displacement of the TM along the sensitivity vector is supplemented with the 3-D TM shape information to derive the true displacement of the TM locally normal to the TM surface. The displacement is then used in an EMA framework to determine natural frequencies, damping and mode shapes of the TMs under the normal and different pathological middle-ear conditions.

Preliminary results show that the damping ratio of the TM at each natural frequency in the normal ear decreases as frequency increases. We also see differences in identified modal shapes and natural frequencies before and after various manipulations. We plan to identify trends in the data associated with different pathologies as well as test the sensitivity and selectivity of these analyses for clinical diagnosis. Due to the large amount of the data obtained from the HDH, Artificial Intelligence (AI) and Data Mining methods will be used to automate the EMA process and assist in the separation of normal and diseased states.

Streptococcus mutans (S. mutans) is a group of cocci bacteria that highly contributes to oral decay. In cases of high potency, it can cause plaque build-up, impaired speech, difficulty chewing, and cavity formation, which contribute to over half of the dental visits in the United States. In severe cases where a biofilm develops on a dental implant, patients will experience pain, swelling, and potential loosening or loss of the titanium implant. S. mutans are also highly resistant to antibiotics, which makes the infections both persistent and difficult to treat. As poor oral hygiene continues to be a global epidemic, it is important to study and characterize the mechanics of the bacteria to develop therapeutic targets to alleviate S. mutans infections. One possible target is modifications of the cell wall. Recent literature suggests that alterations to the biosynthesis pathways of cell wall polysaccharides could lead to new opportunities for therapeutics. The cell wall of S. mutans is high-functioning and complex. It consists of multiple peptidoglycan layers, wall teichoic acid (WTA) containing surface glycopolymers, and a polysaccharide capsule. Using atomic force microscopy (AFM) in combination with fluorescent laser scanning confocal imaging, we compare cell wall deformation with mutants defective in WTA. Furthermore, scanning electron microscopy of both wild type S. mutans and mutant strains reveal differences in surface morphology. Our long-term goal is to determine mechanical and structural properties of bacterial cell walls that contribute to antibiotic resistance in order to preferentially regulate such properties.

Peri-implantitis, a disease formed by subgingival biofilm between dental implants and surrounding tissue, can lead to necrosis or implant loss. The development of an implant surface that promotes osseointegration and deters bacterial biofilm adhesion is paramount to prevent peri-implantitis. A technique to quantify adhesion strengths of biofilms is important to optimize surfaces which prevent bacteria from adhering strongly. The laser spallation technique has been recently adapted to obtain quantitative measures of biofilm adhesion. One key advantage of laser spallation is it results in quantified adhesion strength while using a non-contact high strain rate force. Image analysis can be used to obtain fluence, energy per unit area, at spallation of the biofilm, along with one dimensional wave analysis and finite element analysis, a quantitative interface adhesion strength can be determined for the biofilm-implant interface. In this study, Streptococcus mutans, a gram-positive facultative anaerobe, was chosen because it promotes the attachment and growth of more harmful bacteria. The competition between oral bacteria and cell should also be considered when comparing implant surface characteristics. MG-63 was chosen as it closely mimics osteoblast adhesion. We will demonstrate the competition in adhesion between S. mutans and osteoblast like cells on dental implant mimicking surfaces through the adaptation of the laser spallation technique. This study will lead to the development of dental implant surfaces which promote osseointegration and inhibit biofilm formation. Furthermore, the laser spallation technique will be used to optimize other medical implant surfaces, and surfaces where biofilms have deleterious effects.

The most abundant component of the human umbilical cord, the Wharton Jelly (WJ), is a mucous connective tissue consisting in a spongy network of interlacing collagen fibers that forms a continuous soft skeleton encasing umbilical vessels. The link between WJ structure and the pivotal cord function in providing unimpeded blood flow to the developing fetus is still poorly understood. We performed an advanced biomechanical characterization of human umbilical cord samples by planar equibiaxial tension tests. The intrinsic moiré technique was utilized for that purpose. A 3-D finite element analysis was then implemented to simulate the biomechanical response of the cord.

Streptococcus mutans (S. mutans), a major cariogenic bacterium, has exhibited esterase activities, and has been shown to degrade dental resin composites and adhesives contributing to the risk of developing secondary caries and failed restorations. Changes to the surface morphology of the resin materials have been observed through SEMs; however, they were not quantified. In the current study, we present the development of a laser diffractometer whereby we can apply it to determining quantitatively the surface roughness spectrum of a dental material due to bacteria biodegradation.