Search

Sperm velocity is long known to be an important indicator of sperm quality but without much biophysical theory explaining it. Contact mechanics based modeling was developed here to determine the effects that sperm velocity ([v.sub.s]) and sperm head density ([[rho].sub.s])--which is an indicator of the stiffness of the head, have on the peak sperm-zona pellucida contact pressure ([p.sub.m]) during the early phase of sperm penetration. The modeling identified [v.sub.s] as being more influential on [p.sub.m] compared with the influence that [[rho].sub.s] has, which means that for spermatozoa competing on the same oocyte, greater [v.sub.s] is a more important advantage than higher [[rho].sub.s]. Specifically, [p.sub.m] was more sensitive by a power of 2 to changes in [v.sub.s] than to changes in [[rho].sub.s]. It was further demonstrated that each 0.1 g/cc increase in [[rho].sub.s] (within the physiologically relevant range of 1.3-1.7 g/cc) would be equivalent to just 3% rise in [v.sub.s], indicating again that faster swimming is a better strategy for spermatozoa compared with head stiffening. The modeling hence provided some useful insights regarding sperm biomechanics, which theoretically elucidate the well-recognized importance of sperm velocity measurements as being indicative of sperm quality. [DOI: 10.1115/1.4002857] Keywords: biomechanical cell model, cellular biomechanics, fertilization, motility, sperm quality

A continuum model based on the contact mechanics theory was developed and used for evaluating virus indentation forces at the early stage of membrane invagination, as well as the work of the virus indentation forces and virus-cell contact pressures in a receptor-mediated endocytosis, depending on the virus size and virus/cell stiffnesses. The model indicated that early virus indentation forces are in the order of 1-10 pN and for a given extent of virus engulfment, they increase linearly with the elastic modulus of the host cell and also with the square of the virus radius. The work of invagination at the initial phase of virus endocytosis is in the order of tens of zeptojoules and peak virus-cell contact pressures at this stage are in the order of hundreds of Pascals to several kPa. For a given extent of virus engulfment, peak and average contact pressures increase linearly with the elastic modulus of the host cell but interestingly, they are negligibly affected by the virus size. The present model may be useful in the fields of cellular biomechanics, virology and nanodrug delivery to evaluate mechanical factors during the early phase of membrane invagination. [DOI: 10.1115/1.4001888] Keywords: biomechanical cell model, cellular mechanics, virus engulfment

Deep tissue injury (DTI) is a potentially life-threatening form of pressure ulcer that onsets in muscle tissue overlying bony prominences and progresses unnoticeably to more superficial tissues. To minimize DTI, the efficacy of wheelchair cushions should be evaluated not only based on their performance in redistributing interface pressures but also according to their effects on stress concentrations in deep tissues, particularly muscles. However, a standard bioengineering approach for such analyses is missing in literature. The goals of this study were to develop an algorithm to couple finite element (FE) modeling of the buttocks with an injury threshold for skeletal muscle and with a damage-stiffening law for injured muscle tissue, from previous animal experiments, to predict DTI onset and progression for different patient anatomies and wheelchair cushions. The algorithm was also employed for identifying intrinsic (anatomical) biomechanical risk factors for DTI onset. A set of three-dimensional FE models of seated human buttocks was developed, representing different severities of pathoanatomical changes observed in chronically sitting patients: muscle atrophy and 'flattening' of the ischial tuberosity (IT). These models were then tested with cushions Of different stiffnesses representing products available on the market and semirigid supports. Outcome measures were the percentage of damaged muscle tissue volumes after 90 min and 110 min of simulated continuous immobilized sitting as well as muscle injury rates post-60 min, -90 min, and -110 min of continuous sitting. Damaged muscle volumes grew exponentially with the level of muscle atrophy. For example, simulation of a subject with 70% muscle atrophy sitting on a soft cushion showed damage to 33% of the muscle volume after 90 min of immobilized sitting, whereas a comparable simulation with a nonatrophied muscle yielded only 0.4% damaged tissue volume. The rates of DTI progression also increased substantially with increasing severities of muscle atrophy, e.g., 70% atrophy resulted in 8.9, 2.7, and 1.6 times greater injury rates compared with the 'reference' muscle thickness cases, after 60 min, 90 min, and 110 min of sitting, respectively. Across all simulation cases, muscle injury rate was higher when a 'flatter' IT was simulated. Stiffer cushions increased both the extent and rate of DTI at times shorter than 90 min of continuous sitting, but after 110 min, volumes and rates of tissue damage converged to approximately similar values across the different cushion materials. The present methodology is a practical tool for evaluating the performances of cushions in reducing the risk for DTI in a manner that goes far beyond the commonly accepted measurements of sitting pressures. [DOI: 10.1115/1.3005195] Keywords: pressure ulcer, decubitus, bedsore, wheelchair cushions, rehabilitation

Pressure-related deep tissue injury (DTI) is a life-risking form of pressure ulcers threatening immobilized and neurologically impaired patients. In DTI, necrosis of muscle and enveloping adipose tissues occurs under intact skin, owing to prolonged compression by bony prominences. Modeling the process of DTI in the buttocks requires knowledge on viscoelastic mechanical properties of the white adipose tissue covering the gluteus muscles. However, this information is missing in the literature. Our major objectives in this study were therefore to (i) measure short-term ([H.sub.S]) and long-term ([H.sub.L]) aggregate moduli of adipose tissue covering the glutei of sheep, (ii) determine the effects of preconditioning on [H.sub.S] and [H.sub.L], and (iii) determine the time course of stress relaxation in terms of the transient aggregate modulus H(t) in nonpreconditioned (NPC) and preconditioned (PC) tissues. We tested 20 fresh tissue specimens (from 20 mature animals) in vitro: 10 specimens in confined compression for obtaining the complete H(t) response to a ramp-and-hold protocol (ramp rate of 300 mm/s), and 10 other specimens in swift indentations for obtaining comparable short-term elastic moduli at higher ramp rates (2000 mm/s). We found that [H.sub.S] in confined compression were 28.9 [+ or -] 14.9 kPa and 18.1 [+ or -] 6.9 kPa for the NPC and PC specimens, respectively. The [H.sub.L] property, 10.3 [+ or -] 4.2 kPa, was not affected by preconditioning. The transient aggregate modulus H(t) always reached the plateau phase (less than 10% difference between H(t) and [H.sub.L]) within 2 min, which is substantially shorter than the times for DT1 onset reported in previous animal studies. The short-term elastic moduli at high indentation rates were 22.6 [+ or -] 10 kPa and 15.8 [+ or -] 9.4 kPa for the NPC and PC test conditions, respectively. Given a Poisson's ratio of 0.495, comparison of short-term elastic moduli between the high and slow rate tests indicated a strong deformation-rate dependency. The most relevant property for modeling adipose tissue as related to DTI is found to be [H.sub.L], which is conveniently unaffected by preconditioning. The mechanical characteristics of white adipose tissue provided herein are useful for analytical as well as numerical models of DTI, which are essential for understanding this serious malady. [DOI: 10.1115/1.2800830] Keywords: pressure sore, bedsore, decubitus, fat, stress relaxation

Computational studies of deep pressure sores (DPS) in skeletal muscles require information on viscoelastic constitutive behavior of muscles, particularly when muscles are loaded transversally as during bone-muscle interaction in sitting and lying immobilized patients. In this study, we measured transient shear moduli G(t) of fresh porcine muscles in vitro using the indentation method. We employed a custom-made pneumatic device that allowed rapid (2000 mm/s) 4 mm indentations. We tested 8 gluteus muscles, harvested from 5 adult pigs. Each muscle was indented transversally (perpendicularly to the direction of fibers) at 3 different sites, 7 times per site, to obtain nonpreconditioned (NPC) and preconditioned (PC) G(t) data. Short-term ([G.sub.S]) and long-term ([G.sub.L]) shear moduli were obtained directly from experiments. We further fitted measured G(t) data to a biexponential equation G(t) = [G.sub.1] x exp(-t/[[tau].sub.1])+ [G.sub.2] x exp(-t/[[tau].sub.2]) + [G.sub.[infinity]], which provided good fit, visually and in terms of the correlation coefficients. Typically, plateau of the stress relaxation curves (defined as 10% difference from final [G.sub.L]) was evident ~20 s after indentation. Short-term shear moduli [G.sub.S] (mean NPC: 8509Pa, PC: 5711 Pa) were greater than long-term moduli [G.sub.L] (NPC: 609 Pa, PC: 807 Pa) by about an order of magnitude. Statistical analysis of parameters showed that only [G.sub.2] was affected by preconditioning, while [G.sub.L], [G.sub.S], [G.sub.[infinity]], [[tau].sub.1], [[tau].sub.2], and [G.sub.1] properties were unaffected. Since DPS develop over time scales of minutes to hours, but most stress relaxation occurs within ~20 s, the most relevant property for computational modeling is [G.sub.L] (mean ~700 Pa), which is, conveniently, unaffected by preconditioning. [DOI: 10.1115/1.2264395] Keywords: pressure ulcer, decubitus, striated muscle, viscoelastic mechanical properties, indentation