Vinu Unnikrishnan

Constitutive material modeling of cell: a micromechanics approach.

The variations in mechanical properties of cells obtained from experimental and theoretical studies can be overcome only through the development of a sound mathematical framework correlating the derived mechanical property with the cellular structure. Such a formulation accounting for the inhomogeneity of the cytoplasm due to stress fibers and actin cortex is developed in this work. The proposed model is developed using the Mori-Tanaka method of homogenization by treating the cell as a fiber-reinforced composite medium satisfying the continuum hypothesis. The validation of the constitutive model using finite element analysis on atomic force microscopy (AFM) and magnetic twisting cytometry (MTC) has been carried out and is found to yield good correlation with reported experimental results. It is observed from the study that as the volume fraction of the stress fiber increases, the stiffness of the cell increases and it alters the force displacement behavior for the AFM and MTC experiments. Through this model, we have also been able to find the stress fiber as a likely cause of the differences in the derived mechanical property from the AFM and MTC experiments. The correlation of the mechanical behavior of the cell with the cell composition, as obtained through this study, is an important observation in cell mechanics.

A biofidelic computational model of the female pelvic system to understand effect of bladder fill and progressive vaginal tissue stiffening due to prolapse on anterior vaginal wall

Treatment of anterior vaginal prolapse (AVP), suffered by over 500,000 women in the USA, is a challenge in urogynecology because of the poorly understood mechanics of AVP. Recently, computational modeling combined with finite element method has been used to model AVP through the study of pelvic floor muscle and connective tissue impairments on the anterior vaginal wall (AVW). Also, the effects of pelvic organ displacements on the AVW were studied numerically. In our current work, an MRI-based full-scale biofidelic computational model of the female pelvic system composed of the urinary bladder, vaginal canal, and the uterus was developed, and a novel finite element method framework was employed to simulate vaginal tissue stiffening and also bladder filling due to expansion for the first time. A mesh convergence study was conducted to choose a computationally efficient mesh, and a non-linear hyperelastic Yeohs material model was adopted for the study. The AVW displacements, mechanical stresses, and strains were estimated at varying degrees of bladder fills and vaginal tissue stiffening. Both bladder filling and vaginal stiffening were found to increase the stress concentration on the AVW with varying trends, which have been discussed in detail in the paper. To our knowledge, this study is the first to estimate the individual and combined effects of bladder filling and vaginal tissue stiffening due to prolapse on the AVW. Copyright

A REALISTIC 3D COMPUTATIONAL MODEL OF THE CLOSURE OF SKIN WOUND WITH INTERRUPTED SUTURES

Wounds or cuts are the most common form of skin injuries. While a shallow wound may heal over time, deep wounds often require clinical interventions such as suturing to ensure the wound closure and timely healing. To date, suturing practices are based on a surgeon’s experience and there is no benchmark to what is right or wrong. In the literature, there have been few attempts to characterize wound closure and suture mechanics using simple 2D computational models. In our current work, for the first time, a realistic three-dimensional (3D) computational model of the skin with the two layers, namely the epidermis and dermis, have been developed. A 3D diamond shaped wound with a varying cross-section has been modeled, and interrupted sutures have been placed numerically in multiple steps to close the wound. Nonlinear hyperelastic material properties have been adopted for the skin and a skin pre-stress was applied bi-axially. The force requirements for each suture were estimated numerically using a novel suture pulling technique. The suture forces were found to lie in the range of 0–5 N with a maximum value at the center. Also, the center suture was observed to require an approximately four times pull force compared to the first end suture. All these findings provide important guidelines for suturing. Additionally, the suture force can be approximated as a polynomial function of the displacement. Given a wound geometry, wound depth, skin material properties, skin pre-stress, suture wire material and cross-sectional area, using our computational model, such a relationship can be used to estimate and characterize the suture force requirements accurately. To our knowledge, such a 3D computational model of skin wound closure with interrupted sutures have not been developed till date, and would be indispensable for planning robotic surgeries and improving clinical suturing practices in the future.

Biofidelic human brain tissue surrogates

ABSTRACT Traumatic brain injury (TBI) due to blast exposure or head impacts in accidents or contact sports is one of the most critical and poorly understood areas of research in the 21st century. To date, the unavailability of human brain tissues (grey and white matter especially) due to ethical and biosafety issues has not allowed for much experimental research into the study of the mechanics of brain tissues under impact or dynamic loading. In the current work, for the first time, biofidelic brain tissue surrogates have been developed using a low cost, castable (to any shape or size), two-part silicone-based material system to precisely mimic the nonlinear mechanical properties of both the white and the grey matter. The fabrication methodology involves the iterative mixing of the two parts of silicone at certain mix ratios (by weight) to generate a biomechanical behavior similar to the white and the grey matter tissues, respectively, at two different strain rates (low and high). The nonlinear behavior of these novel brain tissue surrogates have been characterized using five hyperelastic material models. These brain tissue simulant materials would be indispensable not only for the study of TBI, but also to allow doctors to practice brain surgeries (for training purposes) in a clinical setting. Additionally, crucial brain tissue modifications in Alzheimers disease and dementia can be studied in the future with such accessible biofidelic brain tissue surrogate materials.

Experimental study on tissue phantoms to understand the effect of injury and suturing on human skin mechanical properties

Skin injuries are the most common type of injuries occurring in day-to-day life. A skin injury usually manifests itself in the form of a wound or a cut. While a shallow wound may heal by itself within a short time, deep wounds require surgical interventions such as suturing for timely healing. To date, suturing practices are based on a surgeon’s experience and may vary widely from one situation to another. Understanding the mechanics of wound closure and suturing of the skin is crucial to improve clinical suturing practices and also to plan automated robotic surgeries. In the literature, phenomenological two-dimensional computational skin models have been developed to study the mechanics of wound closure. Additionally, the effect of skin pre-stress (due to the natural tension of the skin) on wound closure mechanics has been studied. However, in most of these analyses, idealistic two-dimensional skin geometries, materials and loads have been assumed, which are far from reality, and would clearly generate inaccurate quantitative results. In this work, for the first time, a biofidelic human skin tissue phantom was developed using a two-part silicone material. A wound was created on the phantom material and sutures were placed to close the wound. Uniaxial mechanical tests were carried out on the phantom specimens to study the effect of varying wound size, quantity, suture and pre-stress on the mechanical behavior of human skin. Also, the average mechanical behavior of the human skin surrogate was characterized using hyperelastic material models, in the presence of a wound and sutures. To date, such a robust experimental study on the effect of injury and sutures on human skin mechanics has not been attempted. The results of this novel investigation will provide important guidelines for surgical planning and validation of results from computational models in the future.

Multiscale Nonlocal Thermo-Elastic Analysis of Graphene Nanoribbons

This paper investigates the effect of interfacial thermal characteristics and nonlocal effects on the overall effective thermal property of graphene nanoribbons embedded in low-density polyethylene matrix. It is widely known that apart from the thermal conductivity, the effect of interfacial thermal resistance is the most important factor that determines the thermal efficiency of nanostructures. In this work, we develop nonlocal thermo-elastic properties coupled with a multiscale strategy by means of properties derived from atomistic simulations. Atomistic based thermal properties of graphene nanoribbons are estimated from molecular dynamics (MD) simulations and the effect of inter-tubular spacing on the interfacial thermal characteristics are also studied. The paper also discusses the effect of the nonlocal parameter on the thermal conduction of the composite system using the developed nonlocal thermo-elastic model.

Tissue-fluid interface analysis using biphasic finite element method.

Numerical studies on fluid–structure interaction have primarily relied on decoupling the solid and fluid sub-domains with the interactions treated as external boundary conditions on the individual sub-domains. The finite element applications for the fluid–structure interactions can be divided into iterative algorithms and sequential algorithms. In this paper, a new computational methodology for the analysis of tissue–fluid interaction problems is presented. The whole computational domain is treated as a single biphasic continuum, and the same space and time discretisation is carried out for the sub-domains using a penalty-based finite element model. This procedure does not require the explicit modelling of additional boundary conditions or interface elements. The developed biphasic interface finite element model is used in analysing blood flow through normal and stenotic arteries. The increase in fluid flow velocity when passing through a stenosed artery and the drop in pressure at the region are captured using this method.

Effect of Bladder and Rectal Loads on the Vaginal Canal and Levator Ani in Varying Pelvic Floor Conditions

ABSTRACT The female pelvic floor is a complex system, in which the pelvic organs such as the urinary bladder, rectum, vaginal canal and the pelvic floor muscles interact in different ways over the course of a womens life. To date, the pelvic floor mechanics is poorly understood by doctors and medical practitioners, which is the key to understand the various pelvic floor defects such as child-birth complications and pelvic organ prolapse (POP). Currently, millions of women suffer from child birth trauma and POP in the United States and across the globe. While experimental studies are being performed on human subjects to understand the various qualitative trends in pelvic floor defects, recently, computational modeling has allowed researchers to better understand the mechanics of the pelvis. Based on a recent review on the state of the art knowledge on numerical modeling of pelvic floor mechanics by Chanda et al., a lack of computational study on the interaction between the pelvic organs and the muscles were highlighted. In the current work, a full-scale female pelvic system model (comprising of the urinary bladder, rectum, vaginal canal, uterus and the pelvic floor muscle levator ani) was developed using a magnetic resonance imaging (MRI)-based image segmentation process, and the effect of bladder and rectal loads on the vaginal canal in varying pelvic floor conditions (healthy and prolapsed) were quantified. Nonlinear material models were adopted to simulate pelvic tissue properties and a novel deteriorating material model was developed to simulate pelvic floor muscle degradation in different degrees of prolapse. The results from this study highlight the significance of the inclusion of the pelvic floor muscles in the study of the mechanics of the pelvis. Also, the mechanics of pelvic degradation and its effect on the vaginal walls and levator ani were studied to understand discomfort associated with prolapse. To date, such a detailed study on the interaction of pelvic organs in prolapse has not been conducted, the results of which would be indispensable for better understanding of pelvic floor mechanics and allow doctors to device better surgical planning strategies.

Analysis of Thermal Interfacial Resistance Between Nanofins and Various Coolants

Molecular dynamic simulations are performed to study the thermal interfacial resistance between a (5,5) carbon nanotube (CNT) and different matrix materials (water, ethyl alcohol, and 1-hexene). After the matrix-CNT ensemble is equilibrated to a base temperature of 300K, the temperature of the nanotube is raised to 750K by scaling the velocities of the carbon atoms. The system is then allowed to relax under constant energy. The exponential decay of temperature is used to calculate the thermal interfacial resistance. The interfacial resistance for water, ethyl alcohol and 1-hexene are found to be 2.13 × 10−8, 4.74 × 10−8, and 7.29 × 10−8 m2K/W, respectively, from the analysis.

Multiscale Simulation of Ballistic Composites for Blast Induced Traumatic Brain Injury Mitigation

The effectiveness of helmets in preventing internal damage due to blast waves requires understanding of not just the strength of the helmet material but also its energy absorption characteristics. To understand and develop ballistic helmets with improved protection, it is necessary to develop computational procedures that will enable the accurate modeling of traumatic head injuries as well as the precise measurement of the mechanical properties of composite materials used in helmets. In this study, a multiscale simulation strategy is used to estimate the mechanical characteristics of advanced composite structures with embedded nanostructures in a ballistic material. In most of the previous theoretical works, an analysis dedicated to improving the design of the helmet using composite nano-structures was not included due to a lack of understanding of the interactions of the nano-structures with the matrix materials. In this work, the role of the helmet on the over pressurization and impulse experienced by the head during blast wave is studied. The properties of the nano-composite materials are estimated using molecular dynamics (MD) simulations and the properties are scaled to the macroscopic level using continuum mechanics formulations. Finally, the analysis is also carried out on an unprotected head to compare the results to those obtained when protected by a helmet containing carbon nanotubes. The developed multiscale model can be used to improve the composition of helmets and the understanding of the traumatic effect of blast shock wave, thereby leading to the mitigation and prevention of traumatic head injuries.© 2014 ASME