Finite Element Modeling of Osmotic Swelling in Tissue-Engineered Cartilage

Proteoglycans and Type II collagen represent the two major biochemical constituents of articular cartilage. Collagen fibrils in cartilage resist the swelling pressure that arises from the fixed charges of the glycosaminoglycans (GAGs), and together they give rise to the tissue’s unique load bearing properties. As articular cartilage exhibits a poor intrinsic healing capacity, there is significant research in the development of cell-based therapies for cartilage repair. In some of our tissue engineering studies, we have observed a phenomenon where chondrocyte-seeded hydrogel constructs display cracking in their central regions after significant GAG content has been elaborated in culture. A theoretical analysis was performed to gain greater insights into the potential role that the spatial distribution of proteoglycan and collagen may play in this observed response.

Viscoelastic Properties of Genetically Engineered Silk-Elastin-Like Protein Polymers

Genetic engineering of protein-based materials provides material scientists with high levels of control in material microstructures, properties, and functions [1]. For example, multi-block protein copolymers in which individual block may possess distinct mechanical or biological properties have been biosynthesized [2, 3]. Polypeptide sequences derived from well-studied structural proteins (e.g., collagen, silk, elastin) are often used as motifs in the design and synthesis of new protein-based material, in which new functional groups may be incorporated. In this fashion, we have produced a series of silk-elastin-like proteins (SELPs) consisting of polypeptide sequences derived from silk of superior mechanical strength and elastin that is extremely durable and resilient [2, 4]. Notably, the silk-like blocks are capable of crystallizing to form virtual cross-links between elastin-mimetic sequences, which, in turn, lower the crystallinity of the silk-like blocks and thus enhance the solubility of SELPs. Consequently, SELPs may be fabricated into useful structures for biomedical applications, including drug delivery. In this study, we will characterize viscoelastic properties of SELPs, which are particularly relevant to tissue engineering applications.

Effect of Gait on Patellofemoral Mechanics

Computational modeling of the reconstructed knee is an important tool in designing components for maximum functionality and life. Utilization of boundary conditions consistent with in vivo gait loading in such models enables predictions of knee kinematics and polyethylene damage [1–4], which can then be used to optimize component design. Several recent clinical studies have focused on complications associated with the patellofemoral joint [5–6], highlighting the need to better understand the mechanics of this compartment of total knee arthroplasty (TKA). This study utilizes a computational model to characterize the impact of gait loading on the mechanics of the patella in TKA.

The Role of Knee Extensor Strength in Balance-Restoring Step Initiation and Execution

A stepping response is often used to restore balance following a fall. Using laboratory-induced balance perturbations, various researchers have reported age-related alterations in balance recovery step characteristics including earlier step liftoff time [1; 2], shorter step length [1; 3], and longer step duration [2]. Such age-related changes in the step response may be related to older adults’ reduced strength reserve, which is prominent in the lower extremities [4] and therefore likely plays an important role in balance recovery.

In Vivo Tibiofemoral Contact Kinematics and Contact Forces During Dynamic Weight-Bearing Activities Following Total Knee Arthroplasty

Accurate knowledge of in vivo articular contact kinematics and contact forces is required to quantitatively understand factors limiting life of total knee arthroplasty (TKA) implants, such as polyethylene component wear and implant loosening [1]. Determination of in vivo tibiofemoral contact forces has been a challenging issue in biomechanics. Historically, instrumented tibial implants have been used to measure tibiofemoral forces in vitro [2] and computational models involving inverse dynamic optimization have been used to estimate joint forces in vivo [3]. Recently, D’Lima et al. reported the first in vivo measurement of 6DOF tibiofemoral forces via an instrumented implant in a TKA patient [4]. However this technique does not provide a direct estimation of tibiofemoral contact forces in the medial and lateral compartments. Recently, a dual fluoroscopic imaging system has been used to accurately determine tibiofemoral contact locations on the medial and lateral tibial polyethylene surfaces [5]. The objective of this study was to combine the dual fluoroscope technique and the instrumented TKAs to determine the dynamic 3D articular contact kinematics and contact forces on the medial and lateral tibial polyethylene surfaces during functional activities.

Can the Hip Joint be Modeled Accurately Using Simplified Geometry?

Simplified analytical approaches to estimate hip joint contact pressures using perfectly spherical geometry have been described in the literature (rigid body spring models); however, estimations based on these simulations have not corresponded well with experimental in vitro data. Recent evidence from our laboratory suggests that finite element (FE) models of the hip joint that incorporate detailed geometry for cartilage and bone can predict cartilage pressures in good agreement with experimental data [1]. However, it is unknown whether this degree of model complexity is necessary. The objective of this study was to compare cartilage contact pressure predictions from FE models with varying degrees of simplicity to elucidate which aspects of hip morphology are required to obtain accurate predictions of cartilage contact pressure. Models based on 1) subject-specific (SS) geometry, 2) spheres, and 3) rotational conchoids were analyzed.

CFD Study of Thrombus Formation on Shear Blood Flows by Using Modified Lattice Boltzmann Method and Thrombus Observation

This paper describes development of the prediction method of thrombus formation by Computational Fluid Dynamics (CFD) on pipe orifice shear flows. These shear flows are typical models of the flow in the rotary blood pump. In this investigation, the thrombus formation in blood plasma flow is visualized, and modified lattice Boltzmann method are used to predict the backward forwarding step flow, that is simple model of the orifice flow.

Application of Carbon Nanotubes in Cartilage Tissue Engineering

Carbon nanotubes (CNTs) are cylindrical allotropes of carbon that are nanometers in diameter and posses unique physical properties, positioning them as ideal materials for studying physiology at a single cell level. CNTs have the potential to become a very important component of medical therapeutics, likely acting as (a) drug delivery system [1], (b) existing as an interfacial layer in surgical implants [2,3], or (c) acting as scaffolding in tissue engineering [4,8]. While some studies have explored the use of CNTs as a novel material in regenerative medicine, they have not yet been fully evaluated in cellular systems. One major limitation of CNTs that must be overcome is their inherent cytotoxicity. The goal of this study is to assess the long-term biocompatibility of CNTs for chondrocyte growth. We hypothesize that CNT-based material in tissue engineering can provide an improved molecular sized substrate for stimulation of cellular growth, and structural reinforcement of the scaffold mechanical properties. Here we present data on the effects of CNTs on chondrocyte viability and biochemical deposition examined in composite materials of hydrogels + CNTs mixtures. Also, the effects of CNTs surface functionalization with polyethlyne glycol (PEG) or carboxyl groups (COOH) were examined.

Assessment of Possible Thermal Damage of Tissue due to Atherectomy by Means of a Mechanical Debulking Device

In vitro and cadaver experiments, coupled with numerical simulations, were performed to assess the possibility that orbital atherectomy might cause thermal damage of tissue. The experiments involved debulking operations on a surrogate artery and on the plaque-lined posterior tibial artery of a cadaver. Temperatures and coolant flow rates measured during these experiments enabled a numerical simulation of the debulking of a plaque-lined artery in a living human. The temperature variations from the numerical simulations were used to evaluate a thermal injury index. The resulting values of the index were found to be several orders of magnitude below the threshold value for thermal injury. It is concluded that it is extremely unlikely that the use of an orbital debulking device, the Diamondback 360°™ (Cardiovascular Systems, Inc.), can lead to thermal injury of the artery wall.

Full-Field Viscoelastic Inflation Response of Bovine Cornea

Most previous experimental studies and mechanical cornea models have ignored time-dependence of the cornea’s modulus, with only a few notable exceptions [1–3]. The purpose of the present work was to evaluate the time-dependent properties of cornea tissue independent of scleral contributions in a condition that is as physiologically-relevant as possible without resorting to costly and difficult in vivo characterization. A non-contact 3-dimensional displacement mapping tool was employed to image the entire deformation field across the entire cornea in real-time during pressurization. Unlike prior inflation-based studies, the present study’s unique approach permits dynamic real-time full-field mapping of deformation during inflation for the examination of viscoelasticity, isotropy, and homogeneity.