Strain Gradient Modification of the Mori-Tanaka Model to Predict the Elastic Properties of the Layer by Layer (LBL)Manufactured Polymer/Clay Nanocomposites

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/83580/1/AIAA-2010-2813-592.pd

Effect of implantation on engineered skeletal muscle constructs

The development of engineered skeletal muscle would provide a viable tissue for replacement and repair of muscle damaged by disease or injury. Our current tissue‐engineering methods result in three‐dimensional (3D) muscle constructs that generate tension but do not advance phenotypically beyond neonatal characteristics. To develop to an adult phenotype, innervation and vascularization of the construct must occur. In this study, 3D muscle constructs were implanted into the hindlimb of a rat, along the sciatic nerve, with the sural nerve isolated, transected and sutured to the construct to encourage innervation. Aortic ring anchors were sutured to the tendons of the biceps femoris muscle so that the construct would move dynamically with the endogenous muscle. After 1 week in vivo , the constructs were explanted, evaluated for force production and stained for muscle, nerve and collagen markers. Implanted muscle constructs showed a developing capillary system, an epimysium‐like outer layer of connective tissue and an increase in myofibre content. The beginning of α ‐bungarotoxin clustering suggests that neuromuscular junctions (NMJs) could form on the implanted muscle, given more time in vivo . Additionally, the constructs increased maximum isometric force from 192 ± 41 μN to 549 ± 103 μN (245% increase) compared to in vitro controls, which increased from 276 ± 23 μN to 329 ± 27μN (25% increase). These findings suggest that engineered muscle tissue survives 1 week of implantation and begins to develop the necessary interfaces needed to advance the phenotype toward adult muscle. However, in terms of force production, the muscle constructs need longer implantation times to fully develop an adult phenotype. Copyright © 2012 John Wiley & Sons, Ltd.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98423/1/term537.pd

Fresh and Frozen Tissue-Engineered Three-Dimensional Bone–Ligament–Bone Constructs for Sheep Anterior Cruciate Ligament Repair Following a 2-Year Implantation

Injuries to the anterior cruciate ligament (ACL) often require surgical reconstruction utilizing tendon grafts to restore knee function and stability. Some current graft options for ACL repair are associated with poor long-term outcomes. Our laboratory has fabricated tissue-engineered bone?ligament?bone (BLB) constructs that demonstrate native ligament regeneration and advancement toward native ACL mechanical properties in a sheep ACL reconstruction model. Prior work has shown that freezing BLBs as a method of preservation resulted in similar outcomes compared with fresh BLBs after 6-month implantation. The purpose of this study was to evaluate the long-term efficacy of fresh and frozen BLBs. We hypothesized that both fresh and frozen BLBs would show continued regeneration of structural and functional properties toward those of native ACL after a 2-year implantation. Following removal of the native ACL, fresh (n?=?2) and frozen (n?=?2) BLBs were implanted arthroscopically. After 2 years of recovery, sheep were euthanized and both the experimental and contralateral hindlimbs were removed and radiographs were performed. Explanted knees were initially evaluated for joint laxity and were then further dissected for uniaxial tensile testing of the isolated ACL or BLB. Following mechanical testing, explanted contralateral ACL (C-ACL) and BLBs were harvested for histology. Two years post-ACL reconstruction, fresh and frozen BLBs exhibited similar morphological and biomechanical properties as well as more advanced regeneration compared with our 6-month recovery study. These data indicate that an additional 1.5-year regeneration period allows the BLB to continue ligament regeneration in vivo. In addition, freezing the BLBs is a viable option for the preservation of the graft after fabrication.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140316/1/biores.2016.0032.pd

The Non-Equilibrium Thermodynamics and Kinetics of Focal Adhesion Dynamics

BACKGROUND: We consider a focal adhesion to be made up of molecular complexes, each consisting of a ligand, an integrin molecule, and associated plaque proteins. Free energy changes drive the binding and unbinding of these complexes and thereby controls the focal adhesion's dynamic modes of growth, treadmilling and resorption. PRINCIPAL FINDINGS: We have identified a competition among four thermodynamic driving forces for focal adhesion dynamics: (i) the work done during the addition of a single molecular complex of a certain size, (ii) the chemical free energy change associated with the addition of a molecular complex, (iii) the elastic free energy change associated with deformation of focal adhesions and the cell membrane, and (iv) the work done on a molecular conformational change. We have developed a theoretical treatment of focal adhesion dynamics as a nonlinear rate process governed by a classical kinetic model. We also express the rates as being driven by out-of-equilibrium thermodynamic driving forces, and modulated by kinetics. The mechanisms governed by the above four effects allow focal adhesions to exhibit a rich variety of behavior without the need to introduce special constitutive assumptions for their response. For the reaction-limited case growth, treadmilling and resorption are all predicted by a very simple chemo-mechanical model. Treadmilling requires symmetry breaking between the ends of the focal adhesion, and is achieved by driving force (i) above. In contrast, depending on its numerical value (ii) causes symmetric growth, resorption or is neutral, (iii) causes symmetric resorption, and (iv) causes symmetric growth. These findings hold for a range of conditions: temporally-constant force or stress, and for spatially-uniform and non-uniform stress distribution over the FA. The symmetric growth mode dominates for temporally-constant stress, with a reduced treadmilling regime. SIGNIFICANCE: In addition to explaining focal adhesion dynamics, this treatment can be coupled with models of cytoskeleton dynamics and contribute to the understanding of cell motility

Three-Dimensional Engineered Bone from Bone Marrow Stromal Cells and Their Autogenous Extracellular Matrix

Most bone tissue engineering research uses porous three-dimensional (3D) scaffolds for cell seeding. In this work, scaffold-less 3D bone-like tissues were engineered from rat bone marrow stromal cells (BMSCs) and their autogenous extracellular matrix (ECM). The BMSCs were cultured on a 2D substrate in medium that induced osteogenic differentiation. After reaching confluence and producing a sufficient amount of their own ECM, the cells contracted their tissue monolayer around two constraint points, forming scaffold-less cylindrical engineered bone-like constructs (EBCs). The EBCs exhibited alizarin red staining for mineralization and alkaline phosphatase activity and contained type I collagen. The EBCs developed a periosteum characterized by fibroblasts and unmineralized collagen on the periphery of the construct. Tensile tests revealed that the EBCs in culture had a tangent modulus of 7.5+/-0.5MPa at 7 days post-3D construct formation and 29+/-9MPa at 6 weeks after construct formation. Implantation of the EBCs into rats 7 days after construct formation resulted in further bone development and vascularization. Tissue explants collected at 4 weeks contained all three cell types found in native bone: osteoblasts, osteocytes, and osteoclasts. The resulting engineered tissues are the first 3D bone tissues developed without the use of exogenous scaffolding.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/78137/1/ten.tea.2007.0140.pd

Remodeling of biological tissue: Mechanically induced reorientation of a transversely isotropic chain network

A new class of micromechanically motivated chain network models for soft biological tissues is presented. On the microlevel, it is based on the statistics of long chain molecules. A wormlike chain model is applied to capture the behavior of the collagen microfibrils. On the macrolevel, the network of collagen chains is represented by a transversely isotropic eight chain unit cell introducing one characteristic material axis. Biomechanically induced remodeling is captured by allowing for a continuous reorientation of the predominant unit cell axis driven by a biomechanical stimulus. To this end, we adopt the gradual alignment of the unit cell axis with the direction of maximum principal strain. The evolution of the unit cell axis' orientation is governed by a first-order rate equation. For the temporal discretization of the remodeling rate equation, we suggest an exponential update scheme of Euler-Rodrigues type. For the spatial discretization, a finite element strategy is applied which introduces the current individual cell orientation as an internal variable on the integration point level. Selected model problems are analyzed to illustrate the basic features of the new model. Finally, the presented approach is applied to the biomechanically relevant boundary value problem of an in vitro engineered functional tendon construct.Comment: LaTeX2e, 19 pages, 9 figure

Effects of initial anisotropy on the finite strain deformation behavior of glassy polymers

Solid phase deformation processing of glassy polymers produces highly anisotropic polymer components as a result of the massive reorientation of molecular chains during the large strain forming operation. Indeed, the polymer preform used as the starting materials is usually anisotropic owing to its prior deformation history. The process end product has often been fashioned for a particular application, i.e. to possess an increased flow strength along a particular axis, thereby exploiting the orientation induced anisotropy effects. The fully three-dimensional issues involved in the use of glassy polymer components include anisotropic flow strenghts, limiting extensibilities, and deformation patterns. These characteristics have been altered by the initial forming operation but are obviously not expected to be enhanced in all directions. The presence of anisotropy in structural components may also lead to premature failure or unexpected shear localization. In this report the effects of initial deformation and the associated anisotropies are investigated through uniaxial compression tests on preoriented polycarbonate (PC) and polymethylmethacrylate (PMMA) specimens. The evolving anisotropy is monitored by testing materials preoriented by various amounts of strain and under different states of deformation. The tensorial nature of the anisotropic material is characterized by examining the preoriented material response in three orthogonal directions. A model for the large strain deformation response of glassy polymers has been shown by and [in press] to be well predictive of the evolution of anisotropy during deformation in initially isotropic materials. Here the authors evaluate the ability of the model developed in and [in press] to predict several aspects of the anisotropic response of preoriented materials. Using material properties determined from the characterization of the isotropic material response and a knowledge of the anisotropic state of the preoriented material, model simulations are shown to accurately capture all aspects of the large strain anisotropic response including flow strengths, strain hardening characteristics, cross-sectional deformation patterns, and limiting extensibilities. Although anisotropy has been shown to evolve with temperature and strain rate in , and [in press] and also state of deformation in and [in press], we submit an experimental observation that the subsequent deformation response of preoriented polymers may be predicted using only a measure of optical anisotropy, and not the prior strain or thermal history. Optical anisotropy, as measured for example by birefringence, therefore represents a true internal variable indicative of the evolution of anisotropy with inelastic strain, state of strain, and temperature.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31096/1/0000774.pd

Ogden Material Calibration via Magnetic Resonance Cartography, Parameter Sensitivity, and Variational System Identification

Contemporary techniques in the mechanical calibration of materials that leverage full three-dimensional deformation fields and the weak form of the equilibrium equations face challenges in the numerical solving procedure of the inverse characterization problem. As material models and descriptions differ, so too must the approaches for identifying their system mechanics. The widely-used Ogden material model, comprised of an unknown number of terms of the same mathematical form, presents challenges in interpretability, stability, and parsimony. In turn, we intend to use our estimates to assess and improve our experimental design. Using fully 3D displacement fields acquired in silicone elastomers using our recently-developed magnetic resonance cartography (MR-u) technique on the order of $>20,000$ points per sample, we leverage PDE-constrained optimization as the basis of variational system identification of our material parameters. We incorporate the statistical F-test to maintain parsimony of representation. Using a new decomposition of the deformation field locally into mixtures of biaxial and uniaxial tensile states, we evaluate experiments based on an analytical sensitivity metric, and discuss the implications for future experimental design.Comment: 16 pages, 7 figure

Structure and Functional Evaluation of Tendon–Skeletal Muscle Constructs Engineered in Vitro

During muscle contraction, the integrity of the myotendinous junction (MTJ) is important for the transmission of force from muscle to tendon. We evaluated the contractile and structural characteristics of 3-dimensional (3-D) skeletal muscle constructs co-cultured with engineered self-organized tendon constructs (n = 4), or segments of adult (n = 4) or fetal (n = 5) rat-tail tendon. We hypothesized that the co-culture of tendon and muscle would produce constructs with viable muscle–tendon interfaces that remain intact during generation of force. Construct diameter (lm) and maximum isometric force (µN) were measured, and specific force (kPa) was determined. After measure of force, constructs were loaded at a constant strain rate until failure and surface strains were recorded optically across the tendon, the muscle and the interface and used to determine the tangent modulus (passive stiffness) of the construct. Frozen samples were used for Trichrome Masson staining and immunofluorescent analysis of the MTJ-specific protein paxillin. No differences were observed between the groups with respect to diameter, maximum force, or specific force. The MTJ was robust and withstood tensile loading beyond the physiological strain range. The majority of the constructs failed in the muscle region. At the MTJ, there is an increase in the expression and localization of paxillin. In conclusion, using 3 sources of tendon tissue, we successfully engineered 3-D muscle–tendon constructs with functionally viable MTJ, characterized by structural features and protein expression patterns resembling neonatal MTJs in vivo.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/63387/1/ten.2006.12.3149.pd