Tulane's Department of Biomedical Engineering has a long history of performing a wide variety of research problems using traditional engineering expertise to analyze and solve problems in biology and medicine. Our department has particular expertise in the following biomedical engineering domains*:
Biomaterials include both living tissue and artificial materials used for implantation and to foster cell function. Understanding the properties and behavior of living material is vital in the design of implant materials. The selection of an appropriate material to place in the human body is a complex task, with newer biomaterials incorporating living cells in order to provide a true biological and mechanical match for the living tissue. Research in this area is conducted by Dr. Moore, who studies neuro-generation primarily of the optic nerve (see below).
Biomechanics applies classical mechanics (statics, dynamics, fluids, solids, thermodynamics, and continuum mechanics) to biological or medical problems. It includes the study of motion, material deformation and flows. These can influence the macro-scale and micro-scale stresses that can impact biological function at the organ, cell, and sub-cellular level. Research in this area is conducted by Drs. Ahsan, Anderson, Gaver, Khismatullin, Murfee, and Rice.
Biotransport relates to physical and biological processes that govern molecular and convective transport of substances within biological systems. These transport processes may be passive (convection, diffusion) or active (such as with sodium-potassium pumps), wherein energy is expended to move material against a concentration gradient Research in this area is conducted by Drs. Gaver, Khismatullin, Murfee and Shevkoplyas.
Cell-Tissue and Genetic Engineering utilizes the anatomy, biochemistry and mechanics of cellular and sub-cellular structures in order to understand disease processes and to intervene at very specific sites. With these capabilities, bio-mimetic structures can be fabricated and investigated to understand the basics of physiological (dys)function, or devices can be designed and used to deliver chemical, mechanical or electrical stimuli that can influence cellular processes at precise target locations. This can develop knowledge related to physiological processes in development and disease, or can lead to techniques to promote healing or inhibit disease formation and progression. Research in this area is conducted by Drs. Ahsan, Gaver, Murfee, Moore, and Shevkoplyas.
Devices are used in the diagnosis and treatment of disease. Computers are an essential part of bioinstrumentation, from the microprocessor in a single-purpose instrument used to do a variety of small tasks to the multi-core computers needed to process a large amount of information in a medical imaging system. Microfluidics allows manipulation and analysis of minute amounts of biological samples, and thus enables the design and fabrication of low-cost, miniaturized devices for point-of-care clinical diagnostics. Research in this area is conducted by Professors Anderson, Walker, Rice and Shevkoplyas.
*Definitions adapted from BMES (www.bmes.org)
Biomedical Engineering is highly interdisciplinary, so many of the domains play critical role in research studies being performed at Tulane University. The following provides brief descriptions of several research projects in the department.
Engineering Ventilation Waveforms to Reduce Ventilator-Induced Lung Injury
PI: Professor Gaver
The obstruction of pulmonary airways by a viscous fluid occlusion occurs in a variety of diseases including respiratory distress syndrome (RDS), acute respiratory distress syndrome (ARDS) and asthma. Airway closure contributes to mortality from ventilation-perfusion mismatch and reduced gas transport. The overall goal of this project is to engineer low-volume ventilation waveforms that will open occluded pulmonary airways with minimal damage to sensitive epithelial tissue. In RDS and potentially ARDS, the lining fluid surface tension is elevated due to surfactant deficiency, which increases the pressure necessary to open the occluded airways. The proposed studies will test the hypothesis that engineered pulsatile ventilation waveforms will minimize damage to airway epithelial cells by maximizing surfactant transport and biophysical responses to reduce the damaging mechanical stress imparted on airway epithelium. In this project we couple computational simulations to laboratory experiments to elucidate the interactions between mechanical stresses, transport properties, surfactant biophysical responses and cell damage during the migration of a finger of air through a cylindrical tube as the model system.
Funded by NIH R01-HL81266
Synopsis of the key components of the VILI project. a) The pressure-volume relationship for ventilation. This project addresses issues related to the low-volume portion, where airway closure/reopening can occur. b) An occluded airway that is being cleared by a progressing finger of air. The mechanical properties depend upon c) the local fluid flow and surfactant interactions ('physicochemical hydrodynamics') that influence the reopening behavior and determine the d) mechanical stresses on airway epithelial cells. The goal of this project is to engineer ventilation waveforms to optimize surfactant physicochemical interactions in order to reduce mechanical stresses that can damage pulmonary epithelial cells.
Immobilization Of Developmental Ligands For Mimicking The Developing Optic Chiasm.
PI: Michael Moore
Disorders of the central nervous system are often associated with little or no recovery due to in part to poor neural regenerative capacity, damaging mechanisms that persist after initial neuronal injury, and inhibitory properties intrinsic to myelin. Regenerative medicine strategies that utilize synthetic or natural materials as permissive environments for maximizing axonal extension may not be sufficient for restoration of sight because axons that extend from each retina cross at the optic chiasm and selectively project to both hemispheres of the brain. Some protein ligands activate receptors on retinal ganglion cells to induce directional axon growth in order to form the divergent neural projections during embryogenesis. Development ligands such as these may be able to guide regenerating axons when engineered into biomaterials for tissue engineering. However, there is a critical need for physiologically- and translationally-relevant culture models that support the systematic investigation of structural and molecular parameters that may influence tissue growth. The overall goal of this project is to develop a 3D tissue culture model to study the guidance of retinal neurites in response to engineered cues that mimic the spatial distribution of ligands found at the optic chiasm during development. Specifically, we hypothesize that these developmental ligands, immobilized in a spatially-specific manner within a synthetic three-dimensional matrix, will selectively direct neurite outgrowth from embryonic retinal explants in a structural configuration that mimics the optic chiasm. The techniques developed from this work will allow for the systematic manipulation of the spatial arrangement of structural and molecular cues for directing neuronal growth. Thus, it is anticipated that this work will establish a new experimental platform to study neural growth and guidance and also suggest potential treatment strategies to be explored in future studies.
Schematic of techniques for production of dual-hydrogel constructs with regions of exposed functional groups for protein binding. A) Dynamic mask lithography with a UV light source and digital micromirror device (DMD) will be used to photocrosslink a PEG solution (yellow). B) After immersion in PBS, a photolabile peptide gel will be injected into the crosslinked PEG structures. C) The DMD system will be used to deprotect the modified gel to expose free protein conjugation sites. D) Maleimide-conjugated developmental ligands (red) will bind the photolabile gel only in regions deprotected by UV.
Finite Element Modeling Of Corneal Applanation: Effects Of Mesh Density And Contact Algorithms
PI: Ronald Anderson
The goal of this project is to determine how the finite element modeling considerations of mesh refinement and hard or soft contact algorithms influence calculated corneal contact pressure. To complete this study, models of the cornea were created in ABAQUS 6.4 assuming spherical geometry and hyperelastic material behavior. Increasingly refined meshes of hexahedral elements were examined. Loading consisted of a 15 mmHg pressure applied to the internal corneal surface, and a 1.5 gmf force applied to a glass applanator. Contact was frictionless, subject to the surface normal behaviors of "hard" and "soft" contact, which assumed zero overclosure with infinite contact stiffness and 10 μm overclosure with an exponentially increasing stiffness, respectively. Soft contact represented tear film thickness and epithelial cell layer compression. A conventional convergence test indicated that all meshes were acceptable (< 0.5% error) subject to inflation. While contact pressure did show convergence, trends were not monotonic and comparably weak. It can, however, be concluded that a coarse mesh will result in a significant underestimate of contact pressure. It is surmised that this result reflects a decreasingly accurate inclusion of perimeter elements in the contact area calculation. Soft contact interfacial behavior gave improved solutions, likely attributable to "blurring" of overclosed perimeter nodes accepted in the contact solution. Such a soft contact interface is justifiable in principle due to the presence of a tear film and an epithelial layer overlying the mechanically dominant stroma. This study demonstrated finite element simulations of Goldmann-like tonometric measurements to be notably mesh dependent. Thus, careful mesh design is critical for simulating corneal applanation.
Mechanobiology In Stem/Projenitor Cell Systems
PI: Taby Ahsan
Little is known about the response of stem and progenitor cells to applied mechanical forces. There have been, however, extensive studies in mechanobiology using other cell systems. To maximize understanding of the principles that govern stem/progenitor mechanobiology, we will examine two main hypotheses related to conservation and robustness of mechanoresponses. The conservation hypothesis is that as cells specialize towards a phenotype, fewer signaling pathways can be activated by mechanical cues. To test this hypothesis, mechanoresponses will be compared across stem cells, stem cell derivatives, and differentiated cells. The robustness hypothesis is that certain signaling pathways respond to multiple force types (e.g. shear, tension, compression). The key assumption for this hypothesis is that there exists a set of signaling pathways that are triggered by membrane and/or cytoskeletal deformation, regardless of the applied force type. Single factor studies will allow characterization of known signaling pathways.
In addition, to identify and characterize new mechanoresponsive elements and pathways, there is a need to take a more systems biology-like approach using high throughput assessments (e.g. microarrays, PCR arrays, or proteomics) and modeling software. Furthermore, this unified approach will identify and compensate for elements involved in overlapping pathways that influence multiple cellular processes, such as gp130 which is involved in both the JAK/Stat3 self-renewal pathway and a MAPK differentiation pathway. Overall, the use of bioreactors to apply forces and molecular biology to assess cellular processes allows us to better understand the relationship between specific mechanical cues and cell fate decisions, such as cell cycle, proliferation, viability, apoptosis, and differentiation. This basic science research in mechanobiology will help to predict cellular responses to the mechanical microenvironment both in vivo and in vitro.
Leukocyte Rolling on P-selectin: A 3D Numerical Study of the Effects of Cell Viscosity and PSGL-1 Clustering
PIs: Damir Khismatullin and Professor George Truskey (Duke University)
Rolling leukocytes deform to a teardrop shape and show a large area of contact with endothelium under physiological flow conditions. In vitro, cell deformation cannot be viewed directly, except with special side-view flow channels. Here, we study leukocyte rolling using three-dimensional numerical code of receptor-mediated cell adhesion that is based on a new model for leukocyte rheology, the two-phase Giesekus model with cortical tension, which captures several important features of leukocyte deformation. We demonstrate that the leukocyte cytoplasmic viscosity and the clustering of PSGL-1 molecules on leukocyte microvilli play a critical role in the ability of the leukocyte to roll on P-selectin-coated substrate. Deterministic simulations also indicate that the continuous changes of the leukocyte-endothelium contact area lead to seemingly stochastic variations in the instantaneous velocity of the rolling cell. These variations are similar to what was observed in experiments ("stop and go" motion of leukocytes). Nonlinear regression analysis shows that when the viscosity of leukocyte cytoplasm ranges from 10 to 20 Pa s (100 to 200 poise), the model predicts the velocities and shape changes of rolling leukocytes in vitro and in vivo. This range of viscosity is consistent with step aspiration studies of human neutrophils.
| Computed shape of an adherent leukocyte (side view) for different cytoplasmic viscosities. The channel height is 15 μm. The wall shear stress is 2.5 dyn/cm2. The monocyte has 3112 cylindrical microvilli of initial length 0.5 μm (not shown), distributed uniformly over the membrane. The surface densities of receptor and ligand molecules (P-selectin and PSGL-1) are 320 molec/μm2. The forward and reverse reaction rates are 1.7 μm2/(s·molec) and 1.0 s-1. The equilibrium length of a receptor-ligand complex is 40 nm. The nucleus-to-cytoplasm viscosity ratio is fixed at 2.5. The cytoplasmic and nuclear relaxation times are 0.176 s and 0.2 s, respectively. |
Perivascular Cell Lineage During Microvascular Remodeling
PI: Walter Lee Murfee
Vascular remodeling is a complex continuum that incorporates the formation of new capillaries from pre-existing vessels, termed angiogenesis, capillary acquisition of a perivascular cell coating, referred to as arteriogenesis, and the acquisition of an arterial/venous (A/V) phenotype. Perivascular cells, including both smooth muscle cells (SMCs) and pericytes, are involved in each of these processes. For example, manipulating proper pericyte investment by altering recruitment to the endothelium through PDGFB-PDGFR- signaling can cause lethal microvascular dysfunction, affect vascular patterning during development in some tissues, and affect the formation of new vessels during physiological and pathological angiogenesis. Thus, to fully understand how microvascular networks grow and respond to altered environmental conditions, we must understand the (Figure 1) how perivascular cell types differ along the hierarchy of microvascular networks and from which cell populations do perivascular cells originate (Figure 2). The objectives of this work are to 1) to evaluate the spatial and temporal dynamics of perivascular phenotype states during remodeling of microvascular networks, and 2) to examine the capability of progenitor cells to contribute to microvascular remodeling.
Figure 1: Comparison of Desmin (green) and SM alpha-actin (red) expression along a capillary in the adult rat circulation highlight the need to better understand perivascular cell function.
Figure 2: Image identifying recruitment and engraftment of interstitial cells during remodeling of adult rat microvascular networks. Positive NG2 (a marker for perivascular cells) labeling identifies both cells along vessels and a subset of potential bone marrow derived cells that have altered their phenotype. The mechanisms regulating these cell recruitment and differentiation currently remain unknown.