Narendra K. Simha

Photo-cross-linked hybrid polymer networks consisting of poly(propylene fumarate) and poly(caprolactone fumarate): controlled physical properties and regulated bone and nerve cell responses.

Aiming to achieve suitable polymeric biomaterials with controlled physical properties for hard and soft tissue replacements, we have developed a series of blends consisting of two photo-cross-linkable polymers: polypropylene fumarate (PPF) and polycaprolactone fumarate (PCLF). Physical properties of both un-cross-linked and UV cross-linked PPF/PCLF blends with PPF composition ranging from 0% to 100% have been investigated extensively. It has been found that the physical properties such as thermal, rheological, and mechanical properties could be modulated efficiently by varying the PPF composition in the blends. Thermal properties including glass transition temperature (T g) and melting temperature (T m) have been correlated with their rheological and mechanical properties. Surface characteristics such as surface morphology, hydrophilicity, and the capability of adsorbing serum protein from culture medium have also been examined for the cross-linked polymer and blend disks. For potential applications in bone and nerve tissue engineering, in vitro cell studies including cytotoxicity, cell adhesion, and proliferation on cross-linked disks with controlled physical properties have been performed using rat bone marrow stromal cells and SPL201 cells, respectively. In addition, the role of mechanical properties such as surface stiffness in modulating cell responses has been emphasized using this model blend system.

Effect of Indenter Size on Elastic Modulus of Cartilage Measured by Indentation

Our preliminary indentation experiments showed that the equilibrium elastic modulus of murine tibial cartilage increased with decreasing indenter size: flat-ended 60 deg conical tips with end diameters of 15 microm and 90 microm gave 1.50+/-0.82 MPa (mean+/-standard deviation) and 0.55+/-0.11 MPa, respectively (p<0.01). The goal of this paper is to determine if the dependence on tip size is an inherent feature of the equilibrium elastic modulus of cartilage as measured by indentation. Since modulus values from nonindentation tests are not available for comparison for murine cartilage, bovine cartilage was used. Flat-ended conical or cylindrical tips with end diameters ranging from 5 microm to 4 mm were used to measure the equilibrium elastic modulus of bovine patellar cartilage. The same tips were used to test urethane rubber for comparison. The equilibrium modulus of the bovine patellar cartilage increased monotonically with decreasing tip size. The modulus obtained from the 2 mm and 4 mm tips (0.63+/-0.21 MPa) agreed with values reported in the literature; however, the modulus measured by the 90 microm tip was over two and a half times larger than the value obtained from the 1000 microm tip. In contrast, the elastic modulus of urethane rubber obtained using the same 5 microm-4 mm tips was independent of tip size. The equilibrium elastic modulus of bovine patellar cartilage measured by indentation depends on tip size. This appears to be an inherent feature of indentation of cartilage, perhaps due to its inhomogeneous structure.

Poroviscoelastic cartilage properties in the mouse from indentation

A method for fitting parameters in a poroviscoelastic (PVE) model of articular cartilage in the mouse is presented. Indentation is performed using two different sized indenters and then these data are fitted using a PVE finite element program and parameter extraction algorithm. Data from a smaller indenter, a 15 mum diameter flat-ended 60 deg cone, is first used to fit the viscoelastic (VE) parameters, on the basis that for this tip size the gel diffusion time (approximate time constant of the poroelastic (PE) response) is of the order of 0.1 s, so that the PE response is negligible. These parameters are then used to fit the data from a second 170 mum diameter flat-ended 60 deg cone for the PE parameters, using the VE parameters extracted from the data from the 15 mum tip. Data from tests on five different mouse tibial plateaus are presented and fitted. Parameter variation studies for the larger indenter show that for this case the VE and PE time responses overlap in time, necessitating the use of both models.

Effect of Dermatan Sulfate on the Indentation and Tensile Properties of Articular Cartilage

OBJECTIVE This paper examines the hypothesis that the dermatan sulfate (DS) chain on decorin is a load carrying element in cartilage and that its damage or removal will alter the material properties. METHODS To test this hypothesis, indentation and tensile testing of cartilage from bovine patella were performed before and after digestion with chondroitinase B (cB). Removal of significant amounts of DS by cB digestion was verified by Western blot analysis of proteoglycans extracted from whole and sectioned specimens. Specimens (control and treated) were subjected to a series of step-hold displacements. Elastic modulus during the step rise (rapid modulus) and at equilibrium (equilibrium modulus), and the relaxation function during each step was measured for test (cB and buffer) and control (buffer alone) conditions. RESULTS cB had no effect on any of the viscoelastic mechanical properties measured, either in indentation or tension. CONCLUSION Removing or damaging approximately 50% of the DS had no effect on the mechanical properties, strongly suggesting that DS either carries very low load or no load.

Inverse finite element analysis of indentation tests to determine hyperelastic parameters of soft-tissue layers

Indentation is being used increasingly to probe the mechanical behaviour of cells, the extra-cellular matrix, and organs. Multiple hyperelastic parameters have been extracted from indentation data, but their accuracy has not been evaluated. Consequently, this paper examines whether two non-linear hyperelastic parameters or even a single generalized elastic modulus E can be extracted accurately from atomic force microscopy and nanoindentation force–depth data. Tissue was modelled as incompressible isotropic Mooney–Rivlin (MR), polynomial (POLY), or exponential (EXP) layers. For each model, n = 9 sets of the two hyperelastic parameters (all with R2 > 0.99) were extracted from an artificial ‘benchmark’ by running our inverse finite-element-based extraction algorithm multiple times. Error between the mean extracted E and the known ‘benchmark’ value was 5.5 per cent for an MR layer, 3.6 per cent for a POLY layer, and 20.6 per cent for an EXP layer. Errors in at least one of the two hyperelastic parameters reached 75 per cent for the MR layer, 33 per cent for the POLY layer, and 400 per cent for the EXP layer, indicating that indentation has limited potential to extract two or more hyperelastic parameters accurately. However, the generalized elastic modulus extracted from indentation could be used as a quantitative measure to evaluate the influence of altered processing conditions, pathology, etc.

Mechanical Properties of the Iris Dilator and Stroma Using Nanoindentation

In certain disorders of the eye such as angle-closure glaucoma [1], pigment dispersion syndrome [2], and intra-operative floppy iris syndrome [3] the contour of the iris plays an important role. The active iris contour is determined by a combination of external stresses arising from the flow of the aqueous humor and internal stresses due to the passive and active components of the constituent tissues. For example, in angle closure, the iris bows anteriorly, and the abnormal shape and position of the iris are directly related to the blockage of aqueous humor outflow, increasing the intraocular pressure. While the interaction between the aqueous humor and iris has been studied [4], little is known about the effect of the components of the iris on the contour. The iris is composed of stroma, pigment epithelial cells, and two constituent muscles, the sphincter iridis and dilator pupillae (Fig1).Copyright

Damage of Articular Cartilage due to Mechanical Fatigue and Collagenase Action

Damage to the extracellular matrix (ECM) of articular cartilage is detrimental to its functional load-bearing properties. Cartilage ECM can be damaged chemically by enzymatic cleavage or mechanically either due to impact loads or long term fatigue loads at physiological levels. There is strong evidence that chronic long term loads in conjunction with chemical weakening can lead to osteoarthritic degeneration of cartilage [1, 2].Copyright

Poroviscoelastic properties of mouse cartilage from inverse finite elements and indentation

Transgenic mice offer a novel way to probe structure function relationships in healthy and osteoarthritic cartilage. Indentation is a convenient method to measure mechanical properties of cartilage in the mouse. In order to reduce test data to material properties, test model geometry along with a material model needs to be assumed. Most recent developments support the use of a poroviscoelastic (PVE) model for cartilage. However, using this model makes separation of the flow-dependent and flow-independent viscoelastic parameters challenging. For cartilage from larger animals, Huang [1] showed that tensile tests have negligible flow-dependent response and hence can identify the flow-independent material parameters. A compression experiment can then be used to find only the flow-dependent parameters. However, limited cartilage volumes in mouse do not allow for tension tests, so mouse cartilage is primarily tested by indentation. Mak [2] has shown that fluid flow occurs mainly for times comparable to the gel diffusion time T = a2/Hκ where a is the tip size, H is the aggregate modulus and κ is permeability. Consequently, we propose use of two different sized indenters to separate flow-independent and dependent effects in mouse cartilage. One tip is small enough to make T negligible (say <0.1 s), then relaxation data will probe only the flow-independent response, whereas a second considerably larger tip will probe both flow-dependent and fluid flow effects. The data from the small indenter can be used to fit the flow-independent parameters; the data from the large indenter, in conjunction with parameters from the first fit, can be used to fit the flow-dependent parameters.Copyright