Biomedical subjects

Michael R Neidert

Publications and source records attributed to Michael R Neidert.

4 recordsLinked to original sources

PubMed2006

Tissue-engineered valves with commissural alignment.

We present a novel approach to producing bioartificial valves using the tissue-equivalent method of entrapping cells within a biopolymer gel and using a mold design that presents appropriate mechanical constraints to the cell-induced gel compaction to yield both the fibril alignment and the geometry of a native valve. Bileaflet valves were fabricated from bovine collagen and neonatal human dermal fibroblasts as proof of principle. The resultant valves possessed both commissure-tocommissure alignment of collagen fibers in the leaflets and circumferential alignment in the root. While this alignment was manifested in planar biaxial tensile mechanical properties, histology of the leaflets revealed an aligned collagen matrix but lacking other extracellular matrix (ECM) components present in the native valve. The apparent lack of ECM production by the fibroblasts after contracting and aligning the collagen fibrils is consistent with peak loads during biaxial testing being only approximately 10% of native leaflet values and a 0:1 coupling index that was only approximately 50% of native leaflet values despite exhibiting comparable values for the anisotropy index.

Animals↗

PubMed2003

Cryopreservation of collagen-based tissue equivalents. I. Effect of freezing in the absence of cryoprotective agents.

The effect of freezing on the viability and mechanical properties of tissue-equivalents (TEs) was determined under a variety of cooling conditions, with the ultimate aim of optimizing the cryopreservation process. TEs (a class of bioartificial tissues) were prepared by incubating entrapped human foreskin fibroblasts in collagen gels for a period of 2 weeks. TEs were detached from the substrate and frozen in phosphate-buffered saline using a controlled rate freezer (CRF) at various cooling rates (0.5, 2, 5, 20, and 40 degrees C/min to -80 or -160 degrees C) or in a directional solidification stage (DSS) (5 degrees C/min to -80 degrees C) or slam frozen (>1000 degrees C/min). Viability of the fibroblasts in the TEs was assessed by ethidium homodimer and Hoechst assays immediately after thawing. Uniaxial tension experiments were also performed on an MTS (Eden Prairie, MN) Micro Bionix system to assess the postthaw mechanical properties of the frozen-thawed TEs. Cooling rates of either 2 or 5 degrees C/min using the CRF were optimal for preserving both immediate cell viability and mechanical properties of the TEs, postthaw. By 72 h postthaw, TEs frozen in the CRF at 5 degrees C/min to -80 degrees C showed a slight decrease in cell viability, with a significant increase in tangent modulus and ultimate tensile stress suggesting a cell-mediated recovery mechanism. Both the postthaw mechanical properties and cell viability are adversely affected by freezing to the lower end temperature of -160 degrees C. Mechanical properties are adversely affected by freezing in the DSS.

Cell Culture Techniques↗

PubMedDate not supplied

Cryopreservation of collagen-based tissue equivalents. II. Improved freezing in the presence of cryoprotective agents.

In Part I of this study we determined an optimal cooling rate for cryopreservation of collagen-based tissue equivalents (TEs) that preserves both the postthaw cell viability and mechanical properties, but results in tissue contraction and an overall loss of opacity. The empirically determined optimal cooling rate (5 degrees C/min) was obtained in a freezing medium consisting solely of phosphate-buffered saline (PBS) at physiological concentration (1x). In the present study we report the effect of freezing on TEs in the presence of PBS and two cryoprotective agents (CPAs) (glycerol and dimethyl sulfoxide [Me(2)SO]), at two different concentrations (0.5 and 1.0 M), to two different end temperatures (-80 and -160 degrees C), at a cooling rate of 5 degrees C/min. The controlled rate freezing experiments, postthaw cell viability, and mechanical property measurements were performed as described in Part I of this study. In addition to studying the effect of CPAs on the postthaw properties of TEs, we also investigated (1). the effect of freezing TEs attached to the substrate (as opposed to detached and floating in medium) to determine differences when freezing TEs subject to static mechanical stress via a mechanical constraint to contraction; (2). the effect of freezing glutaraldehyde-fixed TEs to determine differences in freezing-mediated damage to the microstructure; and (3). the effect of freezing more mature TEs that were incubated for 4 weeks in growth factor-supplemented medium as opposed to 2 weeks in basal medium. All TEs frozen at 5 degrees C/min to -80 degrees C in the presence of 0.5 M glycerol or Me(2)SO in PBS were found to be optimally cryopreserved in terms of maintaining opacity and structure as well as cell viability and mechanical properties as compared with unfrozen TEs. The postthaw mechanical properties were adversely affected by freezing to the lower end temperature of -160 degrees C in the presence of CPAs, with the samples frozen in the 1.0 M concentration of CPAs exhibiting a total loss of structural integrity on thawing. Furthermore, TEs frozen attached to the substrate showed decreased opacity and significant contraction as compared with TEs frozen detached from the substrate, as did cross-linked samples frozen without CPA.

Collagen↗

PubMedDate not supplied

Fiber alignment imaging during mechanical testing of soft tissues.

A method to image fiber alignment during mechanical testing of soft tissues was developed based on quantitative polarized light microscopy. Images were acquired after passing light through a rotating polarizer, a tissue sample, and an effective circular analyzer at multiple polarizer positions during uniaxial mechanical testing. The image set was analyzed offline using harmonic analysis to generate an alignment image, which contains the direction and strength of alignment at each image pixel. Alignment images of the entire tissue sample were generated every 3-5 s during the mechanical test allowing stress-strain behavior to be correlated with fiber alignment. Loading of fresh tissue-equivalent samples in the direction normal to the initial direction of fiber alignment revealed a spatially inhomogeneous realignment into the loading direction, with most realignment occurring near the free edges undergoing maximum lateral contraction and prior to significant load developing. Glutaraldehyde-fixed samples, in contrast, showed little realignment until yielding occurred.

Animals↗