Durgadas Bolikal

The support of bone marrow stromal cell differentiation by airbrushed nanofiber scaffolds

Nanofiber scaffolds are effective for tissue engineering since they emulate the fibrous nanostructure of native extracellular matrix (ECM). Although electrospinning has been the most common approach for fabricating nanofiber scaffolds, airbrushing approaches have also been advanced for making nanofibers. For airbrushing, compressed gas is used to blow polymer solution through a small nozzle which shears the polymer solution into fibers. Our goals were 1) to assess the versatility of airbrushing, 2) to compare the properties of airbrushed and electrospun nanofiber scaffolds and 3) to test the ability of airbrushed nanofibers to support stem cell differentiation. The results demonstrated that airbrushing could produce nanofibers from a wide range of polymers and onto a wide range of targets. Airbrushing was safer, 10-fold faster, 100-fold less expensive to set-up and able to deposit nanofibers onto a broader range of targets than electrospinning. Airbrushing yielded nanofibers that formed loosely packed bundles of aligned nanofibers, while electrospinning produced un-aligned, single nanofibers that were tightly packed and highly entangled. Airbrushed nanofiber mats had larger pores, higher porosity and lower modulus than electrospun mats, results that were likely caused by the differences in morphology (nanofiber packing and entanglement). Airbrushed nanofiber scaffolds fabricated from 4 different polymers were each able to support osteogenic differentiation of primary human bone marrow stromal cells (hBMSCs). Finally, the differences in airbrushed versus electrospun nanofiber morphology caused differences in hBMSC shape where cells had a smaller spread area and a smaller volume on airbrushed nanofiber scaffolds. These results highlight the advantages and disadvantages of airbrushing versus electrospinning nanofiber scaffolds and demonstrate that airbrushed nanofiber scaffolds can support stem cell differentiation.

Synthesis, degradation and biocompatibility of tyrosine-derived polycarbonate scaffolds

Polycarbonate terpolymers consisting of desaminotyrosyl-tyrosine alkyl esters (DTR), desaminotyrosyl-tyrosine (DT), and low molecular weight blocks of poly(ethylene glycol) (PEG) are a new class of polymers that have good engineering properties while also being resorbable in vivo. This study is the first evaluation of their (i) degradation behavior, (ii) in vitro cytotoxicity, and (iii) in vivo biocompatibility. Porous, tissue engineering scaffolds were prepared by a combination of solvent casting, porogen leaching and phase separation techniques. The scaffolds (>90% porosity) displayed (i) a bimodal pore distribution with micropores of less than 20 µm and macropores between 200 and 400 µm, (ii) a highly interconnected and open pore architecture, and (iii) a highly organized microstructure where the micropores are oriented and aligned along the walls of the macropores. Molecular weight (number average, Mn) and mass loss were determined in vitro (PBS at 37 °C) for up to 28 days. All three terpolymer compositions were fast degrading and retained only 10% of their initial molecular weight after 21 days, while mass loss during the 28 days was polymer composition-dependent. In vitro biocompatibility of the polymer scaffolds was determined up to 14 days by measuring metabolic activity of MC3T3.E1 (subclone 4) pre-osteoblasts. The outcome showed no statistical difference between cells cultured in monolayer and all tested polymer scaffolds. Robust cell attachment throughout the scaffold volume was observed by confocal microscopy and SEM. The biocompatibility of resorbing scaffolds was evaluated at 12 week in a critical sized defect (CSD) rabbit calvaria model and showed only a minimal inflammatory response. Overall, the results reported here illustrate the potential utility of tyrosine-derived polycarbonate terpolymers in the design of tissue engineering scaffolds.

The fate of ultrafast degrading polymeric implants in the brain

We have recently reported on an ultrafast degrading tyrosine-derived terpolymer that degrades and resorbs within hours, and is suitable for use in cortical neural prosthetic applications. Here we further characterize this polymer, and describe a new tyrosine-derived fast degrading terpolymer in which the poly(ethylene glycol) (PEG) is replaced by poly(trimethylene carbonate) (PTMC). This PTMC containing terpolymer showed similar degradation characteristics but its resorption was negligible in the same period. Thus, changes in the polymer chemistry allowed for the development of two ultrafast degrading polymers with distinct difference in resorption properties. The in vivo tissue response to both polymers used as intraparenchymal cortical devices was compared to poly(lactic-co-glycolic acid) (PLGA). Slow resorbing, indwelling implant resulted in continuous glial activation and loss of neural tissue. In contrast, the fast degrading tyrosine-derived terpolymer that is also fast resorbing, significantly reduced both the glial response in the implantation site and the neuronal exclusion zone. Such polymers allow for brain tissue recovery, thus render them suitable for neural interfacing applications.

Gas-Foamed Scaffold Gradients for Combinatorial Screening in 3D.

Current methods for screening cell-material interactions typically utilize a two-dimensional (2D) culture format where cells are cultured on flat surfaces. However, there is a need for combinatorial and high-throughput screening methods to systematically screen cell-biomaterial interactions in three-dimensional (3D) tissue scaffolds for tissue engineering. Previously, we developed a two-syringe pump approach for making 3D scaffold gradients for use in combinatorial screening of salt-leached scaffolds. Herein, we demonstrate that the two-syringe pump approach can also be used to create scaffold gradients using a gas-foaming approach. Macroporous foams prepared by a gas-foaming technique are commonly used for fabrication of tissue engineering scaffolds due to their high interconnectivity and good mechanical properties. Gas-foamed scaffold gradient libraries were fabricated from two biodegradable tyrosine-derived polycarbonates: poly(desaminotyrosyl-tyrosine ethyl ester carbonate) (pDTEc) and poly(desaminotyrosyl-tyrosine octyl ester carbonate) (pDTOc). The composition of the libraries was assessed with Fourier transform infrared spectroscopy (FTIR) and showed that pDTEc/pDTOc gas-foamed scaffold gradients could be repeatably fabricated. Scanning electron microscopy showed that scaffold morphology was similar between the pDTEc-rich ends and the pDTOc-rich ends of the gradient. These results introduce a method for fabricating gas-foamed polymer scaffold gradients that can be used for combinatorial screening of cell-material interactions in 3D.

Polymer-bound iodosobenzoate reagents for the cleavage of reactive phosphates

Abstract Two polymer-bound iodosobenzoate derivatives have been prepared and found to be phosphorolytically active against both p -nitrophenyldiphenyl phosphate and the nerve agent, Soman.

Synthesis and Characterization of Fatty Acid/Amino Acid Self-Assemblies

In this paper, we discuss the synthesis and self-assembling behavior of new copolymers derived from fatty acid/amino acid components, namely dimers of linoleic acid (DLA) and tyrosine derived diphenols containing alkyl ester pendent chains, designated as “R” (DTR). Specific pendent chains were ethyl (E) and hexyl (H). These poly(aliphatic/aromatic-ester-amide)s were further reacted with poly(ethylene glycol) (PEG) and poly(ethylene glycol methyl ether) of different molecular masses, thus resulting in ABA type (hydrophilic-hydrophobic-hydrophilic) triblock copolymers. We used Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopies to evaluate the chemical structure of the final materials. The molecular masses were estimated by gel permeation chromatography (GPC) measurements. The self-organization of these new polymeric systems into micellar/nanospheric structures in aqueous environment was evaluated using ultraviolet/visible (UV-VIS) spectroscopy, dynamic light scattering (DLS) and transmission electron microscopy (TEM). The polymers were found to spontaneously self-assemble into nanoparticles with sizes in the range 196–239 nm and critical micelle concentration (CMC) of 0.125–0.250 mg/mL. The results are quite promising and these materials are capable of self-organizing into well-defined micelles/nanospheres encapsulating bioactive molecules, e.g., vitamins or antibacterial peptides for antibacterial coatings on medical devices.