Tejal A. Desai

Proteins and cells on PEG immobilized silicon surfaces

Silicon surfaces were modified by covalent attachment of a self-assembled (SA) polyethylene glycol (PEG) film. Adsorption of albumin, fibrinogen, and IgG to PEG immobilized silicon surfaces was studied by ellipsometry to evaluate the non-fouling and non-immunogenic properties of the surfaces. The adhesion and proliferation of human fibroblast and Hela cells onto the modified surfaces were investigated to examine their tissue biocompatibility. Coated PEG chains showed the effective depression of both plasma protein adsorption and cell attachment to the modified surfaces. The mechanisms accounting for the reduction of protein adsorption and cell adhesion on modified surfaces were discussed.

Optical sensing of biomolecules using microring resonators

A biosensor application of vertically coupled glass microring resonators with Q/spl sim/12 000 is introduced. Using balanced photodetection, very high signal to noise ratios, and thus high sensitivity to refractive index changes (limit of detection of 1.8/spl times/10/sup -5/ refractive index units), are achieved. Ellipsometry and X-ray photoelectron spectroscopy results indicate successful modification of biosensor surfaces. Experimental data obtained separately for a bulk change of refractive index of the medium and for avidin-biotin binding on the ring surface are reported. Excellent repeatability and close-to-complete surface regeneration after binding are experimentally demonstrated.

Methods for Fabrication of Nanoscale Topography for Tissue Engineering Scaffolds

Observations of how controlling the microenvironment of cell cultures can lead to changes in a variety of parameters has lead investigators to begin studying how the nanoenvironment of a culture can affects cells. Cells have many structures at the nanoscale such as filipodia and cytoskeletal and membrane proteins that interact with the environment surrounding them. By using techniques that can control the nanoenvironment presented to a cell, investigators are beginning to be able to mimic the nanoscale topographical features presented to cells by extracellular matrix proteins such as collagen, which has precise and repeating nanotopography. The belief is that these nanoscale surface features are important to creating more natural cell growth and function. A number of techniques are currently being used to create nanoscale topographies for cell scaffolding. These techniques fall into two main categories: techniques that create ordered topographies and those that create unordered topographies. Electron Beam lithography and photolithograpghy are two standard techniques for creating ordered features. Polymer demixing, phase separation, colloidal lithography and chemical etching are most typically used for creating unordered surface patterns. This review will give an overview of these techniques and cite observations from experiments carried out using them.

Microfabricated drug delivery systems: from particles to pores

Microfabrication techniques which permit the creation of therapeutic delivery systems that possess a combination of structural, mechanical, and perhaps electronic features may surmount challenges associated with conventional delivery of therapy. In this review, delivery concepts are presented which capitalize on the strengths of microfabrication. Possible applications include micromachined silicon membranes to create implantable biocapsules for the immunoisolation of pancreatic islet cells-as a possible treatment for diabetes-and sustained release of injectable drugs needed over long time periods. Asymmetrical, drug-loaded microfabricated particles with specific ligands linked to the surface are proposed for improving oral bioavailability of peptide (and perhaps protein) drugs. In addition, microfabricated drug delivery systems ranging from transdermal microneedles to implantable microchips will be discussed.

Evaluation of nanostructured composite collagen--chitosan matrices for tissue engineering.

The development of suitable three-dimensional matrices for the maintenance of cellular viability and differentiation is critical for applications in tissue engineering and cell biology. The structure and composition of the extracellular matrix (ECM) has been shown to modulate cell behavior with respect to shape, movement, proliferation, and differentiation. Although collagen and chitosan have separately been proposed as in vitro ECM materials, the influence of chitosan--collagen composite matrices on cell morphology, differentiation, and function is not well studied. To this end, gel matrices of different proportions of collagen and chitosan were examined ultrastructurally and characterized for their ability to regulate cellular activity. A three-chamber system with circulating hydraulic fluids was used to evaluate the gel stability under fluid force. Results indicated that overall matrix integrity increased with the proportion of chitosan. Scanning electron microscopy indicated that the addition of chitosan greatly influences ultrastructure and changes collagen fiber cross-linking, reinforcing the structure and increasing pore size. K562 cells cultured in three-dimensional gels were examined for cell proliferation and differentiation. Although cell proliferation was inhibited with an increasing proportion of chitosan, cell function based on cytokine-release was greatly augmented. Results suggest that a hybrid chitosan--collagen matrix may have potential biological and mechanical benefits for use as a cellular scaffold.

Nanopore Technology for Biomedical Applications

The ability to create well-defined and controlled interfaces has been an area of great interest over the last few years, particularly in the biomedical arena. This paper will describe the development of technology for the fabrication of nanopore membranes, and their operation in biological environments. With monodisperse pores sizes as small as 10 nanometers, these membranes offer advantages in their reproducibility, and their ability to be integrated with controlled biochemical surface modification protocols. A comprehensive review of results in the areas of nanopore and biocapsule microfabrication technologies, biocompatibility of nanomembrane materials, biologically appropriate post-processing protocols (bonding, sterilization), surface modification protocols, and appropriate mass transport models will be presented. The results point to the potential of using such technologies for therapeutic applications including immunoisolation biocapsules, drug delivery devices, and targeted biorecognition platforms.

Microfabricated immunoisolating biocapsules

A microfabricated silicon-based biocapsule for the immunoisolation of cell transplants is presented. The biocapsule-forming process employs bulk micromachining to define cell-containing chambers within single crystalline silicon wafers. These chambers interface with the surrounding biological environment through polycrystalline silicon filter membranes. The membranes are surface micromachined to present a high density of uniform pores, thus affording sufficient permeability to oxygen, glucose, and insulin. The pore dimensions, as small as 20 nm, are designed to impede the passage of immune molecules and graft-borne viruses. The underlying filter-membrane nanotechnology has been successfully applied in controlled cell culture systems (Ferrari et al., 1995), and is under study for viral elimination in plasma fractionation protocols. Here we report the encouraging results of in vitro experiments investigating the biocompatibility of the microfabricated biocapsule, and demonstrate that encapsulated rat neonatal pancreatic islets significantly outlive and outperform controls in terms of insulin-secretion capability over periods of several weeks. These results appear to warrant further investigations on the potential of cell xenografts encapsulated within microfabricated, immunoisolating environments for the treatment of insulin-dependent diabetes.

Biomimetic nanowire coatings for next generation adhesive drug delivery systems

Without bioadhesive delivery devices, complex compounds are typically degraded or cleared from mucosal tissues by the mucous layer.While some chemically modified, microstructured surfaces have been studied in aqueous environments,adhesion due to geometry alone has not been investigated. Silicon nanowire-coated beads show significantly better adhesion than those with targeting agents under shear, and can increase the lift-off force 100-fold. We have shown that nanowire coatings, paired with epithelial physiology, significantly increase adhesion in mucosal conditions.

Long-Term Small Molecule and Protein Elution from TiO2 Nanotubes

In this study, TiO(2) nanotubes of various dimensions were used to elute albumin, a large protein molecule, as well as sirolimus and paclitaxel, common small molecule drugs. The nanotubes controlled small molecule diffusion for weeks and large molecule diffusion for a month. Drug eluted from the nanotubes was bioactive and decreased cell proliferation in vitro. Elution kinetics was most profoundly affected by tube height. This study demonstrates that TiO(2) nanotubes may be a promising candidate for a drug-eluting implant coating.

Fabrication of microtextured membranes for cardiac myocyte attachment and orientation

To understand the role of tissue adaptation to altered physiological states, a more physiologically and dimensionally relevant in vitro model of cardiac myocyte organization has been developed. A microtextured polymeric membrane with micron range dimensions promotes myocyte adhesion through substrate/cell interlocking and, thus, provides a more suitable stretchable matrix for studying overlying cell populations. These microtextured membranes are created using photolithography and microfabrication techniques. Biologically, mechanically, and optically compatible interfaces with specified microarchitecture and surface chemistry have been designed, microfabricated, and characterized for this purpose. Cardiac myocytes plated on these membranes display greater attachment and cell height compared to conventional culture substrates. Advantages of the microtextured membranes include the high degree of reproducibility and the ability to create features on the micron and submicron size scale. Because of the flexibility of substrate material and the ease of creating micron size structures, this technique can be applied to many other physiological and biological systems.