(3df) Synthetic Polypeptide Macromers: Components of a Hydrogel Toolkit for Modeling Cell-Matrix Interactions

The
overarching goal of my research is to design hydrogels for biomedical
applications and as templates with which to investigate cell-matrix
interactions. Cellular functions including DNA synthesis, migration, and
differentiation are co-regulated by both the stiffness of the extracellular
matrix (ECM) and the presence of soluble signaling molecules (growth factors,
cytokines, hormones, etc.): as a postdoctoral scientist working with Professor
Paula T. Hammond (chemical engineering) and Professor Linda G. Griffith
(biological engineering, mechanical engineering) at MIT, I have synthesized and
characterized a set of synthetic polypeptides with tunable size, composition,
and secondary structure as components of a hydrogel toolkit for modeling these
cell-matrix interactions. This system, described further in my abstract submitted to the
Cell-Biomaterial Interactions Session (08B05), provides a level of control that
allows us to design hydrogels with stiff rod-like segments or flexible coil
segments, thus possibly impacting mechanical and other hydrogel properties. I
am investigating the relationships between polypeptide structure and
composition, as well as hydrogel formulation (i.e. mass fraction and cross-link
density) on the physicochemical properties of the resulting hydrogels. These
characterization data will be used to create a library of hydrogel substrates
with systematically and independently varied stiffness, permeability, and
ligand presentation for in vitro study of the fundamental
parameters of cell-matrix interactions.

After obtaining my
bachelor's degree (Lehigh University, B.S. chemical engineering), I developed hydrogel-based
biomaterials with Professor Mark W. Grinstaff at
Boston University. Through my doctoral work (PhD chemistry), I gained expertise
in the design, synthesis, and characterization of hydrogels for ophthalmologic
applications such as in situ gelling surgical adhesives and permanent
corneal implants. One of my research goals was the development of
application-specific hydrogel adhesives to treat a variety of ocular wounds:
the main benefit of these materials is that, unlike sutures, the current standard
of care, hydrogel adhesives can seal wounds without inflicting secondary
trauma, inducing astigmatism, or providing a pathway for infection. In this
project, I formed hydrogel adhesives by mixing a peptide-based dendritic
molecule with bi-functional PEG polymers, which reacted under mild conditions
(i.e. pH 7.4, room temperature) to rapidly form transparent, elastic hydrogels
with high water content. We studied three types of hydrogels ? cross-linked by
formation of either thiazolidine or more robust pseudoproline bonds ? finding that the degradation time of
the hydrogel adhesive could be tuned from 1.5 to 24 weeks depending on the type
of cross-link bond formed. The constituent macromers
and resulting hydrogels did not induce in vitro cytotoxicity and, when
compared to sutures in an ex vivo model, exhibited equal or higher
efficacy in closing corneal wounds. These results demonstrate the feasibility
of hydrogel adhesives for corneal wound repair and highlight the tunable nature
of hydrogel physicochemical properties ? supporting our hypothesis that
hydrogel adhesives can be designed to suit a variety of wounds ranging from
quick-healing corneal incisions to slow-healing corneal transplants.

In addition, I created a hydrogel implant designed to
serve as a space-filling, permanent onlay for repair
of corneal defects caused by trauma or disease. This onlay,
which consisted of two layers: a thin, stiff polyHEMA
hydrogel and a thicker, softer dendrimer hydrogel
layer, was designed to support surface epithelialization, provide a protective
barrier against infection, and promote integration with the surrounding stromal
tissue. Due to their extremely hydrophilic nature, hydrogels do not readily
support the adhesion and spreading of cells and, as such, these materials serve
as neutral templates upon which to engineer specific cell-material interactions
at the implant interface. Our results demonstrated that peptide sequences, such
as arginine-glycine-aspartic acid (RGD), a ubiquitous recognition sequence for α_vβ₃
integrin binding, and ECM proteins, such as collagen, can be covalently
tethered to hydrogels in order to facilitate the adhesion and spreading of
corneal cells. Although additional long-term studies are necessary to refine
the surface modification strategy and fabrication protocol, the promising
optical, mechanical, cell adhesion, and interface properties of the proposed
implant provide motivation for continued investigation into its development as
a functional corneal onlay.

While finishing my
postdoctoral research, I will expand my knowledge of polymer chemistry,
hydrogel characterization, and analysis of cell-biomaterial interactions.
Expertise in these areas is critical for my future research, in which I will
design synthetic scaffolds for in vitro tissue modeling and tissue
engineering applications. I believe it is imperative to conduct applied
research for the design of translational biomaterials in harmony with basic
study of the underlying tissue physiology and pathophysiology. For this reason,
I will develop a research program that includes three main areas: (1) scaffold
design and characterization, (2) examination of cell/scaffold interactions, and
(3) creation and study of relevant in vitro models of both healthy and
diseased tissue.