Biologically-Based Biomaterials to Study Cell and Molecular Function

Self-Mineralizing Organic Templates for Controlling Osteoblast Function

In these studies, we combine materials science strategies with cellular and molecular biology strategies to address questions related to how material chemistry modulates cell function.  The observations that synthetic biomaterials capable of nucleating an apatitic layer on their surface in-situ form a true bond with host tissue has led us to hypothesize that a surface chemistry containing an in-vitro formulated biomimetic mineral will direct bone marrow stem cells down an osteoblast pathway.  Therefore, in an effort to better control the extracellular microenvironment and relate biological output to material chemistry, we have developed biomimetic strategies to efficiently nucleate and grow a bone-like mineral within the pores of functionalized 3D porous polymers via a 1-step, room temperature process (Murphy et al. J. Biomed. Mater. Res. 2000).  The mineral layer is relatively thin (< 5 mm), so porosity is not compromised, resulting in a powerful model system to analyze effects of changes in material chemistry on cellular function, without concurrent changes in material architecture.  This organic template-mediated mineralization process results in 3 pathways of investigation: first, we have a model system for parametrically studying basic aspects of mineral nucleation and growth onto a 3D organic substrate; second, we have a model 3D system for studying the effect of biomaterial alterations on cell and molecular function in a controlled manner; third, we have model system for studying more applied questions related to tissue engineering and regenerative medicine.  Such studies are currently being pursued both in culture and in-vivo (Kohn et al.  J. Biomed. Mater. Res. 2002).  Furthermore, the benign processing conditions (i.e. room temperature, atmospheric pressure) enable incorporation of inductive factors into both the mineral and polymer, facilitating a number of permutations of controlled release (Murphy et al. Biomat. 2000).  Another approach being pursued is the synthesis of organic/inorganic hybrid materials, in which cell-responsive organic constituents are co-precipitated with inorganic phases onto an organic template. 

Functionally-Graded Materials for Controlling Cell Function

A second strategy employed by nature to fulfill heterogeneous functional demands is to synthesize materials with a defined heterogeneity in structure and/or composition. For example, bone has a graded structure from the dense cortical surface toward the inner cancellous material which accomplishes the dual goals of protection and perfusion.  We are therefore developing functionally-graded materials in which organic surface functionalization and mineralization are spatially controlled.  Development of such graded templates is hypothesized to lead to greater spatial control of cell function and may have utility for targeting specific anatomic regions that are in greater need of cellular modulation.

Synthesis, Characterization and Evaluation of "Smart" Biomaterials:

A third biomimetic strategy for developing biomaterials that better regulate biological function is to incorporate environmentally responsive or adaptive constituents into the materials.  Unlike conventional materials, which are designed to assume certain minimal properties that will resist local environmental stimuli, "smart" materials offer the ability to adapt their properties in response to such physical, mechanical or biological stimuli.  For example, by developing “smart” biomaterials, we offer the ability to deliver biological agents in a cyclic fashion (i.e. on-demand).  The hypothesized benefit of release-on-demand is that disease processes that are cyclic in nature could be treated as the need arises.  We have formulated candidate “smart” materials, which include self-buffering polymers that can compensate for localized acid build up during autocatalytic degradation; ion-exchange polymers, in which biological agents are activated by changes in local environmental factors, such as a reduction in pH (Renier, et al. Biomat., accepted), and biodegradable polymers which allow for the local delivery of macromolecules, such as osteogenic proteins, or drugs (Renier et al. J. Biomed. Mater. Res. 1997).  The importance of creating intelligent systems is reinforced by recent data we have generated, which shows that even small shifts in extracellular pH significantly down-regulate the ability stem cells to express markers of the osteoblast phenotype (Kohn et al. J. Biomed. Mater. Res., 2002).

OVERVIEW

Controlled 3-D mineralization

Local pH effects on cell expression