Stefan T. Yohe

Superhydrophobic Materials for Tunable Drug Release: Using Displacement of Air To Control Delivery Rates

We have prepared 3D superhydrophobic materials from biocompatible building blocks, where air acts as a barrier component in a porous electrospun mesh to control the rate at which drug is released. Specifically, we fabricated poly(ε-caprolactone) electrospun meshes containing poly(glycerol monostearate-co-ε-caprolactone) as a hydrophobic polymer dopant, which results in meshes with a high apparent contact angle. We demonstrate that the apparent contact angle of these meshes dictates the rate at which water penetrates into the porous network and displaces entrapped air. The addition of a model bioactive agent (SN-38) showed a release rate with a striking dependence on the apparent contact angle that can be explained by this displacement of air within the electrospun meshes. We further show that porous electrospun meshes with higher surface area can be prepared that release more slowly than control nonporous constructs. Finally, the entrapped air layer within superhydrophobic meshes is shown to be robust in the presence of serum, as drug-loaded meshes were efficacious against cancer cells in vitro for >60 days, thus demonstrating their applicability for long-term drug delivery.

3D superhydrophobic electrospun meshes as reinforcement materials for sustained local drug delivery against colorectal cancer cells.

In this work we expand upon a recently reported local drug delivery device, where air is used as a degradable component of our material to control drug release (J. Am. Chem. Soc. 2012, 134, 2016-2019). We consider its potential use as a drug loaded strip to provide both mechanical stability to the anastomosis, and as a means to release drug locally over prolonged periods for prevention of locoregional recurrence in colorectal cancer. Specifically, we electrospun poly(ε-caprolactone) (PCL) with the hydrophobic polymer dopant poly(glycerol monostearate-co-ε-caprolactone) (PGC-C18) and used the resultant mesh to control the release of two anticancer drugs (CPT-11 and SN-38). The increase in mesh hydrophobicity with PGC-C18 addition slows drug release both by the traditional means of drug diffusion, as well as by increasing the stability of the entrapped air layer to delay drug release. We demonstrate that superhydrophobic meshes have mechanical properties appropriate for surgical buttressing of the anastomosis, permit non-invasive assessment of mesh location and documentation of drug release via ultrasound, and release chemotherapy over a prolonged period of time (>90 days) resulting in significant tumor cytotoxicity against a human colorectal cell line (HT-29).

Functionalized Hydrophobic Poly(Glycerol-co-ε-Caprolactone) Depots for Controlled Drug Release

A limitation to many polymer-based drug delivery systems is the lack of ability to customize a particular polymer composition for tailoring drug release kinetics to a specific clinical application. In this study, we investigated the structure-property effects of conjugating various hydrophobic biocompatible side chains to poly(glycerol-co-caprolactone) copolymers with the goal of achieving prolonged and controlled release of a chemotherapeutic agent. The choice of side chain significantly affected the resulting polymer properties including thermal transitions, relative crystallinity (ΔH(f)), and hydrophobicity. Drug-loaded films cast from solutions of polymer and 10-hydroxycamptothecin demonstrated prolonged release from four to over seven weeks depending upon side chain structure without initial burst release behavior. Use of the stearic acid-conjugated poly(glycerol-co-caprolactone) films afforded substantial anticancer activity in vitro for at least 50 days when exposed to fresh cultures of A549 human lung cancer cells over 24 h intervals, correlating well with the measured drug release kinetics.

Triggered drug release from superhydrophobic meshes using high-intensity focused ultrasound.

Application of high-intensity focused ultrasound to drug-loaded superhydrophobic meshes affords triggered drug release by displacing an entrapped air layer. The air layer within the superhydrophobic meshes is characterized using direct visualization and B-mode imaging. Drug-loaded superhydrophobic meshes are cytotoxic in an in vitro assay after ultrasound treatment.

Invariance and optimality in the regulation of an enzyme.

BackgroundThe Michaelis-Menten equation, proposed a century ago, describes the kinetics of enzyme-catalyzed biochemical reactions. Since then, this equation has been used in countless, increasingly complex models of cellular metabolism, often including time-dependent enzyme levels. However, even for a single reaction, there remains a fundamental disconnect between our understanding of the reaction kinetics, and the regulation of that reaction through changes in the abundance of active enzyme.ResultsWe revisit the Michaelis-Menten equation under the assumption of a time-dependent enzyme concentration. We show that all temporal enzyme profiles with the same average enzyme level yield identical substrate degradation– a simple analytical conclusion that can be thought of as an invariance principle, and which we validate experimentally using a β-galactosidase assay. The ensemble of all time-dependent enzyme trajectories with the same average concentration constitutes a space of functions. We develop a simple model of biological fitness which assigns a cost to each of these trajectories (in the form of a function of functions, i.e. a functional). We then show how one can use variational calculus to analytically infer temporal enzyme profiles that minimize the overall enzyme cost. In particular, by separately treating the static costs of amino acid sequestration and the dynamic costs of protein production, we identify a fundamental cellular tradeoff.ConclusionsThe overall metabolic outcome of a reaction described by Michaelis-Menten kinetics is ultimately determined by the average concentration of the enzyme during a given time interval. This invariance in analogy to path-independent phenomena in physics, suggests a new way in which variational calculus can be employed to address biological questions. Together, our results point to possible avenues for a unified approach to studying metabolism and its regulation.ReviewersThis article was reviewed by Sergei Maslov, William Hlavacek and Daniel Kahn.

Surface Tension Triggered Wetting and Point of Care Sensor Design

Rapid, simple, and inexpensive point-of-care (POC) medical tests are of significant need around the world. The transition between nonwetting and wetted states is used to create instrument-free surface tension sensors for POC diagnosis, using a layered electrospun mesh with incorporated dye to change color upon wetting.