Drugs enmeshed: Superhydrophobic slow delivery

What a tangled web...

X-ray computed tomography can be used to look closely at superhydrophobic polymer meshes. These experimental materials have been shown to trap drug molecules with a barrier of air between them and an external aqueous environment.

In research supported by Boston University, Center for Integration of Medicine & Innovative Technology, Coulter Foundation and the National Institutes of Health, graduate student Stefan Yohe, working for Mark Grinstaff and Yolonda Colson, of the Dana-Farber Cancer Institute/Brigham and Women's Hospital (BWH) Cancer Center, have loaded up with drug molecules a superhydrophobic mesh made from biocompatible polymers.

The preparation involves an electrospinning method of poly(epsilon-caprolactone) with poly(glycerol monostearate-co-epsilon-caprolactone) as a water-repellant, hydrophobic, polymer dopant. The result of this process is a high apparent contact angle material, one that might be described as superhydrophobic, in fact. Superhydrophobic materials, ones in which the contact angle of a water droplet on the surface is determined to be greater than 150 degrees are fairly common in nature. Many plant leaves, most notably those of the lotus, are superhydrophobic, despite being "textured" surfaces, as are the legs, wings and other body parts of many insects.

Materials scientists have spent many years trying to make textured surfaces akin to these natural materials for a wide range of applications. Chemical deposition techniques, lithography borrowed from the microelectronics industry to etch superhydrophobicity on to a material and even treatment with a plasma have been used in this endeavour.

X-ray visualisation

The Boston team was able to visualize directly just how much water penetrates the electrospun mesh, using X-ray computed tomography in an aqueous solution. The addition of an iodinated contrast agent facilitates the formation of distinct images.

The team then tested just how slowly and steadily the drug - a model bioactive compound known as SN-38 - would be released into aqueous solution as well as its performance in standard drug cytotoxicity assays. They demonstrated that the rate of release of the drug correlates directly with the exclusion of the air pockets within the material as water slowly penetrates. The drug release rate remains steady for an extended period, the team explains. Indeed, the entrapped air layer within the superhydrophobic mesh retains its structure even in the presence of blood serum meaning that the material remains efficacious against cancer cells for more than 60 days.

"The ability to control drug release over a 2-3 month period is of significant clinical interest in thoracic surgery with applications in pain management and in the prevention of tumour recurrence after surgical resection," explains Colson. Colson is a thoracic surgeon at BWH who focuses on treating patients with lung cancer. The new polymer mesh approach to treatment could provide a unique way to deliver drugs in a variety of cancer types as well as other diseases, the researchers suggest. Such a long-term slow drug delivery system might be useful in addressing the issue of chronic pain or in the delivery of palliative pharmaceutical care with minimal intervention from a healthcare worker.

The team concludes that it should be possible to make a wide range of analogous systems using a variety of biocompatible polymers and drugs for a range of therapies provided that three conditions are met. First, the material processing technique must be able to produce a porous, bulk material. Secondly, the material must have controlled thickness to allow slow, controlled water penetration. Finally, it must have a high enough apparent contact angle that air is readily trapped within the mesh and is only excluded to allow drug release by the slow penetration of the water.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.