Electroactive Layer-by-Layer Films for Controlled Release and Mechanomutability
Layer-by-layer (LbL) assembly is a versatile technique for fabricating polymer thin films with excellent control over film thickness, morphology, and functionality. My project focuses on the electrochemical manipulation of LbL films containing redox-active Prussian Blue (PB) nanoparticles (Fig. 1 & 2). PB is a multifaceted material that has been used since the 18th century as a dye, and has been researched more recently for its electrochromic, electrocatalytic, magnetic, and ion exchange properties. Interestingly, PB was approved by the FDA in 2003 for treatment of heavy metal and radiation poisoning (Ref. 1). My work utilizes the ability of Prussian Blue to change its surface charge in response to an electric potential. Altering the charge of a component in an LbL film can change the density of ionic crosslinks within the film, allowing for triggered film disassembly or film swelling/shrinking. Two projects are currently underway.
Figure 1: Prussian Blue crystal lattice (left); TEM image of Prussian Blue nanoparticle (center) (Ref. 2); aqueous suspension of Prussian Blue nanoparticles (right)
Electroactive controlled release
The ultimate goal of this project is to fabricate an implantable or transdermal drug delivery device that can be remotely triggered to release precise quantities of drugs on demand. Implantable devices allow localized delivery of drugs that can reduce the required dosage and reduce side effects from systemic delivery. Further, a platform that is responsive to an external stimulus, in this case an electric potential, can allow for active control over drug release. This active control then allows for complex release profiles that can be adjusted for a desired therapeutic response.
PB nanoparticles are negatively charged, and thus can be LbL-assembled with a variety of positively charged species. Upon an applied electric potential of 1.25 V (vs. SCE), the PB switches to its neutral redox state called Prussian Brown. As the particles lose their surface charge, the cohesive electrostatic attractions holding the film together are lost. Ions and water from the surrounding solution enter the film to maintain electroneutrality and in turn solubilize the film components, triggering film disassembly. We have demonstrated the controlled release of the model drug dextran sulfate (Fig. 3). Current work includes delivery of small, hydrophobic drugs.
Figure 2: Electrochemical cell for film characterization and drug release experiments
Figure 3: Release of dextran sulfate from (LPEI/PB/LPEI/Dextran sulfate)30 film (120 nm thick) at constant applied potential (left) and pulsed potential (right) (Ref. 3)
Mechanomutable materials are those that alter their mechanical properties in response to an environmental stimulus. There are a number of examples of such behavior in nature (e.g. sea cucumbers, starfish, etc.). Practical applications of mechanomutable materials could include control over cellular behavior or protein adsorption on surfaces, mechanical motors to perform work, and dynamic coatings for nanoscale devices in general. We have fabricated Prussian Blue-containing LbL films that can reversibly swell and soften in response to an electric potential. These films represent a new class of polymer nanocomposites with electrochemically-tunable mechanical properties.