Research in Dr. Heiden's Group

Research in this laboratory typically involves synthesis, processing, and characterizing polymers and composites or nanoparticles. Most projects involve nanotechnology, sustainability, and/or biomaterials.

Past and recent projects have included stimulus-response nanoparticles for drug delivery; electrospinning of nanofibers for tissue scaffolds; structured nanoparticles for low-leaching wood preservation; biocomposites and biofibers; studies of controlled nanoscale interfaces of rice-hull reinforced composites; and engineered wood- or biopolymer-reinforced composites having improved processing, longevity, moisture resistance, and mechanical properties.

Further Description of Selected Research

Thermally-Responsive Nanoparticles for Drug Delivery

In this project nanoparticles are prepared with a gold core and with a drug-containing organic shell. These nanoparticles respond to heat to release a drug of interest. Below is an example graph showing the change in UV transmission of light (y-axis) as a function of temperatures (x-axis). As the transmission decreases the polymer coil is contracting to give a globule that transmits less light. As that collapse occurs drug is expelled from the polymer. This coil-globule transition is completely reversible as shown by the cooling curve overlapping the heating curve. A portion of a transmission electron microscope image is also shown of a nanoparticle that contains some gold in the core. The gold core can be heated using wavelengths that are safe for human tissues, so nanoparticles with a transition temperature that is above 37 °C (the temperature of the human body) can be heated safely within the body to trigger the drug release.

Use of Controlled Release Nanoparticles to Reduce Leaching of Biocides

The wood products industry is a major component of the US economy. Wood products are used in decks, construction, furniture, fences, etc. All wood products that are used outside must be preserved (treated with various chemicals to protect from biological attack). These preservatives are expensive and also can leach from the wood, especially in warm, wet environments, and the biocide leaching can be detrimental to the environment. We prepared biocide-containing nanoparticles and treated wooden field stakes, and then cut sections from the interior of the field stake. The scanning electron micrographs shown below show the nanoparticles penetrated deeply into the wooden interior. The graph below that shows that wood treated with biocide-containing nanoparticles leached significantly less biocide than wood treated with a similar amount of biocide but not contained within nanoparticles.

Modification and Analysis of Interfaces in Rice Hull Composites (Images and Nanomechanical Testing by Prof. Reza Yassar and Anahita Pakzad)

In this project controlled modifications of composite interfaces (composition and thickness of the interface) between rice hull and different polymer matrices are being prepared and studied. Specimens are provided to Prof. Reza Yassar in Mechanical Engineering and Engineering Mechanics and they are studying the interface and its nanomechanical properties. The Atomic Force Microscopy images shown below were taken by Anahita Pakzad in Adhesion Mode and Topographical Mode. Therefore we can test just the composite interface to correlate interface structure with composite properties.

Rice hull in PVC without a modified interface. (Adhesion mode)	Height image of rice hull in PVC.
Rice hull in PVC with modified interface. (Adhesion mode). Scan size: 20 → 1.5 µm, Adhesion: 0 → 100 nN	Nanomechanical analyses were performed using a peak force-tapping mode in the AFM. At each pixel, the AFM tip generates a small indentation on the sample surface, and then force-separation curves are made that give quantitative nanomechanical values of the surface. (Drawing by Anahita Pakzad).

Electrospinning Nanofibers

Polymer fibers are most commonly formed through the application of a mechanical force to a polymer melt or solution. However, they can also be formed through the application of an electrical force. When electrical forces are used to form nanofibers the method is termed electrospinning. Electrospinning often yields fibers with a diameter below 1 µm. The standard electrospinning process is illustrated in a Flash Animation, and shown in a video a standard electrospinning process for a polymer is shown in these QuickTime Videos (by Jeong Seo Moon).

Electrospinning Animation Flash 7 or higher plugin required. - Get Flash	Electrospinning Video

The polymer solution jet appears as a white streak. It destabilizes and whips to give fibers with a diameter that can range from ~50 nm to several micrometers depending on the polymer, its solvent, and the electrospinning conditions. Adding electrolytes to a polymer solution increases its conductivity and can reduce the diameter of a given polymer nanofiber.

Depending on the polymer and solvent used these nanofibers can be used for applications ranging from filtration to tissue supports (see images below).

	These nanofibers were electrospun for composite reinforcement.
Photograph of microfibers being electrospun.
These nanofibers were electrospun from a calcein-containing solution for use as a tissue scaffold.