It is of great interest to pattern biomaterials on the nanoscale and microscale to control diverse biomedical functions. We utilize soft lithographic technique to nanopattern poly (L-lactic acid) (PLLA) - a biodegradable and biocompatible polymer. We find that the PLLA surfaces nanopatterned with 750 nm pitch nanocup or nanocone arrays exhibit drug release rates 25-30% lower than that of the flat surface, which is counter-intuitive given the nanopattern-induced increase in their surface areas. Based on diffusion and microfluidic meniscus curvature minimization analyses, we attribute the decreased drug release rate to the incomplete wetting of the nanopatterned surface by the fluid.
We demonstrate experimentally-realizable microlens-based light trapping schemes for methyl ammonium lead iodide perovskite solar cells using rigorous scattering matrix simulations. Our simulations utilize commonly studied n-i-p device architecture where microlens array is coupled to the air-glass side of the solar cell. The microlens focuses light in the absorber layer that leads to increase in absorption and enhancement in photocurrent. Our optimal architecture has a period of ~700 nm and microlens height of ~800-1000 nm with absorption and photocurrent enhancement of ~6.5% and 6% respectively, for nearly lossless metal cathodes.
We use rigorous scattering matrix simulations to develop a pathway for synthesizing nanopatterns on polymer surfaces. We consider a system in which the polymer surfaces are initially coated with periodic microspheres in a close packed lattice. When such a lattice is irradiated with a laser of desired wavelength and power, the electric field intensity beneath the spheres is enhanced by more than an order of magnitude, generating localized heating that can remove nanometersize volume of the material in a periodic array of nanocavities. The array of glass spheres can be self-assembled on any curved surface, and these spheres can be utilized as an optical lens to focus light energy within the surface. Our simulations show that the depth and size of the nano-cavities depends critically on the size of microspheres.
Nanoscale patterned structures composed of biomaterials exhibit great potential for the fabrication of functional biostructures. In this paper, we report cost-effective, rapid, and highly reproducible soft lithographic transfer-molding techniques for creating periodic micro- and nano-scale textures on poly (L-lactic acid) (PLLA) surface. These artificial textures can increase the overall surface area and change the release dynamics of the therapeutic agents coated on it. Specifically, we use the double replication technique in which the master pattern is first transferred to the PDMS mold and the pattern on PDMS is then transferred to the PLLA films through drop-casting as well as nano-imprinting. The ensuing comparison studies reveal that the drop-cast PLLA allows pattern transfer at higher levels of fidelity, enabling the realization of nano-hole and nano-cone arrays with pitch down to ~700 nm. The nano-patterned PLLA film was then coated with rapamycin to make it drug-eluting.
Organic solar cells have rapidly increasing efficiencies, but typically absorb less than half of the incident solar spectrum. To increase broadband light absorption, we rigorously design experimentally realizable solar cell architectures based on dual photonic crystals. Our optimized architecture consists of a polymer microlens at the air-glass interface, coupled with a photonic-plasmonic crystal at the metal cathode. The microlens focuses light on the periodic nanostructure that generates strong light diffraction. Waveguiding modes and surface plasmon modes together enhance long wavelength absorption in P3HT-PCBM. The architecture has a period of 500 nm, with absorption and photocurrent enhancement of 49% and 58%, respectively.
Biological applications can benefit from nanoscale texturing of materials for biomedical functions. Texturing of
biomaterials can increase the available surface area so that they can be coated with larger doses of therapeutic agents.
We demonstrate nano-texturing of poly (L-lactic acid) (PLLA) – a prototypical material commonly used for drug-eluting
coronary stents and as a template for cell growth. A master pattern consisting of a periodic array was transferred to a
PDMS mold. Drop-casting PLLA achieves the best transfer of patterns, with nanoarrays of holes with pitch ~700 nm.
Nanoimprinting the PLLA films results in shallower and less resolved features.