Anand Kadiyala

Nanotopographical Modulation of Cell Function through Nuclear Deformation

Although nanotopography has been shown to be a potent modulator of cell behavior, it is unclear how the nanotopographical cue, through focal adhesions, affects the nucleus, eventually influencing cell phenotype and function. Thus, current methods to apply nanotopography to regulate cell behavior are basically empirical. We, herein, engineered nanotopographies of various shapes (gratings and pillars) and dimensions (feature size, spacing and height), and thoroughly investigated cell spreading, focal adhesion organization and nuclear deformation of human primary fibroblasts as the model cell grown on the nanotopographies. We examined the correlation between nuclear deformation and cell functions such as cell proliferation, transfection and extracellular matrix protein type I collagen production. It was found that the nanoscale gratings and pillars could facilitate focal adhesion elongation by providing anchoring sites, and the nanogratings could orient focal adhesions and nuclei along the nanograting direction, depending on not only the feature size but also the spacing of the nanogratings. Compared with continuous nanogratings, discrete nanopillars tended to disrupt the formation and growth of focal adhesions and thus had less profound effects on nuclear deformation. Notably, nuclear volume could be effectively modulated by the height of nanotopography. Further, we demonstrated that cell proliferation, transfection, and type I collagen production were strongly associated with the nuclear volume, indicating that the nucleus serves as a critical mechanosensor for cell regulation. Our study delineated the relationships between focal adhesions, nucleus and cell function and highlighted that the nanotopography could regulate cell phenotype and function by modulating nuclear deformation. This study provides insight into the rational design of nanotopography for new biomaterials and the cell-substrate interfaces of implants and medical devices.

Modeling of a 3-D tunable photonic crystal for camouflage coating

Photoresponsive polymers undergo structural changes (expansion or contraction) upon photoirradiation[1]. This group of polymers when polymerized with arrays of colloidal particles self-assembled into a crystalline colloidal array (CCA) forming a polymerized CCA (PCCA). The functionality of the CCA is enhanced by polymerizing it, due to the inclusion of specific properties of the polymer. Combining the properties of both, significant change in the lattice size is observed under external stimuli resulting in an optical response of the PCCA (either a blue or red shift of the spectrum)[2],[3]. Novel tunable nanophotonic devices that enable a camouflage behavior could be realized using this technique. Camouflage behavior is achieved when an object blends into the environment, making it indiscernible from its surroundings. One approach to achieving such behavior in an object is to reflect only the wavelength that is predominant in the range of wavelengths incident from its surroundings. In this work we model and simulate a 3D polymerized photonic crystal structure which has the potential to exhibit this behavior. Simulations are performed to model the dynamic band gap tuning of the 3-D photonic crystal using the MIT Photonic Bands (MPB) and OptiFDTD optical modeling tools. These results lend a key understanding to the design of PhCs that exhibit dynamic band gap tuning, and how they can be applied in device designs.

Fabrication of two-dimensional visible wavelength nanoscale plasmonic structures using hydrogen silsesquioxane based resist

In recent years, the global market for biosensors has continued to increase in combination with their expanding use in areas such as biodefense/detection, home diagnostics, biometric identification, etc. A constant necessity for inexpensive, portable bio-sensing methods, while still remaining simple to understand and operate, is the motivation behind novel concepts and designs. Labeled visible spectrum bio-sensing systems provide instant feedback that is both simple and easy to work with, but are limited by the light intensity thresholds required by the imaging systems. In comparison, label-free bio-sensing systems and other detection modalities like electrochemical, frequency resonance, thermal change, etc., can require additional technical processing steps to convey the final result, increasing the system’s complexity and possibly the time required for analysis. Further decrease in the detection limit can be achieved through the addition of plasmonic structures into labeled bio-sensing systems. Nano-structures that operate in the visible spectrum have feature sizes typically in the order of the operating wavelength, calling for high aspect ratio nanoscale fabrication capabilities. In order to achieve these dimensions, electron beam lithography (EBL) is used due to its accurate feature production. Hydrogen silsesquioxane (HSQ) based electron beam resist is chosen for one of its benefits, which is after exposure to oxygen plasma, the patterned resist cures into silicon dioxide (SiO2). These cured features in conjunction with nanoscale gold particles help in producing a high electric field through dipole generation. In this work, a detailed process flow of the fabrication of square lattice of plasmonic structures comprising of gold coated silicon dioxide pillars designed to operate at 560 nm wavelength and produce an intensity increase of roughly 100 percent will be presented.

Improvement in the Light Extraction of Blue InGaN/GaN-Based LEDs Using Patterned Metal Contacts

We demonstrate a method to improve the light extraction from an LED using photonic crystal (PhC)-like structures in metal contacts. A patterned metal contact with an array of Silicon Oxide (SiOx) pillars (440 nm in size) on an InGaN/GaN-based MQW LED has shown to increase output illumination uniformity through experimental characterization. Structural methods of improving light extraction using transparent contacts or dielectric photonic crystals typically require a tradeoff between improving light extraction and optimal electrical characteristics. The method presented here provides an alternate solution to provide a 15% directional improvement (surface normal) in the radiation profile and ~ 30% increase in the respective intensity profile without affecting the electrical characteristics of the device. Electron beam patterning of hydrogen silesquioxane (HSQ), a novel electron beam resist is used in patterning these metal contacts. After patterning, thermal curing of the patterned resist is done to form SiOx pillars. These SiOx pillars aid as a mask for transferring the pattern to the p-metal contact. Electrical and optical characterization results of LEDs fabricated with and without patterned contacts are presented. We present the radiation and intensity profiles of the planar and patterned devices extracted using Matlab-based image analysis technique from 200 μm (diameter) circular unpackaged LEDs.

Modeling of novel hybrid photonic crystal structures involving cured hydrogen silsesquioxane pillars for improving the light extraction in light-emitting diodes

The Solid-State Lighting (SSL) industry utilizes semiconductor based light-emitting diodes (LEDs) as core elements of light sources. LED lighting has several advantages over conventional incandescent bulbs; however, device-level issues such as material quality, low quantum efficiencies, and low light extraction efficiencies still exist. Many techniques have been explored to provide improvement in the area of LED light extraction. Improvement in light extraction efficiency, through the use of integrated optical components such as photonic crystals, is critical for the improvement in the overall efficiency of the device. Fabrication and integration of PhCs into LEDs with little or no degradation in device’s electrical characteristics is an important accomplishment to be considered. Use of electron beam lithography and novel electron beam resists like hydrogen silsesquioxane will allow advancements toward achieving this goal. The unique chemical properties of HSQ allows transformation of the patterned resist into silicon dioxide. This leads to hybrid PhC structures that contain the cured form of HSQ and other materials of interest in an LED. In this work, novel hybrid PhC structures in square and triangular lattice configurations will be modeled to improve light extraction in blue InGaN/GaN based LEDs (λ=465 nm) and attain an optimal structure. Feature sizes from 100 nm to 465 nm will be modeled and the effect of the patterned structure (band gap and/or diffraction) on the light extraction will be studied and analyzed. Simulation data from frequency domain and time domain engines in MPB and OptiFDTD respectively will be analyzed and presented.

Co-molding of nanoscale photonic crystals and microfluidic channel

Photonic crystals are nanofabricated structures that enhance light as it is passed through the constructed design. These structures are normally fabricated out of silicon but have shown to be an improvement if fabricated from a more cost effective material. Photonic crystals have uses within biosensing as they may be used to analyze DNA and other analytes. Microfluidic channels are used to transport different analytes and other samples from one end to another. Microfluidics are used in biosensing as a means of transport and are typically fabricated from biocompatible polymers. Integrated together, the photonic crystals and microfluidic channels would be able to achieve better sensing capabilities and cost effective methods for large scale production. Results will be shown from the co-molding.

A process for co-molding a visible-wavelength photonic crystal and microfluidic channel for biosensing applications

Rapid DNA analysis systems show promise for reduced DNA analysis times and can be used by untrained operators in point-of-use applications. Throughput improvements can be gained by reducing the polymerase chain reaction (PCR) cycle count, which is used in conventional DNA processing to amplify the DNA to an easily measurable amount. A Photonic Crystal (PhC) can be integrated within a microfluidic channel to enhance fluorescence emission, enabling a reduction in PCR cycling. Most PhCs are fabricated using serial top-down fabrication techniques, resulting in a structure that is challenging to integrate with microfluidic system components. Here, we present a co-integration process for fabricating a Silicon master mold consisting of a visible range PhC lattice and a microfluidic channel. This process can be used to co-fabricate microscale channel and nanoscale lattice structures in polymer or thermoplastic materials. Two dimensional visible range PhCs are fabricated by patterning electron beam resist via E-Beam Lithography (EBL). The patterned features (100-300nm features with 200-450nm pitch) are cured to a glass-like material that is used as a direct etch mask for Reactive Ion Etching. A 200μm wide and 25μm high ridge “strip” is fabricated around the PhC region using Photolithography and Deep RIE etching to form the completed channel and lattice mold. Results indicating the quality of nanoscale features resulting from the molding process in Polydimethylsiloxane (PDMS) will be discussed.

The Modeling, Fabrication, and Optical Characterization of Silicon and Polymer- based Photonic Crystals for Biosensing Applications

Two-dimensional, nano-scale photonic crystals (PhCs) in silicon and biocompatible polymer materials, such as: Polydimethylsiloxane (PDMS) and epoxy, are potential core structures in ultra-sensitive biosensors enhancing fluorescence emission in the near IR and visible range. A triangular PhC lattice (r/a = 0.33) of silicon pillars suspended in toluene was designed to enhance emission in the near-IR range. We present here a 27-fold enhancement of PbS-Quantum-Dot emission at 1100 nm. Moving to more biocompatible materials, we also present frequencydomain modeling results demonstrating partial photonic bandgaps for triangular PhC lattices in PDMS and epoxy. The existence of these bandgaps suggests that PhCs in polymer materials could potentially enhance visible-range fluorescence emission and become co-integrated with other on-chip components, such as microfluidic channels and optical waveguides, to produce costeffective biosensors.