Pit Fee Jao

Immersion Lithographic Patterning of Electrospun Nanofibers for Carbon Nanofibrous Microelectrode Arrays

Immersion photolithography of electrospun SU-8 nanofibers followed by carbonization has been demonstrated for 3-D carbon nanofibrous microelectrodes. The process and resultant structures offer unique advantages: 1) precise patterning of nanofibrous microstructures via refractive index matching immersion photolithography; 2) good cell/neuron adhesion characteristic of the resultant scaffold attributed to nanomorphology in 100s nanometer scale; 3) high electrical conductivity of the carbonous microelectrodes appropriate for neural stimulation and signal detection; and 4) biocompatibility of the carbon nanofibers. Immiscible refractive index matching liquid, such as oil (noil = 1.47), has been applied to the nonhomogeneous nanofiber stack consisting of SU-8 nanofibers (nSU-8 = 1.67) and air (nair = 1) before ultraviolet light exposure, which greatly suppresses optical diffraction and scattering effects, resulting in high aspect ratio 3-D nanofibrous microstructures and enhanced patterning resolution. The aspect ratio of the fabricated 3D structures is increased from 0.26 to 0.89 and the contrast ratio from 0.56 to 0.96, compared with ones from the nonimmersion process. Ray tracing simulation taking into account diffraction effects in nanofibrous media has been discussed. Microelectrode arrays consisting of integrated nanofibers and thin-film carbon structures are implemented for in vitro neuron culture experiments. Experimental results of structures surface roughness, electrical conductivity, and cell viability on them are detailed.

Fabrication of an all SU-8 electrospun nanofiber based supercapacitor

Supercapacitors (SCs) as energy storage devices are advantageous in their rapid charge/discharge capabilities and their immense charge storage capacity. Two important components of a SC are the electrically conductive electrodes (anode and cathode) and an electrically non-conductive separator between the two electrodes. This paper details a fabrication process for nanofibrous carbon electrodes and a nanoporous polymer separator using all SU-8 based electrospinning and post electrospinning processes, such as lithographical patterning, conversion of the nanofibrous polymer to carbon structures using heat treatment (carbonization) and their assembly to complete a SC. The process produces immensely porous electrodes with good conductivity; it is scalable and economical compared with the carbon nanotube electrode approach. High throughput tube nozzle electrospinning for nanofiber (NF) production and its photolithographical patterning have been employed to facilitate manufacturability. The dependence of the NF morphology on the carbonization temperatures is studied. Also, SC testing and characterization are discussed.

Fabrication of nanoporous membrane and its nonlithographic patterning using Electrospinning and Stamp-thru-mold (ESTM)

The Electrospinning and Stamp-thru-mold (ESTM) technique, an integrated fabrication process which incorporates the versatility of the electrospinning process for nanofiber fabrication with the non-lithographic patterning ability of the stamp-thru-mold process is introduced. In-situ multilayer stacking of orthogonally aligned nanofibers, ultimately resulting in a nanoporous membrane, has been demonstrated using orthogonally placed collector electrode pairs and an alternating bias scheme. The pore size of the nanoporous membrane can be controlled by the number of layers and the deposition time of each layer. Nonlithographic patterning of the fabricated nanoporous membrane is then performed by mechanical shearing using a pair of pre-fabricated micromolds. This patterning process is contamination free compared to other photo lithographical patterning approaches. The ability to pattern on different substrates has been tested with and without oxygen plasma surface treatment. In vitro tests of ESTM poly-lactic-co-glycolic acid (PLGA) nanofibers verify the biocompatibility of this process. Simulation by the COMSOL Multiphysics tool has been conducted for the analysis of electrospun nanofiber alignment.

Fabrication of carbon nanofibrous microelectrode array (CNF-MEA) using nanofiber immersion photolithography

Microelectrode arrays (MEAs) are widely used for stimulating and receiving electrical signals between human and machines and for in vitro neural study. This work demonstrates the fabrication process of nanofibrous 3D microelectrodes using immersion lithography. Oil immersion negates the diffraction effects intrinsic in the photopatterning of electrospun nanofibers to give increased aspect ratio microarchitectures. Nanofiber electrode resistivity is characterized and its performance compared to that of carbon thin film. In vitro testing of electrodes are performed using E18 cortical neurons and analyzed for cell density and cell viability.

Nanomanufacturing of large area carbon nanofibers using tube nozzle electrospinning (TNE), lithography and carbonization processes

This paper reports on the tube nozzle electrospinning (TNE) technique for the large scale production of electrospun nanofibers with a single plug-and-play unit to be added to the conventional electrospinning setup; lithographical micropatterning of the nanofibers; and carbonization of the patterned nanofibers. Nozzle architectures with diameters of 0.2mm and 0.5mm with 1mm and 0.5mm spacing are implemented and tested for nanofiber productivity, porosity, and uniformity. Electrospun SU8 nanofibers are achieved with no beads at an operating voltage of 10kV and a tip-collector-distance (TCD) of 7.5cm. Nanofiber morphologies are characterized with different operating conditions. The diameter of nanofibers ranges from 287nm to 566nm. Nanofibers with a porosity of 29.1% are accomplished with a single two minute electrospinning cycle and an average pore size of 0.31μm2. The TNE approach incorporated with a microcontrolled stage produces a uniform deposition of electrospun SU8 nanofiber on a 6” × 8” collector substrate in a semi-automated process. Studies on nozzle diameter and spacing for maximum liquid throughput, porosity, and uniformity of deposited nanofibers are discussed. Lithographical patterning and carbonization of the electrospun nanofibers are performed as post electrospun processes to produce patterned carbon nanofibers.

Scale of Carbon Nanomaterials Affects Neural Outgrowth and Adhesion

Carbon nanomaterials have become increasingly popular microelectrode materials for neuroscience applications. Here we study how the scale of carbon nanotubes and carbon nanofibers affect neural viability, outgrowth, and adhesion. Carbon nanotubes were deposited on glass coverslips via a layer-by-layer method with polyethylenimine (PEI). Carbonized nanofibers were fabricated by electrospinning SU-8 and pyrolyzing the nanofiber depositions. Additional substrates tested were carbonized and SU-8 thin films and SU-8 nanofibers. Surfaces were O2-plasma treated, coated with varying concentrations of PEI, seeded with E18 rat cortical cells, and examined at 3, 4, and 7 days in vitro (DIV). Neural adhesion was examined at 4 DIV utilizing a parallel plate flow chamber. At 3 DIV, neural viability was lower on the nanofiber and thin film depositions treated with higher PEI concentrations which corresponded with significantly higher zeta potentials (surface charge); this significance was drastically higher on the nanofibers suggesting that the nanostructure may collect more PEI molecules, causing increased toxicity. At 7 DIV, significantly higher neurite outgrowth was observed on SU-8 nanofiber substrates with nanofibers a significant fraction of a neurons size. No differences were detected for carbonized nanofibers or carbon nanotubes. Both carbonized and SU-8 nanofibers had significantly higher cellular adhesion post-flow in comparison to controls whereas the carbon nanotubes were statistically similar to control substrates. These data suggest a neural cell preference for larger-scale nanomaterials with specific surface treatments. These characteristics could be taken advantage of in the future design and fabrication of neural microelectrodes.

A carbon nanofiber (CNF) based 3-D microelectrode array for in-vitro neural proliferation and signal recording

Microelectrode arrays (MEAs) are commonly utilized for stimulating and recording extracellular electrical signals including both local field and action potentials in both in-vitro and in-vivo neural studies. This work demonstrates the feasibility of 3D microelectrode arrays using a novel carbon nanomaterial, electrospun carbon nanofiber (CNF). CNF MEAs impedance is characterized and compared to that of carbon nanotube MEAs and commercial TiN MEAs. An in-vitro culture of CNF electrodes are performed using E18 cortical neurons and analyzed for cell interaction. With these electrodes we are able to detect extracellular neural signals including action potentials from single neurons from an array of CNF electrodes embedded within the substrate of an MEA.

Fabrication of Self-Packaged Seamless Nanoporous SU-8 Microchannels

Fabrication of a self-packaged partial semicircular nanoporous microchannel is demonstrated using electrospinning, self-limiting ultraviolet lithography, and thermal-reHow packaging processes with SU-8. The fabrication process offers unique advantages of microresolution channel patterning, alignment free seamless integration of nanofibers in a microchannel by self-packaged reHow, and easy formation of 3-D nanoporous channels. A fabricated nanoporous microchannel with a channel length of 7 mm, a channel width of 73.85 μm, and a channel height of 34.65 μm shows slow Huidic How through the channel, functioning as an effective nanochannel with an equivalent channel height of 800 nm and a radius of 857 nm, and therefore, its reduction ratios of the channel height and radius are ~97.7% and 97.8%, respectively.

Mechano-active tissue scaffold system based on a magnetic nanoparticle embedded nanofibrous membrane

A mechano-active nanofibrous scaffold system for in vitro active cell culture is fabricated and demonstrated using electrospun nanofibers with magnetic nanoparticles embedded, and an electromagnet. The electrospun nanofiber consists of polycaprolactone and iron oxide nanoparticles. The magnetic nanofibrous membrane is held by micromachined printing circuit board (PCB) O-rings and remotely actuated by an electromagnet, which generates alternating current (AC) magnetic fields. The scaffold provides mechanical stress and strain on culturing cells in response to external AC magnetic fields. The mechanical properties of the magnetic nanoporous membrane including the density, porosity, and effective Youngs modulus are characterized. Cell viabilities on the nanofibrous membrane with and without magnetic nanoparticles embedded have been tested.