Hae Woon Choi

Micropatterning and characterization of electrospun poly(ε-caprolactone)/gelatin nanofiber tissue scaffolds by femtosecond laser ablation for tissue engineering applications.

Experimental investigations aimed at assessing the effectiveness of femtosecond (FS) laser ablation for creating microscale features on electrospun poly(ε‐caprolactone) (PCL)/gelatin nanofiber tissue scaffold capable of controlling cell distribution are described. Statistical comparisons of the fiber diameter and surface porosity on laser‐machined and as‐spun surface were made and results showed that laser ablation did not change the fiber surface morphology. The minimum feature size that could be created on electrospun nanofiber surfaces by direct‐write ablation was measured over a range of laser pulse energies. The minimum feature size that could be created was limited only by the pore size of the scaffold surface. The chemical states of PCL/gelatin nanofiber surfaces were measured before and after FS laser machining by attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectroscopy and X‐ray photoelectron spectroscopy (XPS) and showed that laser machining produced no changes in the chemistry of the surface. In vitro, mouse embryonic stem cells (mES cells) were cultured on as‐spun surfaces and in laser‐machined microwells. Cell densities were found to be statistically indistinguishable after 1 and 2 days of growth. Additionally, confocal microscope imaging confirmed that spreading of mES cells cultured within laser‐machined microwells was constrained by the cavity walls, the expected and desired function of these cavities. The geometric constraint caused statistically significant smaller density of cells in microwells after 3 days of growth. It was concluded that FS laser ablation is an effective process for microscale structuring of these electrospun nanofiber tissue scaffold surfaces. Biotechnol. Bioeng. 2011; 108:116–126.

Structuring electrospun polycaprolactone nanofiber tissue scaffolds by femtosecond laser ablation

Meshes of electrospun (ES) polycaprolactone (PCL) and polyethylene terephthalate nanofiber meshes were structured by ablation of linear grooves with a scanned femtosecond laser. Focus spot size, pulse energy, and scanning speed were varied to determine their affects on groove size and the characteristics of the electrospun fiber at the edges of these grooves. The femtosecond laser was seen to be an effective means for flexibly structuring the surface of ES PCL scaffolds. Femtosecond ablation resulted in much more uniformly ablated patterns compared to Q-switched nanosecond pulse laser ablation. Also, the width of the ablated grooves was well controlled by laser energy and focus spot size, although the grooves were significantly larger than the spot size. Also, some melting of fibers was observed at the edges of grooves. These affects were attributed to optical radiation from laser-induced plasma at higher pulse energies and melting of fibers at laser fluences lower than the ablation threshold. The ablatio...

Femtosecond laser micromachining of dielectric materials for biomedical applications

Techniques for microfluidic channel fabrication in soda-lime glass and fused quartz using femtosecond laser ablation and ablation in conjunction with polymer coating for surface roughness improvement were tested. Systematic experiments were done to characterize how process variables (laser fluence, scanning speed and focus spot overlap, and material properties) affect the machining feature size and quality. Laser fluence and focus spot overlap showed the strongest influence on channel depth and roughness. At high fluence, the surface roughness was measured to be between 395 nm and 731 nm RMS. At low fluence, roughness decreased to 100 nm–350 nm RMS and showed a greater dependence on overlap. The surface roughness of laser ablation was also dependent on the material properties. For the same laser ablation parameters, soda-lime glass surfaces were smoother than fused quartz. For some applications, especially those using quartz, smoother channels are desired. A hydroxyethyl methacrylate (HEMA) polymer coating was applied and the roughness of the coated channels was improved to 10–50 nm RMS.

Femtosecond laser bulk micromachining of microfluid channels in poly(methylmethacrylate)

Internal channels in a polymer are widely used in biotechnology applications such as DNA stretching and in devices such as micrototal analysis systems and lab on a chip systems. For manufacturing prototype devices, femtosecond pulsed laser energy has been used to implement a convenient direct write bulk-machining process in glass. In this technique, the laser beam is focused inside of a transparent material, resulting in the ablation of an internal channel. Initial experiments for internal channel fabrication in a poly(methylmethacrylate) (PMMA) polymer revealed a significant problem with clogging of channels by debris and rough, fractured channel walls. In this article, we describe a new method to fabricate internal channels in PMMA using femtosecond pulsed laser energy and a gas-assisted material removal concept. Relatively smooth channels with a minimum diameter of 2 μm, a maximum diameter of 20 μm, and a maximum length of 10 mm were achieved with this technique.

Femtosecond laser micromachining and application of hot embossing molds for microfluid device fabrication

Femtosecond laser micromachining was used to create microfluid channel patterns on AISI 304L stainless steel to fabricate hot embossing mold patterns. To characterize the machining process, measurements of single pulse ablation spots with pulse energies from 13 to 500 nJ were used to determine ablation threshold as 0.19 J/cm2. The volume of material removed per pulse increased steadily with pulse energy but ablation efficiency relative to the evaporation enthalpy of removed material had its maximum value of 2% at 65 nJ. Molds with fluid channel features 12 μm deep and as narrow as 8 μm wide could be machined by laser ablation. A mold with channels 75 μm wide and 10 μm deep was machined and used to emboss a microfluidic micromolecular magnetic separator pattern in polymethyl macryolate polymer.

Direct write patterning of ito film by femtosecond laser ablation

Indium tin oxide (ITO) is a commonly used conducting transparent oxide film (CTO) used in flat panel display applications. The ITO needs to be patterned during panel fabrication to form pixels and to repair defects. Direct write laser ablation is sometimes employed for this purpose and it is important that the substrate material and remaining ITO be affected as little as possible by the laser ablation. In this investigation, femtosecond laser ablation of ITO was studied to identify laser processing parameters which cleanly ablated ITO with a minimum of damage to a glass substrate and surrounding ITO. The material used for the experiments was glass with a deposited ITO film thickness of approximately 150 nm. The Ti:Sapphire chirp pulse amplified femtosecond laser used for the experiments had a wavelength of 775nm and produced pulses with a duration of 150 fs at a rate of 2 kHz. The pulse energy was attenuated with thin film polarizers and was focused by a 10x microscope objective with NA=0.6 and 25.4 mm focal length achromatic lens, producing a calculated minimum spot sizes of 2.5 µm and 6.5 µm respectively. The fiber-based femtosecond laser used for experiments had a wavelength of 1045 nm and pulses with a duration of 500 fs at a rate of 100 kHz. It was focused with a 20x microscope objective to spot sizes of 3.6 µm and 6.3 µm. Ablation was carried out at a sufficiently high panel scanning speed that single ablation spots could be studied. The pulse energy was adjusted to determine feasible spot diameters and depths which could be ablated into the ITO without damaging the glass substrate. Ablation of lines without glass damage was also demonstrated. The beam from the Ti:Sapphire laser was also used focused at the tip of an atomic force microscope (AFM) and the feasibility of nanomachining was demonstrated with this apparatus.Indium tin oxide (ITO) is a commonly used conducting transparent oxide film (CTO) used in flat panel display applications. The ITO needs to be patterned during panel fabrication to form pixels and to repair defects. Direct write laser ablation is sometimes employed for this purpose and it is important that the substrate material and remaining ITO be affected as little as possible by the laser ablation. In this investigation, femtosecond laser ablation of ITO was studied to identify laser processing parameters which cleanly ablated ITO with a minimum of damage to a glass substrate and surrounding ITO. The material used for the experiments was glass with a deposited ITO film thickness of approximately 150 nm. The Ti:Sapphire chirp pulse amplified femtosecond laser used for the experiments had a wavelength of 775nm and produced pulses with a duration of 150 fs at a rate of 2 kHz. The pulse energy was attenuated with thin film polarizers and was focused by a 10x microscope objective with NA=0.6 and 25.4 mm fo...

Optical scattering in electrospun poly(ε-caprolactone) tissue scaffolds

Optical scattering in electrospun poly(e-caprolactone) (ES-PCL) nanofibers was studied. Femtosecond laser beams with wavelengths of 775 and 387.5 nm were directed onto PCL nanofiber meshes of different thicknesses, and the reflection and transmission were measured by using an integrating sphere. Meshes were prepared by electrospinning PCL in acetone and dichloromethane (DCM). The absorption and scattering coefficients of the samples were calculated using a three-flux scattering approximation. The PCL/acetone meshes had finer fibers, smaller pore size, and 50% larger scattering coefficients than the PCL/DCM meshes. In addition, somewhat higher scattering coefficients were measured at shorter wavelength in both PCL/Ace and PCL/DCM nanofibers. However, in all cases, scattering coefficients were 15 to 30 times the absorption coefficients; thus, scattering was the dominant factor in optical attenuation in both types of meshes and at both wavelengths.

Electrical discharges at small gap lengths stimulated by femtosecond laser pulses

The electrical characteristics of current-controlled electrical discharges formed in micro- and nano-scale gaps between sharpened metal cathodes and metal anodes were studied. When electrical potential was applied to the tip, transient electrical discharges were observed to form at a field of about 3×108V/m. Irradiation of the tip/gap/substrate area with a femtosecond laser beam was found to decrease the field required for spark initiation. The length of current-controlled discharges increased with voltage. The surface modification at the gold film anodes was found to be well-controlled when current was regulated and the sharpened metal cathode tips were not much modified by discharges.

Micropatterning and characterization of electrospun PCL/gelatin nanofiber tissue scaffolds by femtosecond laser ablation

PCL/gelatin is a biodegradable polymer blend with good biocompatibility, mechanical, physical and chemical properties that is a promising material for use in fabrication of electrospun (ES) nanofiber tissue engineering scaffolds. Although ES nanofiber scaffolds provide a favorable extracellular matrix, microscale surface patterns are needed for study and control of cell adhesion, proliferation and migration. However, micro-patterning of ES polymer/biomaterial blended nanofibers with available techniques is difficult due to its fibrous surface texture and biomaterial content. In this work, the effects of direct-write femtosecond laser ablation on electrospun PCL/gelatin nanofiber tissue scaffold surface morphology and chemistry and resulting growth rate of deposited cells was investigated. Scanning electron microscope (SEM) images showed that laser ablation did not change the statistical distributions of fiber diameter and surface porosity. Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR)...

Femtosecond laser micromachining of metal surfaces for lubrication enhancement

High interface friction is a primary cause for adhesive pickup in cold forging and extrusion of aluminum alloys and of wear of sliding surfaces in thrust bearings and seals. This paper describes investigations of femtosecond laser texturing for reduction of interface friction on sliding surfaces. Direct-write femtosecond pulsed laser micromachining is used to create microgroove patterns with varying size and density on metal forming dies. Ablation of patterned textures using a Digital Light Processor (DLP) programmable mask and a homogenized femtosecond laser beam is also studied. The achievable resolution and depth profiles and material removal rate for laser micromachining of lubrication-enhancing features is characterized. In metal forming tests, the effectiveness of various laser–machined patterns for enhancing interface lubrication is determined.High interface friction is a primary cause for adhesive pickup in cold forging and extrusion of aluminum alloys and of wear of sliding surfaces in thrust bearings and seals. This paper describes investigations of femtosecond laser texturing for reduction of interface friction on sliding surfaces. Direct-write femtosecond pulsed laser micromachining is used to create microgroove patterns with varying size and density on metal forming dies. Ablation of patterned textures using a Digital Light Processor (DLP) programmable mask and a homogenized femtosecond laser beam is also studied. The achievable resolution and depth profiles and material removal rate for laser micromachining of lubrication-enhancing features is characterized. In metal forming tests, the effectiveness of various laser–machined patterns for enhancing interface lubrication is determined.