Martin B.G. Jun

An experimental evaluation of an atomization-based cutting fluid application system for micromachining

An atomization-based cutting fluid application system is developed for micro-end milling. The system was designed to ensure spreading of the droplets on the workpiece surface based on the analysis of the atomized droplet impingement dynamics. The results of the initial experiments conducted to examine the viability of the system show that the cutting forces are lower and tool life is significantly improved with the atomized cutting fluids when compared to dry and flood cooling methods. Also, application of atomized cutting fluid is found to result in good chip evacuation and lower cutting temperature. Experiments were also conducted to study the effect of fluid properties on cutting performance, and the results show that cutting fluids with lower surface tension and higher viscosity perform better in terms of cutting forces.

Fabrication of poly (ϵ-caprolactone) microfiber scaffolds with varying topography and mechanical properties for stem cell-based tissue engineering applications

Highly porous poly (ϵ-caprolactone) microfiber scaffolds can be fabricated using electrospinning for tissue engineering applications. Melt electrospinning produces such scaffolds by direct deposition of a polymer melt instead of dissolving the polymer in a solvent as performed during solution electrospinning. The objective of this study was to investigate the significant parameters associated with the melt electrospinning process that influence fiber diameter and scaffold morphology, including processing temperature, collection distance, applied, voltage and nozzle size. The mechanical properties of these microfiber scaffolds varied with microfiber diameter. Additionally, the porosity of scaffolds was determined by combining experimental data with mathematical modeling. To test the cytocompatability of these fibrous scaffolds, we seeded neural progenitors derived from murine R1 embryonic stem cell lines onto these scaffolds, where they could survive, migrate, and differentiate into neurons; demonstrating the potential of these melt electrospun scaffolds for tissue engineering applications.

Atomic force microscope probe-based nanometric scribing

Miniaturization of machine components is recognized by many as a significant technological development for a vast spectrum of products. An atomic force microscope (AFM) probe that can exert forces onto a variety of engineering materials is used to perform mechanical scribing at the nanoscale. The success of nanomechanical machining at such fine scales is based on the understanding of microstructural machining mechanics. This paper investigates the cutting behaviour in the nanoscale of a chromium workpiece by using a retrofitted commercial AFM with an acoustic emission sensor, in order to scratch the surface and measure forces. The calibration procedure for acquiring the forces is discussed. The cutting force model, which incorporates the flow stress and friction coefficient in the nano-scale machining, is also presented.

Electrospun biomaterial scaffolds with varied topographies for neuronal differentiation of human-induced pluripotent stem cells

In this study, we investigated the effect of micro and nanoscale scaffold topography on promoting neuronal differentiation of human induced pluripotent stem cells (iPSCs) and directing the resulting neuronal outgrowth in an organized manner. We used melt electrospinning to fabricate poly (ε-caprolactone) (PCL) scaffolds with loop mesh and biaxial aligned microscale topographies. Biaxial aligned microscale scaffolds were further functionalized with retinoic acid releasing PCL nanofibers using solution electrospinning. These scaffolds were then seeded with neural progenitors derived from human iPSCs. We found that smaller diameter loop mesh scaffolds (43.7 ± 3.9 µm) induced higher expression of the neural markers Nestin and Pax6 compared to thicker diameter loop mesh scaffolds (85 ± 4 µm). The loop mesh and biaxial aligned scaffolds guided the neurite outgrowth of human iPSCs along the topographical features with the maximum neurite length of these cells being longer on the biaxial aligned scaffolds. Finally, our novel bimodal scaffolds also supported the neuronal differentiation of human iPSCs as they presented both physical and chemical cues to these cells, encouraging their differentiation. These results give insight into how physical and chemical cues can be used to engineer neural tissue.

An acoustic emission-based method for determining contact between a tool and workpiece at the microscale

An acoustic emission-based touch-off detection system has been developed to determine contact between a rotating microtool and a workpiece surface with micron-level accuracy. The system has been implemented on an existing three-axis microscale machine tool. The system has been tested with microendmills as small as 50 jam in diameter and microdrills as small as 254 μm in diameter. The accuracy of the system has been found to depend on tool geometry and workpiece surface characteristics and is generally on the order of I μm. An analytical model has been constructed to predict touch-off detection error. The calibrated model has been shown to predict surface overshoot and undershoot trends quite well. Simulations have shown that touch-off error is dominated by part surface roughness.

Development of a glial cell-derived neurotrophic factor-releasing artificial dura for neural tissue engineering applications

Encapsulated electrospun nanofibers can serve as an artificial dura mater, the membrane that surrounds the brain and spinal cord, due to their desirable drug delivery properties. Such nanofiber scaffolds can be used to deliver drugs such as glial cell-derived neurotrophic factor (GDNF). GDNF promotes the survival of both dopaminergic and motor neurons, making it an important target for treatment of central nervous system injuries and disorders. This work focuses on designing a novel class of encapsulated poly(ε-caprolactone) (PCL) nanofiber scaffolds with different topographies (random and aligned) that generate controlled release of GDNF to potentially serve as a suitable substitute for the dura mater during neurosurgical procedures. Random and aligned scaffolds fabricated using solution electrospinning were characterized for their physical properties and their ability to release GDNF over one month. GDNF bioactivity was confirmed using a PC12 cell assay with the highest concentrations of released GDNF (∼341 ng mL-1 GDNF) inducing the highest levels of neurite extension (∼556 μm). To test the cytocompatibility of aligned GDNF encapsulated PCL nanofibers, we successfully seeded neural progenitors derived from human induced pluripotent stem cells (hiPSCs) onto the scaffolds where they survived and differentiated into neurons. Overall, this research demonstrates the potential of such substrates to act as artificial dura while delivering bioactive GDNF in a controlled fashion. These scaffolds also support the culture and differentiation of hiPSC-derived neural progenitors, suggesting their biocompatibility with the cells of the central nervous system.

Using mathematical modeling to control topographical properties of poly (ε-caprolactone) melt electrospun scaffolds

Melt electrospinning creates fibrous scaffolds using direct deposition. The main challenge of melt electrospinning is controlling the topography of the scaffolds for tissue engineering applications. Mathematical modeling enables a better understanding of the parameters that determine the topography of scaffolds. The objective of this study is to build two types of mathematical models. First, we modeled the melt electrospinning process by incorporating parameters such as nozzle size, counter electrode distance and applied voltage that influence fiber diameter and scaffold porosity. Our second model describes the accumulation of the extruded microfibers on flat and round surfaces using data from the microfiber modeling. These models were validated through the use of experimentally obtained data. Scanning electron microscopy (SEM) was used to image the scaffolds and the fiber diameters were measured using Quartz-PCI Image Management Systems® in SEM to measure scaffold porosity.

Design and fabrication of auxetic stretchable force sensor for hand rehabilitation

Using a melt electrospinning technique, stretchable force sensors were designed for use in an application of hand rehabilitation. The main purpose of this study was to verify that the use of auxetic sensors improved hand rehabilitation practices when compared to their absence. For this study, novel stretchable poly (-caprolactone) (PCL) force sensors were fabricated into the following formations: auxetic microfiber sheets (AMSs), auxetic solid sheets (ASSs), microfiber sheets (MSs), and solid sheets (SSs). A femtosecond laser device was used to make an auxetic structure in the MSs and SSs. Subsequently, these sensors were coated with gold particles to make them conductive for the electrical current resistance assays. Through the cycles of applied stress and strain, auxetic structures were able to retain their original shape once these forces have been dissipated. This stretchable sensor could potentially measure applied external loads, resistance, and strain and could also be attachable to a desired substrate. In order to verify the workability and practicality of our designed sensors, we have attempted to use the sensors on a human hand. The AMS sensor had the highest sensitivity on measuring force and resistance among the four types of sensors. To our knowledge, this is the first study to form a stretchable force sensor using a melt electrospinning technique.

Biomaterial Strategies for Delivering Stem Cells as a Treatment for Spinal Cord Injury

Ongoing clinical trials are evaluating the use of stem cells as a way to treat traumatic spinal cord injury (SCI). However, the inhibitory environment present in the injured spinal cord makes it challenging to achieve the survival of these cells along with desired differentiation into the appropriate phenotypes necessary to regain function. Transplanting stem cells along with an instructive biomaterial scaffold can increase cell survival and improve differentiation efficiency. This study reviews the literature discussing different types of instructive biomaterial scaffolds developed for transplanting stem cells into the injured spinal cord. We have chosen to focus specifically on biomaterial scaffolds that direct the differentiation of neural stem cells and pluripotent stem cells since they offer the most promise for producing the cell phenotypes that could restore function after SCI. In terms of biomaterial scaffolds, this article reviews the literature associated with using hydrogels made from natural biomaterials and electrospun scaffolds for differentiating stem cells into neural phenotypes. It then presents new data showing how these different types of scaffolds can be combined for neural tissue engineering applications and provides directions for future studies.

A Submicron Multiaxis Positioning Stage for Micro- and Nanoscale Manufacturing Processes

A piezoelectrically driven, submicron XY-positioning stage with multiprocess capability is developed and then integrated into two micro/nanoscale manufacturing processes to improve their performance. The design is based on the HexFlex™ mechanism but is modified to improve structural robustness using a combination of factorial design, linear programming, and finite element analysis. Performance analysis reveals travel ranges of 16 μm (X-axis) and 8 μm (Y-axis), positioning accuracies of 87 nm (X-axis) and 92 nm (Y-axis), and overall stiffnesses of 32 N/μm (X-axis) and 36 N/μm (Y-axis). A comparison of microfluidic channels manufactured with a micromachine tool (mMT) alone and with the stage stacked on the mMT shows an improvement in feature accuracy from 870 nm to 170 nm. The stage is integrated with an electrochemical deposition setup. Nanowire structures with sharp angles are fabricated. The diameter of these nanowires shows an improvement in uniformity by decreasing the standard deviation of diameter variation from 2.088 μm to 0.009 μm.