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1. Atomistic Investigation of Material Deformation Behavior of Polystyrene in Nanoimprint Lithography

   https://doi.org/10.3390/surfaces3040043  

   Odujole, Jahlani ; Desai, Salil ( December 2020 , Surfaces)

   null (Ed.)

   This research investigates deformation behavior of polystyrene (PS) as a thermoplastic resist material for the thermal nanoimprint lithography (T-NIL) process. Molecular dynamics modeling was conducted on a PS substrate with dimensions 58 × 65 × 61 Å that was imprinted with a rigid, spherical indenter. The effect of indenter size, force, and imprinting duration were evaluated in terms of indentation depth, penetration depth, recovery depth, and recovery percentage of the polymer. The results show that the largest indenter, regardless of force, has the most significant impact on deformation behavior. The 40 Å indenter with a 1 µN of force caused the surface molecules to descend to the lowest point compared to the other indenters. An increase in indenter size resulted in higher penetration depth, recovery depth, and recovery percentage. Higher durations of imprint cycle (400 fs) resulted in plastic deformation of the PS material with minimal recovery (4 Å). The results of this research lay the foundation for explaining the effect of several T-NIL process parameters on virgin PS thermoplastic resist material. 

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2. Molecular Dynamics Study of the Quenching Effect on Direct Nanoimprint of Gold

   Abhaysinh Gaikwad, Jahlani Clarke ( May 2019 , Proceedings of the 2019 IISE Annual Conference)

   Nanoimprinting of gold has the potential to make significant impact in the fabrication of miniaturized devices including optical devices, photonic crystals and biochemical sensors. The deformation behavior and recrystallization of gold structures has profound impact on the accurate replication of the imprinted pattern. An important step in the nanoimprint lithography (NIL) process is the cooling of the mold and resist assembly to room temperatures. The rate of cooling, commonly termed as quenching determines the final shape of the NIL features. Molecular dynamics simulations were implemented to study the nanoimprinting of gold substrates using a silicon mold. Simulations were conducted at room temperature (298 K), gold recrystallization (473K) and melting point (1000K) temperatures. The effect of different cooling rates (quenching) on the deformation behavior and spring back of gold was investigated. These include 0.01ns, 0.05ns, and 0.2ns time steps of quenching at 1fs per time step. The modified embedded atom method (MEAM) potential was used for the system molecules. Fast cooling at 298 K and 473 K whereas, intermediate cooling at 1000 K resulted in good pattern transfer. High fidelity nanoscale features were achieved with a gradient cooling proportional to the initial heating temperatures. The findings of this research provide insight on the effect of quenching in the material deformation and spring back during direct metal nanoimprinting. 

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3. Development of a Resilience Parameter for 3D-Printable Shape Memory Polymer Blends

   https://doi.org/10.3390/ma16175906  

   Cavender-Word, Truman J. ; Roberson, David A. ( September 2023 , Materials)

   The goal of this paper was to establish a metric, which we refer to as the resilience parameter, to evaluate the ability of a material to retain tensile strength after damage recovery for shape memory polymer (SMP) systems. In this work, three SMP blends created for the additive manufacturing process of fused filament fabrication (FFF) were characterized. The three polymer systems examined in this study were 50/50 by weight binary blends of the following constituents: (1) polylactic acid (PLA) and maleated styrene-ethylene-butylene-styrene (SEBS-g-MA); (2) acrylonitrile butadiene styrene (ABS) and SEBS-g-MA); and (3) PLA and thermoplastic polyurethane (TPU). The blends were melt compounded and specimens were fabricated by way of FFF and injection molding (IM). The effect of shape memory recovery from varying amounts of initial tensile deformation on the mechanical properties of each blend, in both additively manufactured and injection molded forms, was characterized in terms of the change in tensile strength vs. the amount of deformation the specimens recovered from. The findings of this research indicated a sensitivity to manufacturing method for the PLA/TPU blend, which showed an increase in strength with increasing deformation recovery for the injection molded samples, which indicates this blend had excellent resilience. The ABS/SEBS blend showed no change in strength with the amount of deformation recovery, indicating that this blend had good resilience. The PLA/SEBS showed a decrease in strength with an increasing amount of initial deformation, indicating that this blend had poor resilience. The premise behind the development of this parameter is to promote and aid the notion that increased use of shape memory and self-healing polymers could be a strategy for mitigating plastic waste in the environment.

    

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4. Molecular Dynamics Investigation of the Deformation Mechanism of Gold with Variations in Mold Profiles during Nanoimprinting

   https://doi.org/10.3390/ma14102548  

   Gaikwad, Abhaysinh ; Desai, Salil ( May 2021 , Materials)

   null (Ed.)

   Understanding the deformation behavior during nanoimprint lithography is crucial for high resolution patterning. Molecular dynamics modeling was implemented to investigate the effect of different mold profiles (cylindrical, rectangular, and spherical) on the von Mises stress, lattice dislocations, and material deformation. Relatively higher von Mises stress (1.08 × 107 Pa) was observed for the spherical mold profile compared to the rectangular and cylindrical profiles due to the larger surface area of contact during the mold penetration stage of NIL. Substantial increases in the von Mises stress were observed for all the mold geometries during the mold penetration stage. The von Mises stresses had a reduction in the relaxation and mold retrieval stages based on the rearrangement of the gold atoms. The lattice dislocation during the deformation process revealed the formation of the BCC structure which further reverted to the FCC structure after the mold retrieval. The polyhedral template matching (PTM) method was used to explain the retention of the FCC structure and subsequent ductile behavior of the substrate. The cylindrical mold had the lowest percentage spring back in both of the orthogonal directions and thus replicated the mold profile with high-fidelity as compared to the spherical and rectangular molds. The findings of this research can aid the design of molds for several applications. 

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5. Laser Fabrication of Polymeric Microneedles for Transdermal Drug Delivery

   Aldawood, Faisal ; Desai, Salil ; Andar, Abhay ( January 2021 , Proceedings of the 2021 IISE Annual Conference)

   null (Ed.)

   Microneedles provide a transdermal pathway for drug delivery, cosmetic infusion, vaccine administration, and disease diagnostics. Microneedle fabrication relies on the interplay of several variables which include design parameters, material properties, and processing conditions. In this research, our group explores the effect of design parameters and process variables for laser ablation of microneedles within a Polymethyl methacrylate (PMMA) mold. An Ytterbium laser (200W) was utilized to study the effect of five inputs factors (laser power, pulse width, number of repetitions, laser waveform, and interval time between laser pulses) on two output factors (diameter and height) of the fabricated microneedles. Polydimethylsiloxane (PDMS) polymer was cast within the PMMA microneedle mold. Scanning electron microscopy (SEM) was employed to image topographical features of the microneedles. Further, mechanical testing of the microneedles was conducted to evaluate the buckling load and deformation behavior of the microneedle array. A 20W pulse laser with trapezoidal waveform resulted in optimal microneedle topography with an aspect ratio of 1.2. ANOVA results (α = 0.05) depicted that laser power and number of repetitions were significant factors determining the geometrical features of the microneedle array. This research establishes a framework for the design and manufacturing of customized microneedles for precision medicine. 

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