Polymer Pillars Research Articles

Flexible FPI sensor fabricated by fiber end photopolymerization and integration for high sensitivity gas pressure sensing

This work presents a new design framework for highly sensitive fiber gas pressure sensor. We construct a Flexible Fabry–Perot Interferometer (F-FPI), which consists of three polymer pillars and an end cap, on a fiber end face. A unique fiber-end photopolymerization and integration (FEPI) technique is adopted for fabricating the device. The varied gas pressure in the enclosed space not only changes the refractive index (RI) of the gas but also the length of the pillars. Therefore the difference in the optical path length of the FPI is more sensitive to pressure changes. Experimental results indicate that the F-FPI realizes a gas pressure sensitivity up to 79.0 pm/kPa in the range of 0–200 kPa. In addition, the Vernier effect is introduced to amplify the sensitivity to −409.7 pm/kPa. The temperature compensation is achieved by cascading a Fiber Bragg Grating (FBG) behind the F-FPI. Such a design framework provides a new idea for highly sensitivity gas pressure sensing.

Tailored silver coated polymer pillar metasurfaces for near-infrared refractive index sensing applications

In this work, we report the design, numerical simulation, and fabrication of plasmonic metasurfaces consisting of two-dimensional array arranged polymer pillars on glass substrate coated by a thin silver layer. Silver coated polymer pillar metasurfaces with various silver thicknesses are designed and their optical characteristics analyzed with a finite-difference time-domain (FDTD) method. The designed structures for near-infrared refractive index (RI) sensing show the sensitivities and figure-of-merits (FOM) of ∼1438.3 nm/RIU and ∼71.5 RIU−1, for gaseous media, and ∼1444.0 nm/RIU and ∼59.2 RIU−1 for liquid media, respectively. By introducing and applying the fabrication processes of 3D laser direct writing combined with sputtering technique, we have demonstrated the facile implementation of the plasmonic metasurfaces working in the near-infrared region without using the precise fabrication techniques. This work will provide general guidelines and useful approaches for plasmonic metasurface device design.

Pick and Place of Sensitive Chips with Vacuum-Free Gecomer® Tools

To cope the lack of suitable pick and place methods for sensitive chips, we present a new process using a gripper tool from the company INNOCISE called Gecomer®. The tool, made of polymer pillars, is using Van der Waals forces to create adhesion between the gripping tool and the chip surface. Hence, the tool works without any vacuum. While placing, the Van der Waals interactions are reduced by buckling those pillars resulting in a decreased contact area. The challenge is to find the right parameters to avoid a re-gripping of the chip during the Gecomer®-movement away from the chip. In this paper we present results on the placement accuracy including the reproducibility compared to a standard vacuum tool for different process parameters. For that, we used a pick and place machine with built-in inspection camera to examine the deviation from the target position. It is revealed that both vacuum and polymer gripper have a similar deviation of approx. ±15 μm. Hence, the placing accuracy is not negatively influenced by the Gecomer® tool and depends only on the machine parameters. Furthermore, we analyze the surfaces of the chips, whether there are any residues of the gripping tool. In a first optical inspection, no damages or residues could be found. The new vacuum-free pick and place process has the potential to open new opportunities in the assembly of sensitive (MEMS)- chips regarding reproducibility, accuracy, and process throughput.

Lab-on-fiber: laser-induced micro-cavity for a relative humidity measurement.

The lab-on-fiber design philosophy is the foundation for creating high-performance integrated fiber sensors. Hence, this Letter proposes an ultra-compact Fabry-Perot interferometer (FPI) based on a laser-induced micro-cavity (LIMC-FPI) on a fiber end for measuring relative humidity. To our knowledge, this novel approach, named the fiber-end photopolymerization (FEP) technique, is applied to create a micro-cavity. Specifically, a pair of humidity-sensitive polymer pillars and a resin end cap obtained by FEP are integrated to generate the cavity. As the ambient humidity changes, the pillars lengthen or shorten, resulting in the spectral evolution of the LIMC-FPI. A typical humidity sensitivity of 0.18 nm/%RH is obtained experimentally. For monitoring the human breathing process, the LIMC-FPI is responsive in the breathing frequency range of 0.2 to 0.5 Hz, allowing a response and recovery time of less than 0.388 s and 1.171 s, respectively. This work introduces a fresh and cost-effective approach for developing lab-on-fiber concept-based sensors.

Fabrication of amino and fluorine functionalized graphene-based polymer composites to enhance the electromechanical conversion efficiency of TENGs for energy-harvesting applications

The two main factors hindering the practical utilization of triboelectric nanogenerators (TENGs) are low power density and high internal impedance. To address this issue, TENGs with high electromechanical conversion efficiency must be developed. Several techniques have been investigated to improve the TENGs output performance, including surface modifications (physical or chemical) and embedded charge-trap nanomaterials (dielectric or conductive). Herein, we introduce the novel amino-functionalized reduced graphene oxide (AFGO) and graphite-fluorinated polymer (FG) pillars to enhance the tribo-positivity and tribo-negativity of polyurethane (PU) and Ecoflex (EC), respectively. The surface positive-potential of PU is increased almost 2.2 times (from 201 V to 480 V) and negative-potential of Ecoflex is increased 2.1 times (from −848 V to −1813 V) with incorporating AFGO and FG pillars. Functionalized graphene and graphite-based pillars synergistically enhance surface charges (owing to the presence of –NH2 and -F terminating groups) and minimize triboelectric loss (owing to their conductive nature). The effects of the AFGO and FG contents on the electrical performance were studied systematically. The optimized PUAFGO2/EC15FG-TENG exhibited the maximum electrical output performance with an open-circuit potential of 434 V, a short-circuit current of 11. 2 µA, a power density of 1.18 Wm−2 at a load of 40 MΩ, and a charge density of 17.5 µCm−2. The PUAFGO2/EC15FG-TENG showed a mechanical conversion efficiency of 95% and successfully operated 33 LEDs and a stopwatch. Owing to this effectivity, even under low pressures, with a high sensitivity of 9.5 VPa−1 and high mechanical stability, the proposed TENG device could be used as a biomechanical energy harvester in the near future.

Inkjet‐Printed Flexible Semitransparent Solar Cells with Perovskite and Polymeric Pillars

Flexible Solar Cells In article number 2200988, Naresh Kumar Pendyala, Shlomo Magdassi, and Lioz Etgar present the fabrication of flexible and semitransparent perovskite-based solar cells made by an inkjet printing process using polymer pillars for existing windows as retrofitting process. Designed by Ehsan Faridi and Ehsan Keshavarzi - Inmywork Studio.

Influence of Oblique Angle Deposition on Porous Polymer Film Formation.

In this study, we applied oblique angle deposition to a modified initiated chemical vapor deposition (iCVD) process to synthesize porous poly(methacrylic acid) (PMAA) films. During the modified iCVD process, frozen monomer molecules are first captured on a cooled substrate, then polymerization occurs via a free radical polymerization mechanism, and finally, the excess monomer is sublimated, resulting in a porous polymer film. We found that delivering the monomer through an extension at an oblique angle resulted in porous films with three morphological regions. Region 1 is located nearest to the monomer extension outlet and consists of porous polymer pillars; region 2 consists of densified pillars, which occur due to the recapturing and polymerization of the sublimated monomer; and region 3 is located furthest from the monomer extension outlet and consists of dendritic structures, which occur due to low monomer concentration. We investigated the role of substrate temperature and monomer deposition time on the growth process. We found that changing the extension angle influenced the location of the regions and the film coverage across the substrate. Our results provide useful guidelines for tuning the structures within porous polymer films by varying the angle of monomer delivery.

Gecko-Inspired Adhesives with Asymmetrically Tilting-Oriented Micropillars.

The anisotropic adhesion behavior of the gecko is closely related to their feet and is comprised of keratinous hairs, setae, where van der Waals forces permit attachment and detachment during locomotion. Previous research either achieved only isotropic adhesion behaviors or involved the complicated photolithography method. Here, we reported a simple way to achieve the anisotropic adhesion behaviors of gecko-inspired adhesives, consisting of micropillars with asymmetrically tilted orientation via the 3D printing technique. The adhesive forces of structured polymer pillars achieved 4-fold stronger, compared to controls with the plain surface. The anisotropic adhesion behavior is presented on the patterned surface and is two times stronger along the gripping direction compared to the releasing direction on the adhesives, which is attributed to the asymmetric stress distributions at the edges, as well as the stresses resulting from the moment with the sheared top. The finite element analysis is applied to demonstrate the stress distributions and displacement variations. This work provides the insight into the design and fabrication of gecko-inspired adhesives with anisotropic adhesion behaviors in practical applications.

3D Printing-Assisted Soft Capacitive Inclinometers for Simultaneous Monitoring of Tilt Angles and Directions

This study presents capacitive-type inclinometers composed of flexible polymer pillars and dome-shaped roof frames that were manufactured using the three-dimensional (3D) printing method. Polylactic acid (PLA) filaments and acrylonitrile butadiene styrene (ABS) filaments were printed by the fused deposition modeling type 3D printer to fabricate the dome-shaped roof frames and the polymer curing molds, respectively. The operating principle of the inclinometer was to detect the change in capacitance between the helix-shaped electrode coiled around the polymer pillar and the built-in electrode in the roof frame. When the inclinometer was tilted, the polymer pillar was bent, and the physical distance between each electrode was changed with respect to the tilt angle and direction. Therefore, the tilt angles and directions were simultaneously estimated by distinguishing the capacitance and peak capacitance, respectively. The results of the experiments revealed that the inclinometer using the polymer pillar that was electrically connected to a standard weight and the roof frame with a roof angle of 45° exhibited a higher sensitivity (1.391 pF at a tilt angle of 40°) compared to those using roof angles of 90° and 135°. This study supports the use of 3D printing technology for the facile manufacturing of inclinometers that can detect tilt angles and directions simultaneously, which is not achievable with conventional inclinometers.

High Aspect Ratio and Light-Sensitive Micropillars Based on a Semiconducting Polymer Optically Regulate Neuronal Growth.

Many nano- and microstructured devices capable of promoting neuronal growth and network formation have been previously investigated. In certain cases, topographical cues have been successfully complemented with external bias, by employing electrically conducting scaffolds. However, the use of optical stimulation with topographical cues was rarely addressed in this context, and the development of light-addressable platforms for modulating and guiding cellular growth and proliferation remains almost completely unexplored. Here, we develop high aspect ratio micropillars based on a prototype semiconducting polymer, regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT), as an optically active, three-dimensional platform for embryonic cortical neurons. P3HT micropillars provide a mechanically compliant environment and allow a close contact with neuronal cells. The combined action of nano/microtopography and visible light excitation leads to effective optical modulation of neuronal growth and orientation. Embryonic neurons cultured on polymer pillars show a clear polarization effect and, upon exposure to optical excitation, a significant increase in both neurite and axon length. The biocompatible, microstructured, and light-sensitive platform developed here opens up the opportunity to optically regulate neuronal growth in a wireless, repeatable, and spatio-temporally controlled manner without genetic modification. This approach may be extended to other cell models, thus uncovering interesting applications of photonic devices in regenerative medicine.

Data reduction of friction factor, permeability and inertial coefficient for a compressible gas flow through a milli-regenerator

A regenerator of a Stirling machine alternately absorbs and releases heat from and to the working fluid which allows to recycle rejected heat during theoretical isochoric processes. This work focuses on a milli-regenerator fabricated with a multiple jet molding process. The regenerator is a porous medium filled with a dense pillar matrix. The pillars have a geometrical lens shape. Two metallic layers (chromium and copper) are deposited on the polymer pillars to increase heat transfer inside the regenerator. We performed experiments on different milli-regenerators corresponding to three porosities (ε = 0.80, 0.85 and 0.90) under nitrogen steady and oscillating compressible flows (oscillating Reynolds number in the range 0 < Reω < 60 and Reynolds number based on the hydraulic diameter ReDh,max<6000) for different temperature gradients (ΔT < 100°C). Temperature, velocity and pressure experimental measurements are performed with microthermocouples (type K with 7,6 µm diameter), hotwires and miniature pressure sensors, respectively. We identified a threeterm composite correlation equation for the friction factor based on a Darcy-Forchheimer flow model that best-fit the experimental data. In steady and oscillating flows permeabilities and inertial coefficients are of the same magnitude order. Inertial coefficients decrease when the porosities increase.

UV LED Assisted Printing Platform for Fabrication of Micro-Scale Polymer Pillars

Ultraviolet (UV) assisted printing has emerged as a promising additive manufacturing (AM) technology, which puts UV light sources with focused emission in high demand. Unfortunately, current ultraviolet light-emitting diodes (UV LEDs) are not able to provide adequately focused UV emission required for printing. Thus, a UV LED package with focused UV emission was proposed and fabricated using microelectromechanical system (MEMS) technology in this study. An optical model of the UV LED package with focused UV emission was developed. Based on the designed model, a silicon reflector stacked on another was achieved. By assembling the stacked silicon reflector and hemispherical quartz lenses, the UV LED package with focused UV emission was successfully achieved. A narrow beam with a beam angle of 16° between half maximum intensity was achieved, while the intensity at 0° was greater than 1000 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at the focal point. The UV LED curing system using two focused UV LED packages was successfully developed based on the optical simulation results. Polymer pillars were fabricated by instantaneous UV curing process with synchronized UV curable polymer jetting process in one shot, and micro-scale polymer pillars with a diameter of 20 μm were achieved. Thanks to the high intensity and focused UV emission, polymer pillars can be formed in as little as 100 ms. 13600 polymer pillars were printed on a silicon wafer with uniform diameter and geometry in a single dispensing cycle. The UV LED assisted printing platform was successfully demonstrated for fabrication of micro-scale 3D structures in a fast speed with smooth printing process.

Nanogap Plasmonic Structures Fabricated by Switchable Capillary‐Force Driven Self‐Assembly for Localized Sensing of Anticancer Medicines with Microfluidic SERS

AbstractNanogap plasmonic structures, which can strongly enhance electromagnetic fields, enable widespread applications in surface‐enhanced Raman spectroscopy (SERS) sensing. Although the directed self‐assembly strategy has been adopted for the fabrication of micro/nanostructures on open surfaces, fabrication of nanogap plasmonic structures on complex substrates or at designated locations still remains a grand challenge. Here, a switchable self‐assembly method is developed to manufacture 3D nanogap plasmonic structures by combining supercritical drying and capillary‐force driven self‐assembly (CFSA) of micropillars fabricated by laser printing. The polymer pillars can stay upright during solvent development via supercritical drying, and then can form the nanogap after metal coating and subsequent CFSA. Due to the excellent flexibility of this method, diverse patterned plasmonic nanogap structures can be fabricated on planar or nonplanar substrates for SERS. The measured SERS signals of different patterned nanogaps in fluidic environment show a maximum enhancement factor ≈8 × 107. Such nanostructures in microchannels also allow localized sensing for anticancer drugs (doxorubicin). Resulting from the marriage of top‐down and self‐assembly techniques, this method provides a facile, effective, and controllable approach for creating nanogap enabled SERS devices in fluidic channels, and hence can advance applications in precision medicine.

Enhanced Fields in Mirror-Backed Low-Index Dielectric Structures

This Letter reports the localization of optical fields in dielectric nanopillar arrays on a metal film. Large-area arrays of tall (>450 nm) polymer pillars were patterned on an optically thick gold...

Effect of oblique polymer pillars on spreading and elongation of rat mesenchymal stem cells

Stiffness and anisotropy of culture substrates are important factors influencing the cell behavior and their responses to external stimuli. Herein, we report a fabrication method of oblique polymer pillars which allow modulating both stiffness and anisotropy of the substrate for spreading and elongation studies of Rat Mesenchymal Stem Cells (RMSCs). Poly (Lactic-co-Glycolic Acid) (PLGA) has been chosen to produce micro-pillars of different heights and different pitches using a combined method of soft-lithography and hot embossing. The stiffness of such pillar substrates varies over a large range so that RMSCs show effectively different spreading behaviors which are also sensitive to the inclining angle of the pillars. Our results showed that with the increase of the pillar height the area of cell spreading decreases but the cell elongation aspect ratio increases. Moreover, cells preferentially elongate along the direction perpendicular to that of the pillars’ inclining, which is in agreement with the calculated anisotropy of the pillar substrate stiffness.

High-Aspect-Ratio Semiconducting Polymer Pillars for 3D Cell Cultures.

Hybrid interfaces between living cells and nano/microstructured scaffolds have huge application potential in biotechnology, spanning from regenerative medicine and stem cell therapies to localized drug delivery and from biosensing and tissue engineering to neural computing. However, 3D architectures based on semiconducting polymers, endowed with responsivity to visible light, have never been considered. Here, we apply for the first time a push-coating technique to realize high aspect ratio polymeric pillars, based on polythiophene, showing optimal biocompatibility and allowing for the realization of soft, 3D cell cultures of both primary neurons and cell line models. HEK-293 cells cultured on top of polymer pillars display a remarkable change in the cell morphology and a sizable enhancement of the membrane capacitance due to the cell membrane thinning in correspondence to the pillars’ top surface, without negatively affecting cell proliferation. Electrophysiology properties and synapse number of primary neurons are also very well preserved. In perspective, high aspect ratio semiconducting polymer pillars may find interesting applications as soft, photoactive elements for cell activity sensing and modulation.

Tipping the scales toward new color-changing device

A common aquarium fish, the neon tetra, has given researchers inspiration for a new type of color-shifting device (ACS Nano 2019, DOI: 10.1021/acsnano.9b00822). The colorful fish (Paracheirodon innesi) produces iridescence through light bouncing off stacks of guanine platelets in its scales. By tilting the platelets, the fish can change the spacing between them and the wavelength of light they reflect. Chih-Hao Chang of North Carolina State University and colleagues built a device that mimics this mechanism. First they molded a siloxane copolymer containing iron oxide nanoparticles into a block studded with 1 µm tall pillars spaced 2 µm apart. They then surrounded the pillars with more nanoparticles dispersed in water and topped the device with a layer of poly(dimethylsiloxane). When they applied a magnetic field, the nanoparticles in the water arranged themselves into 20 µm high columns on top of the polymer pillars. Changing the angle of the magnetic field

Growth of Polystyrene Pillars in Electric Field.

Surface patterning on polymer films, which is a self-assembly process under the action of external and/or internal impetus, has a variety of applications, including drug delivery and flexible electronics. In this work, we study the growth of polystyrene pillars in the electric field for different combinations of annealing temperature, film thickness, and electrode separation (electric field intensity). There are five stages for the growth of the polystyrene pillars for all the configurations used in this work, including a nucleation stage, a linear growth stage, an acceleration stage in the pillar length prior to the contact between the top surface of a pillar and the upper electrode, a radial growth stage after the contact, and a stationary stage without further growth of the pillar. In the linear growth stage, there exist linear relationships between the pillar length and the annealing time and between the square of the pillar diameter and the annealing time. The activation energies for the rate processes controlling the radial growth and the length growth in the linear growth stage are 30.2 and 25.3 kJ/mol, respectively. There are two rate processes controlling the radial growth of the pillars: one is the field-induced flow of polymer through the polymer film to the roots of pillars and the other is the coalescence of pillars. The activation energy for the coalescence is 16.5 kJ/mol. The results obtained in this work offer a practical route to control the geometrical dimensions of polymer pillars through the processing parameters.

Clustering and Self‐Recovery of Slanted Hydrogel Micropillars

AbstractSlanted high‐aspect‐ratio polymer pillars are studied for their unique properties such as unidirectional spreading of liquid, directional adhesions, or alignment of cells, where the pillars are in constant contact with water or in a humid environment. These pillars, however, tend to cluster upon water evaporation due to the capillary force and lowered modulus of the pillars. Here, spontaneous recovery of clustered slanted hydrogel pillars to their original shape is presented by exploiting the modulus change of hydrogel materials during water evaporation. The clustering and recovery of the slanted hydrogel micropillars are monitored in situ by optical microscopy and environmental scanning electron microscopy. To elucidate sequential clustering and recovery mechanism, the adhesion force between the pillars and the restoring force is compared. Finally, the dynamic change of optical transparency is exploited as the result of switching between clustering and recovery of the slanted micropillars for display. The study of the deformation and recovery of slanted hydrogel pillars will offer insights into geometrical and material designs in water‐based applications.

Large-Area Fabrication of Droplet Pancake Bouncing Surface and Control of Bouncing State.

Superhydrophobic pillar arrays, which can generate the droplet pancake bouncing phenomenon with reduced liquid-solid contact time, have huge application prospects in anti-icing of aircraft wings from freezing rain. However, the previously reported pillar arrays, suitable for obtaining pancake bouncing, have a diameter ≤100 μm and height-diameter ratio >10, which are difficult to fabricate over a large area. Here, we have systematically studied the influence of the dimension of the superhydrophobic pillar arrays on the bouncing dynamics of water droplets. We show that the typical pancake bouncing with 57.8% reduction in contact time with the surface was observed on the superhydrophobic pillar arrays with 1.05 mm diameter, 0.8 mm height, and 0.25 mm space. Such pillar arrays with millimeter diameter and <1 height-diameter ratio can be easily fabricated over large areas. Further, a simple replication-spraying method was developed for the large-area fabrication of the superhydrophobic pillar arrays to induce pancake bouncing. No sacrificial layer was needed to reduce the adhesion in the replication processes. Since the bouncing dynamics were rather sensitive to the space between the pillars, a method to control the contact time, bouncing shape, horizontal bouncing direction, and reversible switch between pancake bouncing and conventional bouncing was realized by adjusting the inclination angle of the shape memory polymer pillars.