Photosensitive Polymer Research Articles

A NEW EQUIVALENT CIRCUIT SCHEME FOR GROUNDED BACK-TO-BACK GCPW-MS-GCPW TRANSITIONS FABRICATED ON A THIN LOW-K SUBSTRATE

We hereby present a new equivalent circuit model including both lumped and distributed elements for GCPW-MS transitions (GCPW for Grounded CoplanarWaveguide and MS for Microstrip). In order to validate the modelling results, such transitions have been fabricated on a 20 μm-thick BCB (Benzocyclobutene resin) substrate using grounding pads including via-holes of different diameters. The study focused on the impact of the via-hole diameter on the performance of the transition and more specifically on its bandwidth. The transitions were made using a simple technological process based on photosensitive polymer. ADS simulation data of the new equivalent circuit model were in very good agreement with measured S-parameters. Both theoretical and experimental results have shown that the bandwidth of such a transition can reach up to 100 GHz using via-holes of 900 μm diameter.

Optical Trapping of Photosoftened Solid Polymers

We present evidence of optical trapping of solid polymer films by a tightly focused near-infrared (NIR) laser beam (785 nm) when the polymer is softened by green laser (532 nm) irradiation, which is uniformly distributed over the film surface. While the whole film is softened by the green laser, only the part which is subjected to the NIR laser is trapped. The film, which is transparent in the NIR range and strongly absorbs the visible light, is deposited on a glass substrate. The NIR laser is focused at the sample plane and attracts the polymer to the laser focus, by the optical gradient force, during the green light irradiation, indicating that molecular mobility is strongly reduced and the optical gradient force of the NIR laser is inefficient in moving the polymer below the glass-transition temperature of the polymer without photosoftening. The observations are consistent with the theory of photochemical tweezing, which rests on the assumption that photosensitive polymers, containing azo dyes, are softened, i.e., molecular mobility is enhanced, by photoisomerization of the azo dyes during green light irradiation, and are moved in gradients of light fields. We also found that this two-color laser tweezing scheme is sensitive to light polarization, owing to the polarization sensitivity of the azo dye chromophores.

Greyscale lithography-based fabrication of thin polymeric cantilevers for MEMS/NEMS applications

Highly photosensitive epoxy-based polymer SU8 is becoming extensively used for MicroElectro-Mechanical Systems (MEMS) and microfluidic devices not only as a negative photoresist but also as the basic constitutional material because of the easy fabrication of devices with tailored dimensions from µm to mm sizes. Here, we have shown a one-step process for the fabrication of thin cantilever arrays of thickness upto 1-2 µm using MASKLESS GRAYSCALE LITHOGRAPHY (MGL). Microfabricated polymeric disposable cantilevers can be utilized for label-free biochemical sensing with desirable advantages such as high sensitivity due to low Young’s modulus, low cost compared to its Silicon counterpart and ease in reproducible fabrication. Microcantilever arrays of different shapes were fabricated reproducibly by optimizing the process parameters. Arrays of suspended cantilevers can be used for the fabrication of chemical/biochemical sensors as SU-8 is highly resistant to chemicals and biocompatible making it a suitable material for such sensors.

Model for athermal enhancement of molecular mobility in solid polymers by light.

Molecular motion in polymers is frozen below the glass transition temperature T_{g} and changes of viscoelastic functions are most spectacular near T_{g}. Exceptional enhancement of molecular mobility and a decrease of polymer viscosity, by several orders of magnitude, down to the viscous flow regime, are observed way below T_{g} by light absorption. Relaxation processes, which take decades to centuries in some high-T_{g} polymers, are reduced to minute timescales by sub-T_{g} light absorption. Here we develop a model for this intriguing albeit spectacular action of light on glass forming materials and we propose experiments to relate light absorption to materials properties. The model provides a solution to a long-lasting problem of how molecular mobility is enhanced in solid polymers by photoisomerization and provides a tool for a better understanding of the relationship between light absorption and material properties and developing photosensitive polymers for light to mechanical energy transduction.

Bioprinting within live animals.

Biomaterial structures can be printed with near-infrared light within the tissues of living animals by using photosensitive polymer hydrogels.

Influence of Structural Defects on the Properties of Metamagnetic Shape Memory Alloys

The production of μ-particles of Metamagnetic Shape Memory Alloys by crushing and subsequent ball milling process has been analyzed. The high energy involved in the milling process induces large internal stresses and high density of defects with a strong influence on the martensitic transformation; the interphase creation and its movement during the martensitic transformation produces frictional contributions to the entropy change (exothermic process) both during forward and reverse transformation. The frictional contribution increases with the milling time as a consequence of the interaction between defects and interphases. The influence of the frictional terms on the magnetocaloric effect has been evidenced. Besides, the presence of antiphase boundaries linked to superdislocations helps to understand the spin-glass behavior at low temperatures in martensite. Finally, the particles in the deformed state were introduced in a photosensitive polymer. The mechanical damping associated to the Martensitic Transformation (MT) of the particles is clearly distinguished in the produced composite, which could be interesting for the development of magnetically-tunable mechanical dampers.

Formation of half-period surface relief gratings in azobenzene containing polymer films

We study the peculiar response of photo-sensitive polymer films irradiated with a certain type of interference pattern where one interfering beam is S-polarized, while the second one is P-polarized. The polymer film, although in a glassy state, deforms following the local polarization distribution of the incident light, and a surface relief grating (SRG) appears whose period is half the optical one. All other types of interference patterns result in the matching of both periods. The topographical response is triggered by the alignment of photo-responsive azobenzene containing polymer side chains orthogonal to the local electrical field, resulting in a bulk birefringence grating (BBG). We investigate the process of dual grating formation (SRG and BBG) in a polymer film utilizing a dedicated set-up that combines probe beam diffraction and atomic force microscopy (AFM) measurements, and permits acquiring in situ and in real-time information about changes in local topography and birefringence. We find that the SRG maxima appear at the positions of linearly polarized light (tilted by 45° relative to the grating vector), causing the formation of the half-period topography. This permits to inscribe symmetric and asymmetric topography gratings with sub-wavelength period, while changing only slightly the polarization of one of the interfering beams. We demonstrate an easy generation of sawtooth profiles (blazed gratings) with adjustable shape. With these results, we have taken a significant step in understanding the photo-induced deformation of azo-polymer films.

Realizing Dynamic Diffraction Gratings Based on Light-Direct Writing of Responsive 2D Ordered Patterns

A dynamic diffraction grating (DDG) can realize on-demand in-situ regulation of light diffraction and is highly necessary in the next-generation optical devices; however, its fabrication remains challenging because of difficulties in generating a responsive pattern in the bulk material or on the surface. In this study, a facile and general approach to construct DDG is developed based on the near-infrared radiation (NIR)-driven responsive 2D ordered surface pattern, which is fabricated by light-direct writing of wrinkles on a thin film of a photosensitive supramolecular polymer network comprised of a copolymer containing amino groups, anthracene carboxylic acid (ANA), and carbon nanotubes. The resulting surface pattern, which exhibits a height dependence on the NIR irradiation, can be used as the DDG whose optical diffraction can be regulated in-situ by NIR. Furthermore, because of the presence of a reversible ANA photodimer and hydrogen bonding in the supramolecular network, the patterned surface can be erased and rewritten by UV light with different wavelengths and self-healed by NIR irradiation. The results demonstrate a simple strategy for DDGs that will have a broad application in the fields of optics, electronics, and intelligent sensing.

Self-Limiting Polymer Exposure for Vertical Processing of Semiconductor Nanowire-Based Flexible Electronics

In this work, we demonstrate a vertical processing method to fabricate nanowire (NW)-based devices. This method combines the strong light absorption ability caused by the NW geometry and exposure to dose-dependent clearance properties of a photo-sensitive polymer. By embedding NW arrays in a polymer, the NW light absorption leads to self-limited exposure and selective removal of the polymer. This optical and self-limited exposure pattern definition method can replace more expensive processing equipment, such as reactive ion etching and the use of a mask aligner. Excitingly, this method can be used to enable peel-off of NW arrays from their parent substrate, opening up opportunities to fabricate flexible NW array devices.

Intravital three-dimensional bioprinting.

Fabrication of three-dimensional (3D) structures and functional tissues directly in live animals would enable minimally invasive surgical techniques for organ repair or reconstruction. Here, we show that 3D cell-laden photosensitive polymer hydrogels can be bioprinted across and within tissues of live mice, using bio-orthogonal two-photon cycloaddition and crosslinking of the polymers at wavelengths longer than 850 nm. Such intravital 3D bioprinting-which does not create by-products and takes advantage of commonly available multiphoton microscopes for the accurate positioning and orientation of the bioprinted structures into specific anatomical sites-enables the fabrication of complex structures inside tissues of live mice, including the dermis, skeletal muscle and brain. We also show that intravital 3D bioprinting of donor-muscle-derived stem cells under the epimysium of hindlimb muscle in mice leads to the de novo formation of myofibres in the mice. Intravital 3D bioprinting could serve as an in vivo alternative to conventional bioprinting.

Polymer-derived SiOC ceramic lattice with thick struts prepared by digital light processing

Compared with the photosensitive ceramic slurry containing ceramic particles, the photosensitive preceramic polymer was more suitable for the digital light processing (DLP) printing of ceramics. However, the strut thickness of the DLP 3D printed polymer-derived ceramics is always limited due to the serious deformation caused by the aggregation of the enormous by-product gases during the pyrolysis process. Herein, a novel photosensitive epoxy-acrylic siloxane was employed as the matrix for the digital light processing and a non-photosensitive liquid siloxane was introduced as an additive to generate micro-sized channels during pyrolysis, consequently ensuring that the by-product gases can be smoothly released through these channels. With the increase of the pyrolysis temperature, those micro-sized channels would shrink, leaving some nano-pores (or uneven surface) in the pyrolyzed SiOC components. Results show that the DLP 3D printed SiOC lattices with the addition of liquid siloxane show a better macro-appearance and macro-mechanical property with scarifying its micro-hardness. We believed that the method of promoting the release of by-product gases by introducing a non-photosensitive siloxane can be further applied to other DLP 3D printed polymer-derived ceramics.

Effect of Thermal Cycling on the Thermal and Mechanical Properties of Dielectric Materials

Photosensitive polymers or dielectric materials such as polyimides (PIs) have been used in fan-out wafer-level packaging process (FOWLP) as a redistribution layer for quite some time. In the condensed 2-D and 3-D packaging processes, the dielectric materials are applied multiple times to create as many as four or five redistribution layers. At the end of the process, the first layer may experience up to four times of heat cycles in which the polymer properties may change. However, the effect of heat cycles on properties of dielectric film as well as their contributions to the wafer-level warpage is still unclear. To understand the changes of materials’ properties after several heat cycles, the thermal and thermo-mechanical properties of negative-tone and positive-tone types of dielectric films are characterized by using the thermomechanical analysis (TMA), dynamic mechanical analysis (DMA), and thermogravimetric analysis (TGA). The glass transition temperature ( ${T}_{g}$ ), coefficient of linear thermal expansion (CTE), and modulus (storage, ${E}'$ , and loss, ${E}''$ ) are analyzed and compared. In addition, the morphology of the films before and after heat cycles is also analyzed.

Synthesis of optothermal responsive polymers by thiol-ene click reaction and their aggregation behavior

Thermosensitive and photosensitive polymers (PNIPAM-Azo) were synthesized by thiol-ene click reaction with PNIPAM with terminal thiol groups (PNIPAM-SH) and azobenzene compounds containing a double bond. Different catalysts were used in the reaction to investigate their catalytic effect and a photocatalyst was found to have the best effect. The aggregation behavior of PNIPAM-Azo in aqueous solution was studied, which showed that PNIPAM-Azo had good surface activity in aqueous solution, and could form different aggregates at different concentration. Meanwhile PNIPAM-Azo exhibited good light response and thermal response, only the LCST of PNIPAM-Azo increased a little after long time ultraviolet irradiation.

Novel bioceramics from digital light processing of calcite/acrylate blends and low temperature pyrolysis

Porous bioceramic composites with complex shapes were easily fabricated starting from fine calcite (CaCO3) particles (<10 μm) immersed in a liquid photocurable polymer. The building of 3D structures, layer-by-layer, was achieved by means of Digital Light Processing (DLP), using a commercial stereo-lithography 3D printer. The pyrolysis of the hardened photosensitive polymer, in nitrogen atmosphere (at 500 °C), left calcite powders embedded in an amorphous carbon matrix, without any degradation of the main phase, which retained its biocompatibility, according to the cell culture studies. In addition, calcite was successfully converted into hydroxyapatite (HAp), by simple immersion of samples in a phosphatizing bath (aqueous solution of sodium phosphate), for 2 weeks, obtaining HAp-pyrolytic carbon composite scaffolds. The developed structures, after phosphatization, exhibited an adequate strength-to-density ratio (compressive strength of 1.4 MPa with porosity of about 80 vol%).

Design, fabrication, and characterization of SU-8/carbon black nanocomposite based polymer MEMS acceleration sensor

Design, fabrication, and characterization of SU-8/carbon black (CB) nanocomposite based single axis acceleration sensor is presented. The entire sensor made of SU-8 photosensitive polymer is fabricated using the process spin pattern anchor release device. Thanks to the low young’s modulus of SU-8 polymer, the cantilever beams used to support the proof mass are very flexible, which resulted in increased sensitivity. A composite layer of CB nanoparticles mixed with the SU-8 polymer is sandwiched between two SU-8 2002 layers in the cantilever beam. This nanocomposite layer acts as a piezoresistive layer and is used to measure the induced stress and strain. The piezoresistors are patterned in a full bridge Wheatstone arrangement to cancel the effect on temperature variation. Maximum stress locations are identified through Finite Element -Modeling based simulations under accelerations up to 100 g. The natural frequency, stiffness, and sensitivity of the acceleration sensor are characterized using the techniques of nanoindenter and Laser Doppler Vibrometer. The measured natural frequency is 1.884 kHz. The relative change in resistance per unit strain of the prototype is measured using I–V measurements, which gives a gauge factor of 13.05.

Utilizing photosensitive polymers to evaluate UV radiation exposures in different plasma chamber configurations

Monitoring vacuum ultraviolet (UV/VUV) emission in plasma systems is challenging as it requires specialized diagnostic systems or sensors to be compatible with these devices. This study addresses different reactor configurations and plasma chemistries with various levels of UV emission and their effects on a known set of polymers. First, the effect of He plasma treatment of organic underlayer (NFC-1400) and e-beam resist (hydrogen silsesquioxane) on line-space patterning of polysilicon at sub-100 nm pitch was investigated. By applying He plasma exposure, both before and after patterning of the optical underlayer, a significant improvement in line edge roughness (LER) from 2.5 to 1.4 nm was observed. To understand the plasma treatment mechanisms, polymers were exposed to synchrotron VUV light at 62 nm (or 20.0 eV), which coincides with the He VUV emission range (50–60 nm), followed by etching of poly-Si, and were found to have similar LER results. The refraction index measurements for both the optical underlayer and the e-beam resist revealed the key finding that the polymers absorption maxima corresponded exactly to the wavelength of ∼60 nm. Second, the impact of inductively coupled and microwave plasma configurations on etch rates and chemical properties of photoresists were investigated. Identifying specific photon-induced modifications on polymers can help detect UV/VUV emission in the plasma and decouple ion and photon effects on materials. Poly(methyl methacrylate) and poly(4-vinylphenol)-based photoresists were exposed to argon (Ar) and nitrogen (N2) plasmas. Surface and elemental analyses confirmed that plasma effects on chemical modifications, surface roughness, and etch rate were significantly higher for poly(methyl methacrylate) compared to poly(4-vinylphenol)-based photoresists. Detailed elemental and molecular structure analyses of these polymers showed relatively higher damage to both species caused from the inductively coupled plasma, which is ultimately correlated with a higher UV/VUV emission.

Development of photosensitive phosphonic acid derivatives for the fabrication of organic thin film transistors

New materials, particularly those that are environmentally friendly, are much sought after. These can be created by manipulating existing materials. For example, the surface of materials can be changed with surface control technology using self-assembled monolayers (SAMs). This can be done to create materials that can be used in a variety of applications, such as in electronics, as biosensors and for wettability control. Professor Kazuo Yamaguchi, based at the Department of Chemistry, Kanagawa University, Japan, has extensive experience in this field and is working with a team of researchers to shed light on this area. His research focuses on topics such as the synthesis of photosensitive molecules and polymers, and their application as materials in electronics and biological sciences.

A hybrid P3HT-Graphene interface for efficient photostimulation of neurons

Graphene conductive properties have been long exploited in the field of organic photovoltaics and optoelectronics by the scientific community worldwide. We engineered and characterized a hybrid biointerface in which graphene is coupled with photosensitive polymers, and tested its ability to elicit light-triggered neural activity modulation in primary neurons and blind retina explants. We designed such a graphene-based device by modifying a photoactive P3HT-based retinal interface, previously reported to rescue light sensitivity in blind rodents, with a CVD graphene layer replacing the conductive PEDOT:PSS layer to enhance charge separation. The new graphene-based device was characterized for its electrochemical features and for the ability to photostimulate primary neurons and blind retina explants, while preserving biocompatibility. Light-triggered responses, recorded by patch-clamp in vitro or MEA ex vivo, show a stronger light-transduction efficiency when the neurons are interfaced with the graphene-based device with respect to the PEDOT:PSS-based one. The possibility to ameliorate flexible photo-stimulating devices via the insertion of graphene, paves the way for potential biomedical applications of graphene-based neuronal interfaces in the context of retinal implants.

Quantitative Study of Morphological Features of Stem Cells onto Photopatterned Azopolymer Films.

In the last decade, the use of photolithography for the fabrication of structured substrates with controlled morphological patterns that are able to interact with cells at micrometric and nanometric size scales is strongly growing. A promising simple and versatile microfabrication method is based on the physical mass transport induced by visible light in photosensitive azobenzene-containing polymers (or azopolymers). Such light-driven material transport produces a modulation of the surface of the azopolymer film, whose geometry is controlled by the intensity and the polarization distributions of the irradiated light. Herein, two anisotropic structured azopolymer films have been used as substrates to evaluate the effects of topological signals on the in vitro response of human mesenchymal stem cells (hMSCs). The light-induced substrate patterns consist of parallel microgrooves, which are produced in a spatially confined or over large-scale areas of the samples, respectively. The analysis of confocal optical images of the in vitro hMSC cells grown on the patterned films offered relevant information about cell morphology—i.e., nuclei deformation and actin filaments elongation—in relation to the geometry and the spatial extent of the structured area of substrates. The results, together with the possibility of simple, versatile, and cost-effective light-induced structuration of azopolymers, promise the successful use of these materials as anisotropic platforms to study the cell guidance mechanisms governing in vitro tissue formation.

Polarization controlled fine structure of diffraction spots from an optically induced grating

We report on the remote control of the fine structure of a diffraction spot from optically induced dual gratings within a photosensitive polymer film. The material contains azobenzene in the polymer side chains and develops a surface relief under two-beam holographic irradiation. The diffraction of a polarized probe beam is sensitive to the orientation of the azobenzene groups forming a permanently stored birefringence grating within the film. We demonstrate that the fine structure of the probe diffraction spot switches from a Gaussian to a hollow or a hollow to a “Saturn”-like structure by a change in polarization. This makes it potentially useful in photonic devices because the beam shape can be easily inverted by an external stimulus.