Search

A functional polymer-matrix composite (PMC) with controllable material properties is emerging as a promising material for the smart sensors in structural health monitoring (SHM). A GnF/PDMS composite is developed as a new functional PMC by blending graphite nanoflakes (GnFs) with polydimethylsiloxane (PDMS) where GnF and PDMS are used as a reinforcing/conductive filler and an elastic host matrix, respectively. We characterize the mechanoelectrical property-controllable GnF/PDMS composite, mainly focusing on the following issues: (i) determination of the best solvent for the preparation of a GnF/PDMS composite solution, (ii) exploration of changes in the mechanoelectrical properties of the functional PMC induced by variations in the aspect ratio (AR) and concentration of GnF. Among 9 kinds of common solvents, benzene shows both high GnF dispersibility and excellent PDMS compatibility, therefore being chosen as the optimal solvent. Variations in GnF AR (223–1017) and GnF concentration (1.0–25.0 wt.%) lead to significant changes in the elastic modulus, fracture strain, electrical conductivity, and gauge factor of the functional PMC in the ranges of 0.39–13.8 MPa, 0.09 to 2.54, 6.97 × 10−6 to 221.0 S/m, and 6 to about 37,000, respectively. The empirical models for predicting the mechanoelectrical properties of the functional PMC are also intensively studied. Our GnF/PDMS composite is expected to be used as a functional PMC for the development of smart sensors that detect deformation and fracture in structures.

The graphite nanoflake (GnF)-reinforced polydimethylsiloxane (PDMS) composites (GnF/PDMS composites) are developed as new polymer matrix composites (PMCs) with controllable mechanoelectrical properties. Here, we investigate the effect of GnF concentration on the mechanoelectrical properties (i.e., elastic modulus, fracture strain, and conductivity) of GnF/PDMS composites; the change in the surface morphology of GnF/PDMS composites caused by a variation in GnF concentration is also explored. The mechanoelectrical properties are measured by performing tensile tests on the GnF/PDMS composite specimens with different GnF concentrations of 5.0, 10.0, 12.5, 15.0, 20.0, and 25.0 wt.%. The surface morphology is analyzed in terms of internal void formation and surface roughness. The elastic modulus is measured to be in the range of 1.62 to 13.8 MPa which is proportional to GnF concentration, while the fracture strain and electrical conductivity are respectively characterized to be in ranges of 0.09 to 2.09 and 0.3 to 221.0 S/m which are in inverse proportion to GnF concentration. An increase in GnF concentration leads to increases in internal voids’ amount and surface roughness. The GnF/PDMS composites can be used as sensing materials for detecting both small and large deformations in a variety of engineering applications.

The GnF/SU-8 composites are new polymer matrix composites (PMCs) composed of graphite nanoflakes (GnFs) bound together by SU-8 photoresist. The PMCs therefore have excellent ultraviolet (UV) photopatternability and high electrical properties. In spite of the unique material properties of GnF/SU-8 composites, much still remains uncertain about their controllability in both UV photopatternability and electrical properties. Here, we investigate 7 kinds of GnF/SU-8 composites having different GnF concentrations of 5.0 to 25.0 wt.% to characterize the changes in the UV photopatternability (i.e., polymerized thickness and photopattern quality) and electrical conductivity of GnF/SU-8 composites caused by a variation in GnF concentration. The polymerized thickness of GnF/SU-8 composites is measured to be in the range of 4.06 to 23.99 μm, which is inversely proportional to GnF concentration and also directly proportional to UV dose (i.e., 345 to 3,450 mJ/cm2) because of the screening effect of GnF existed in the composites; the photopattern quality at the edge is in inverse proportion to GnF concentration. An increase in GnF concentration leads to a significant change in the electrical conductivity of GnF/SU-8 composites in a proportional way (up to 25.34 S/m). The GnF/SU-8 composites are expected to be widely used as UV photopatternable and electrically conductive PMCs for diverse engineering applications.

A mosquito is known to precisely and easily insert its proboscis to the human skin by pressing down a labium and vibrating a fascicle bundle. Its advanced skin-piercing mechanisms indicate that skin resistance to the insertion of needle-like objects can be changed by the application of mechanophysical stimuli. Here, we characterize the effect of the application of mechanophysical stimuli on skin resistance to microneedle insertion to find clues for inserting a microneedle in a deep and precise fashion with low force. Microneedles with a diameter of 60-140µm are inserted at a velocity of 0.1-2.0mm/s to full-thickness porcine skins while either uniaxial/equibiaxial stretch of 0-20% or mechanical vibration at a frequency of 1 to 1000Hz and an amplitude of 1-10µm is applied to the skins as static or dynamic mechanophysical stimulus, respectively. The values of force and depth at two events of skin puncture and maximum penetration are measured to explore changes in skin resistance induced by the application of external stimuli. The static mechanophysical stimulus applied to the skin mainly affects the precision of microneedle insertion; the application of dynamic mechanophysical stimulus controls the value and deviation of skin resistance to microneedle insertion. The application of mechanophysical stimuli, inspired from a mosquito, therefore allows a microneedle to be deeply and easily inserted to the skin in a controlled way. The findings will have broad impacts on microneedle-mediated applications and lead to an in-depth understanding of skin biomechanics.

Diverse surface/bulk treatments have been introduced to overcome the interfacial limitations of pristine (or untreated) PDMS, thus extending the possible applications of PDMS in micro/nano device development. Despite of extensive efforts, the temporal wetting change of PDMS induced by surface/bulk treatments still remains incompletely understood. We prepared 3 kinds of physicochemically treated PDMS blocks using widely used surface/bulk treatments-3D interconnected pore network formation, biocompatible surfactant (i.e., Silwet L-77) addition, and combination of both. Their wetting nature was characterized by measuring the time profile of water contact angle. A 3D interconnected pore network formation produced a time-invariant decrease in PDMS wettability; a surfactant addition increased the PDMS wettability in a time-variant way; a combination of pore network formation and surfactant addition had a combined effect. The measurement led to the successful development of a model for predicting the temporal wetting change in PDMS caused by variances in pore size and surfactant concentration. The accuracy of our model was verified by comparing experimental results with model predictions. This model will result in better understanding of polymer interface.

A skin-perforable dissolving microneedle is a promising mediator for painlessly delivering active pharmaceutical compounds across the skin. All the microneedle manufacturing processes so far, however, are much sensitive to input variation and unfavorable for make-to-order approach. Here, a robust method for fabricating mass-customizable master molds is developed to prepare sharp-tipped biodegradable polymer microneedles. Our approach combines the predrying and chip casting (PCC) of an ultrathick photoresist layer with a substrateless, inclined, and rotational exposure (SIR exposure). The PCC achieves the uniform reduction of solvent across the photoresist thickness which is critically required for the formation of a sharp tip; the SIR exposure creates master molds whose geometry is easily customizable and virtually insensitive to a variation in ultraviolet (UV) exposure dose. A theoretical model for the spatiotemporal distribution of UV dose under SIR exposure is established to show the technological superiority of our method. Next, our method's applicability is proven by fabricating a set of poly(lactic-co-glycolic) acid (PLGA) microneedles and performing both porcine skin penetration test and their in vitro degradation test. Our approach is verified to be robust in manufacturing mass-customizable molds for skin-perforable dissolving microneedles and to have high compatibility with almost all existing biodegradable polymers. The findings of this study lead to both a significant growth of dissolving microneedle-mediated drug delivery and better understanding of drug release kinetics.

A successful development of high-performance multilevel microdevices is attributed to the outstanding physical properties of a polycrystalline silicon (poly-Si) film which can be tailored by its process conditions. Here, we select deposition temperature and post-deposition annealing treatment as critical process parameters, followed by investigating their effects on both structure properties and chemical-mechanical planarization (CMP) manufacturability of a poly-Si film. Experimental samples are prepared through low-pressure chemical vapor deposition (LPCVD) poly-Si deposition at different temperatures of 545–625 °C and thermal annealing treatment at 1050 °C under nitrogen atmosphere. At first, the poly-Si sample is inspected to characterize changes in macroscopic structure properties caused by the critical process parameters, together with changes in its surface morphology and grain boundary density. Next, the sample is polished to quantify changes in poly-Si CMP manufacturability such as material removal rate (MRR) and surface roughness. A relation between changes in microscopic features and changes in macroscopic properties is also discussed in depth. A poly-Si film deposited at 625 °C and then thermally annealed at 1050 °C shows significant improvement in both structure and CMP manufacturing aspects. The findings of this paper can potentially have a significant impact on developing poly-Si-based multilevel microdevices.

There is an explosion of interest in characterizing cell adhesion process and the mechanochemical players that is critical to a wide range of physiological and pathological consequences. Much, however, still remains uncertain and controversial about the quantitative measurement of cell adhesion strength (i.e., cell-substratum adhesion force, cell-cell adhesion force, etc.). The main reason for this deficit is a lack of well-established and comprehensive experimental methods that quantitatively measure cell adhesion strength at cellular and even molecular levels. This leads to the need for conducting a literature review on the methods for characterizing the adhesion force of anchorage-dependent cells. In this review article, we discern and critique available techniques for measuring cell adhesion strength that is required for an adherent cell (or cells) of interest to detach from the neighboring cell or the substratum. This review focuses on recent advances in the technique for measuring cell adhesion strength to weigh up the strengths and weaknesses of each technique, together with a brief description for the biological aspects of cell adhesion process. The summary of this article might help us both to develop a standard method for measuring cell adhesion strength and to broaden and deepen our understanding of cell adhesion process.

The reversible migration of adatoms along a basal plane of graphene under electrical bias is experimentally demonstrated. Single-layer graphene oxide with partial oxygen adatom coverage is utilized for this demonstration. The intensity ratio of G and G' Raman modes is used to determine the oxygen adatoms migration. Finally, reversible wettability property of graphene due to oxygen adatom migration is demonstrated.

CMOS utilizing high mobility III–V channels on Si substrates is expected to be one of the promising devices for high performance and low power integrated systems in the future technology nodes, because of the enhanced carrier transport properties. In addition, Tunnel FETs (TFETs) using III–V channel materials are regarded as one of the most promising steep slope devices for the ultra-low power applications. In this presentation, we address the critical issues for realizing III–V MOSFETs and TFETs on the Si CMOS platform. Viable technologies of MOS channel, gate stack, source/drain and tunnel junction formation are introduced for satisfying the device requirements. The electrical properties of ultrathin body InAs MOSFETs, InGaAs/Ge CMOS, InAs/GaSb CMOS, InGaAs TFETs and InGaAs/GaAsSb TFETs are presented.

Much still remains uncertain and controversial about the mechanophysical parameters involved in graphite exfoliation. Here, we quantify the expansion characteristics of natural graphite in the microwave-irradiated exfoliation where KMnO4 and HNO3 are used as an oxidant and an intercalant, respectively. As a mark of the degree to which graphite becomes exfoliated, the expansion ratio of graphite is investigated as a function of mix ratio by weight of 3 ingredients (i.e., graphite, KMnO4, and HNO3), mixing time, and graphite physical properties. The findings of this study will lead to a better understanding of graphite exfoliation and make it possible to achieve the industrial-scale production of defect-free graphene.

We study the impact of gate stacks on the electrical characteristics of Zn-diffused source In x Ga1− x As tunneling field-effect transistors (TFETs) with Al2O3 or HfO2/Al2O3 gate insulators. Ta and W gate electrodes are compared in terms of the interface trap density (D it) of InGaAs MOS interfaces. It is found that D it is lower at the W/HfO2/Al2O3 InGaAs MOS interface than at the Ta/HfO2/Al2O3 interface. The In0.53Ga0.47As TFET with a W/HfO2 (2.7 nm)/Al2O3 (0.3 nm) gate stack of 1.4-nm-thick capacitance equivalent thickness (CET) has a steep minimum subthreshold swing (SS) of 57 mV/dec, which is attributed to the thin CET and low D it. Also, the In0.53Ga0.47As (2.6 nm)/In0.67Ga0.33As (3.2 nm)/In0.53Ga0.47As (96.5 nm) quantum-well (QW) TFET supplemented with this 1.4-nm-thick CET gate stack exhibits a steeper minimum SS of 54 mV/dec and a higher on-current (I on) than those of the In0.53Ga0.47As TFET.