Yiin-Kuen Fuh

Pattern transfer of aligned metal nano/microwires as flexible transparent electrodes using an electrospun nanofiber template

Due to the scarcity and high cost of indium, the predominant use of indium tin oxide (ITO) films as transparent electrodes has attracted great attention for finding a potential replacement, such as solution-processed networks of carbon nanotubes, graphene, or silver nanowires (NWs). More recently, the use of electrospun copper NWs as high-performance electrodes with a high aspect ratio of 100,000 and 90% transmittance at 50 Ω/sq was experimentally achieved. However, the fabrication route of the Cu nanofiber (NF) web includes two high temperature processes (calcined 2 h in air at 500 °C and annealed 1 h in hydrogen at 300 °C). In this paper, we propose a new method to obtain metal nano/microwires to be used as flexible transparent electrodes by using electrospun NF templates and the dry pattern transfer process. Our proposed method is advantageous because we can easily tune the conductivity and transmittance (T) via sputtering time in minutes without the need for time-consuming high temperature thermal steps. Here, we comprehensively show the transferred high performance transparent electrodes with platinum (Pt)-coated NW electrodes with a facile and scalable electrospinning combined sputtering process. Pt-coated NWs have high aspect ratios of up to 5000 and, when sputtered with Pt, reduce junction resistance, which results in high T at low sheet resistance, e.g. 90% at 131 Ω/sq. The Pt-coated NW electrodes also show great flexibility and stretchability, which easily surpass the brittleness of ITO films.

Massively parallel aligned microfibers-based harvester deposited via in situ, oriented poled near-field electrospinning

In this study, we demonstrate a direct-write, in situ poled polyvinylidene fluoride power generator via near-field electrospinning and fully encapsulated on a flexible substrate. An unique polarity alignment and a total of 500 microfibers continuously deposited in parallel and serial configurations are capable of producing a peak output voltage of ∼1.7 V and the current of ∼300 nA, which is two to three orders of magnitude increase in both voltage/current outputs when compared with near-field electrospinning setup of a single nanofiber and the similar amount of microfibers with postpoling treatment.

Hybrid Energy Harvester Consisting of Piezoelectric Fibers with Largely Enhanced 20 V for Wearable and Muscle-Driven Applications

We present a polyvinylidene fluoride (PVDF) nanogenerator (NG) with advantages of direct writing and in situ poling via near-field electrospinning (NFES), which is completely location addressable and substrate independent. The maximum output voltage reached 20 V from the three layers piled NGs with serial connections, and the maximum output current can exceed 390 nA with the parallel integration setup. Linear superposition and switching polarity of current and voltage tests were validated by the authentic piezoelectric output. Nanofiber (NF)-based devices with a length ∼5 cm can be easily attached on the human finger under folding-releasing at ∼45°, and the output voltage and current can reach 0.8 V and 30 nA, respectively. This work based on NFs can potentially have a huge impact on harvesting various external sources from mechanical energies.

Rapid in-process measurement of surface roughness using adaptive optics

We present an in-process measurement of surface roughness by combining an optical probe of laser-scattering phenomena and adaptive optics for aberration correction. Measurement results of five steel samples with a roughness ranging from 0.2 to 3.125 μm demonstrate excellent correlation between the peak power and average roughness with a correlation coefficient (R(2)) of 0.9967. The proposed adaptive-optics-assisted system is in good agreement with the stylus method, and error values of less than 8.7% are obtained for average sample roughness in the range of 0.265 to 1.119 μm. The proposed system can be used as a rapid in-process roughness monitor/estimator to further increase the precision and stability of manufacturing processes in situ.

A highly flexible and substrate-independent self-powered deformation sensor based on massively aligned piezoelectric nano-/microfibers

In this study, we demonstrate highly flexible and substrate-independent piezoelectric nano-/microfiber (NMF) arrays that have the potential to function as a self-powered active deformation sensor. The fabricated hybrid structure of a sensor/power generator (PG) is realized via direct deposition of near-field electrospun and in situ poled polyvinylidene fluoride (PVDF) NMF on a Cu-foil electrode of thickness ∼200 μm. The NMF-based active deformation sensor has been successfully deposited on four different flexible substrate materials including paper and fully encapsulated with comparable electrical output performance, demonstrating the superior functionality of substrate-independent deposition of NMF arrays. Capable of integrating into a fabric such as a waving flag due to its high flexibility and excellent conformability, the NMF-based device can serve as an active deformation sensor under ambient wind-speed and the feasibility of efficiently converting the flutter motion into electricity is also demonstrated. This low-cost, simple structure, high sensitivity and good environmentally friendly NMF based PG is a very promising material/technology for practical energy harvesting devices and self-powered sensors and capable of scavenging very small wind power or mechanical induced vibration.

Direct-write, Well-aligned Chitosan-Poly(ethylene oxide) Nanofibers Deposited via Near-field Electrospinning

A continuous near-field electrospinning (NFES) process has been demonstrated to be able to achieve direct-write and well-aligned chitosan/poly(ethylene oxide) nanofibers. The ability to precisely control and deposit chitosan-based nanofibers in a direct-write manner is favorable in manipulating cells attachment and proliferation at a preferred position. Experimental results show that fiber diameters can be reliably controlled in the range of 265–1255 nm by adjusting various operating parameters of the NFES processes. These prescribed patterns of nanofibers exceed tens of centimeters long and complex configurations such as grid arrays and arc shapes are assembled at specified separations as small as 5 μm. FTIR analysis reveals that NFES nanofibers have a similar morphology and composition as conventional electrospinning counterpart and constitute all components formerly present in the polymer solution. The versatile functionality to fabricate chitosan-based nanofibers with controllable size and directional alignment as well as highly ordered and customized patterns may represent an ideal candidate of a functional biomaterial and in tissue-engineering scaffolds that are predominantly representative of extracellular matrix (ECM).

Highly flexible self-powered sensors based on printed circuit board technology for human motion detection and gesture recognition.

In this paper, we demonstrate a new integration of printed circuit board (PCB) technology-based self-powered sensors (PSSs) and direct-write, near-field electrospinning (NFES) with polyvinylidene fluoride (PVDF) micro/nano fibers (MNFs) as source materials. Integration with PCB technology is highly desirable for affordable mass production. In addition, we systematically investigate the effects of electrodes with intervals in the range of 0.15 mm to 0.40 mm on the resultant PSS output voltage and current. The results show that at a strain of 0.5% and 5 Hz, a PSS with a gap interval 0.15 mm produces a maximum output voltage of 3 V and a maximum output current of 220 nA. Under the same dimensional constraints, the MNFs are massively connected in series (via accumulation of continuous MNFs across the gaps ) and in parallel (via accumulation of parallel MNFs on the same gap) simultaneously. Finally, encapsulation in a flexible polymer with different interval electrodes demonstrated that electrical superposition can be realized by connecting MNFs collectively and effectively in serial/parallel patterns to achieve a high current and high voltage output, respectively. Further improvement in PSSs based on the effect of cooperativity was experimentally realized by rolling-up the device into a cylindrical shape, resulting in a 130% increase in power output due to the cooperative effect. We assembled the piezoelectric MNF sensors on gloves, bandages and stockings to fabricate devices that can detect different types of human motion, including finger motion and various flexing and extensions of an ankle. The firmly glued PSSs were tested on the glove and ankle respectively to detect and harvest the various movements and the output voltage was recorded as ∼1.5 V under jumping movement (one PSS) and ∼4.5 V for the clenched fist with five fingers bent concurrently (five PSSs). This research shows that piezoelectric MNFs not only have a huge impact on harvesting various external sources from mechanical energy but also can distinguish different motions as a self-powered active deformation sensor.

Direct-write, highly aligned chitosan-poly (ethylene oxide) nanofiber patterns for cell morphology and spreading control

Near-field electrospinning has been demonstrated to be able to achieve direct-write and highly aligned chitosan nanofibers (CNF) with prescribed positioning density. Cell spreading in preferential direction could be observed on parallel-aligned nanofibers, and the CNF patterns were capable of guiding cell extension when the distances between them are 20 and 100 μm, respectively. Alignment of the cells was characterized according to their elongation and orientation using the fast Fourier transform data and binary image analysis. Parallel CNF indicates that the alignment values sequentially increased as a function of positioning density such that incrementally more aligned cells were closely related to the increasing CNF positioning density. These maskless, low-cost, and direct-write patterns can be facily fabricated and will be a promising tool to study cell-based research such as cell adhesion, spreading, and tissue architecture.

High-Throughput Production of Nanofibrous Mats via a Porous Materials Electrospinning Process

A facile method is presented for the electrospinning of multiple polymer jets into nanofibers. The experiments in this study electrified 7 wt% polyethylene oxide (PEO) and 10 wt% poly(vinylidene fluoride) (PVDF) solutions and adopted porous materials (bars with various dimensions) to enhance the productivity of the electrospinning process. The proposed electrospinning mechanism can be used to mass produce nanofibers at a relatively lower voltage (D.C. 6∼7 kV) and obtain a remarkable increase in throughput. The experimental results showed that the jets per area were on the order of 0.85∼1.5 jets/mm2, which is one to two orders of magnitude higher than the conventional single needle electrospinning process and can easily surpass the magnetic needleless method by a factor of 3.3–5.8. The proposed method of using porous materials as electrospinning devices (nozzles) should contribute to the advancement of next-generation, large-scale electrospinning systems for nanofiber fabrication.

A Novel In Situ Roughness Measurement Based on Spatial Average Analysis of Binary Speckle Image

This study proposes a novel optical technique and method for in-situ roughness measurement. The speckle image was obtained by illuminating a laser beam and the reflected laser pattern image from a surface was binarizd and examined. The intensity distribution of binary image utilizes the combined effects of speckle and scattering phenomena. A new parameter of intensity distribution of binary image, Sd BD has been proposed and the surface roughness parameter Ra of machined surfaces (ground) were correlated experimentally. Measurement results demonstrate an excellent correlation between the SdBD and Ra with correlation coefficient of 0.9706. The practicality of the proposed method to in-situ roughness measurement was applied to six samples from roughness Ra 0.2 to 6.25μm (0.3 λ and 10 λ, where λ is diode laser wavelength) of steel through grinding process.