How Nanofluidic Chips Were Fabricated By Near-field Electrospinning (NFES)

In recent years, nanofluidic chips have attracted lots of interests because of their high integration. As a typical technology with great compatibility and low cost, NFES has displayed great potential in the preparation of nanofluidic chips. In 2007, Lee et al. demonstrated the integration of chip-to-chip fluidic connectors, as shown in Fig. 9.21A, via NFES to the wire bonding technique in integrated circuit manufacturing. Orderly direct-written fibers were deposited with position precision of better than 10 mm to connect two separated chips, which served as the sacrificial material. Then, the coating process and sacrificial layer etching process were followed to fabricate micro/nanofluidic channels with inner diameter about 0.05-5 mm. Based on NFES, Wang et al. presented complicated wave-shape and grid pattern channels under the predesigned movement of substrate, of which the fabrication process is shown in Fig. 9.21B. Fuh utilized NFES to generate well-aligned and addressable nanofiber arrays, which were used as the master to prepare polydimethylsiloxane stamps. The nanofluidic channel was sealed by bonding with the same polymer or a microscope slide, as shown in Fig. 9.21C, which shows reliable and repeatable performance in the nanofluidic test.

How To Make Direct-written 3D Structures

Many complex 3D nanofibrous structures with high aspect ratio have been built based on NFES. Lee and Kim proposed a method to fabricate a freestanding nanowall (Fig. 9.20C) with direct-written nanofibers. To control the whipping instability of the electrical nanojets, a conductive microline on the insulating plate was used as the grounded collector to focus the electrical field. In this way, a jet with 180 nm diameter and 30 mm/s velocity can be used to construct a nanowall of 4.5 mm height and 220 mm length. Han et al. applied electrohydrodynamic printing for 3D microstructures. A droplet will print on the top of the previous droplet once the nozzle is fixed at a certain location, which results in a high aspect ratio of pillars. A circular tube with a height of 40-60 mm and wall thickness of 6 mm was printed, as shown in Fig. 9.20D. Compared with traditional 3D printing, the resolution was improved by 1 or 2 orders of magnitude.

How To Use Auxiliary Methods To Print 2D Complex Patterns

To print 2D patterns with complex contours, some methods have been suggested to ameliorate NFES. Zhu et al. designed an auxiliary electrode applied with an AC electrical field to regulate the deposition of wavy fibers in NFES, as illustrated in Fig. 9.19A. An AC voltage was applied to control the wavy amplitude and generating frequency of the fibers. The results show that a continuous wavy fiber can be deposited on the collector when the distance between nozzle and collector increases to 4 mm. The amplitude of wavy fibers increases with increasing AC voltage and the frequency of wavy fibers equals the AC frequency. Lee et al. used a guide ring located 0.03 mm below the nozzle and a pin-type electrode that served as the grounded electrode to focus the jet on the substrate, as shown in Fig. 9.19B. The setup is usable for increasing the axial electrical field without a significant change in the radial electrical field, which can improve the stability of a microsized jet in the cone-jet mode and reduce the breakup of the jet. In this way, various complex 2D patterns can be printed onto photographic paper with average line width of 130 mm.

How To Do SHEATH GAS-ASSISTED ELECTROHYDRODYNAMIC DIRECT WRITING

To further improve the deposition behavior, a core-shell-shaped spinneret has been designed and in this design a sheath gas goes out from the peripheral channel to travel around the jet. The sheath gas provides an additional stretching and focusing force on the ejecting jet, which is beneficial to overcoming interference from the surrounding environment and gaining precise micropatterns. He et al. utilized sheath gas to fabricate micro/nanostructures under a lower applied voltage. With the help of the stretching force stemming from the sheath gas, the initiation voltage and sustaining voltage decrease obviously. Low applied voltage is helpful to restrain the instability of the printing process and promote the integration fabrication of micro/nanodevices. The average diameter of the micro/nanostructure decreases from 21.58 mm to 505.58 nm when the assisted gas pressure increases to 50 kPa. In addition, based on the same setup, Zheng et al. investigated the patterned deposition behavior of a charged jet. With the help of a sheath gas, the surrounding interference can be weakened and the charged jet can be free from the influence of the microstructures. Fig. 9.16 shows that precise complex micropatterns such as parallel lines and grids can be direct written with position precision to less than 5 mm.

How To Make Micro/nanofibrous Structures Direct Written On An Insulating Substrate

As a mass of charge accumulates on the deposited nanofiber, it is difficult to achieve patterned deposition on the insulating substrate. Zheng et al. demonstrated direct writing of orderly micro/nanofibrous structures on a flexible insulating polyethylene terephthalate (PET) substrate utilizing an AC electrical field. The charge transfer characteristics in the jet are changed and the Coulomb repulsive force from the residual charges on the deposited structure is reduced with the help of the applied AC voltage, as shown in Fig. 9.11A and B. Thus a stable jet can be built up and orderly structures, as shown in Fig. 9.11C, can be collected on the PET substrate. From the experimental results, the minimum motion velocity of substrate required to direct write a straight line is 700 mm/s and the line widths of direct-written fibrous structures are in the range of 10-40 μm.

How to Do ALTERNATING CURRENT ELECTROHYDRODYNAMIC DIRECT WRITING

Due to the strong Coulomb repulsive force, direct writing of conductive patterns on an insulating substrate is of great difficulty for NFES. An AC electrical field has been introduced to change the transfer characteristics of the charge along the jet, by which the Coulomb repulsive force can be weakened and the stability of the charged jets can be improved. Nguyen and Byun used a nozzle that was not connected electrically to overcome the electrical breakdown in a conventional NFES system. As shown in Fig. 9.9, an AC voltage is applied to an extraction electrode and the reflection of charged droplets due to patterned geometry on the substrate decreases owing to the patterned geometry on the substrate. Under the AC voltage, positively and negatively charged droplets can be obtained. With the alternation of positive and negative voltage, the jet will be turned to an electrically neutral state, which is helpful for the continuous ejection of droplets even at the peak signal of voltage. Based on the single AC potential setup, dots with sizes ranging from 10 to 30 mm were generated on the substrate. Zheng et al. investigated the effects of process parameters on the microdroplet ejection behaviors under the AC electrical field. The deposition frequency increases and the droplet diameter decreases with increasing AC voltage frequency. In addition, the deposition frequency and droplet diameter increase with increasing duty cycle and solution supply rate. Based on the aforementioned research, Liu et al. printed a bead-on-string structure under an AC electric field. The positive voltage drags out more solution and form beads, while the negative pulse voltage provides the opposite force to stretch the jet into nanofibrous structures between two adjacent beads. The stability of the jet can be enhanced by increasing the voltage frequency. As the voltage frequency increases from 10 to 60 Hz, the diameter of the bead structure decreases from 200 to 110 mm, as presented in Fig. 9.10.

How To Fabricate ZnO Gas Sensor Using Near-Field Electrospray

In addition, a thin-film ZnO gas sensor was fabricated by using near-field electrospray, as shown in Fig. 9.8. Comb electrodes with a large contact area were printed to increase the sensitivity of micro/nanosensors. The electrospray micro/nanoparticles were deposited over the electrodes, which would be heated and oxidized to form a ZnO semiconductor at 500℃ afterward. The experimental results showed that the fabricated sensor displays high sensitivity because of the small diameter and high specific surface area of the electrospray particles, indicating a new promising method for the integration fabrication of micro/nanodevices.