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 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.