Nano Today
Volume 35, December 2020, 100942
Journal home page for Nano Today

Electro-Hydrodynamic Direct-Writing Technology toward Patterned Ultra-Thin Fibers: Advances, Materials and Applications

https://doi.org/10.1016/j.nantod.2020.100942Get rights and content

Highlights

  • Evolvement of electrohydrodynamic direct-writing (EHDDW) technology is introduced.

  • Basic principle, typical apparatus, commonly used materials and controlling parameters during the directing process are discussed.

  • Latest advances of EHDDW applications in fabricating smart materials, electronics and biomedicals are overviewed.

  • Remaining challenges and perspectives towards EHDDW are summarized and outlooked.

Abstract

Fibers, having a large aspect ratio, have become an essential material in human life since the dawn of civilization. Lots of efforts have been made in controlling the fine structure and architecture of fibers for diverse applications. However, great technological challenges remain on patterning fibers with diameters down to tens of nanometers into the desired structure through conventional methods. Electro-hydrodynamic direct-writing (EHDDW) technology shows great potential in depositing the highly aligned micro/nanofibers in a noncontact, direct, and controllable manner which can achieve a real-time adjustment and individually accurate control even on flexible, curved substrates. In this review, beginning with a brief introduction to the history of EHDDW, we first discuss its basic principle and typical apparatus. We continue with a highlight of its rise over the past decades as a powerful technology for the production of nanofibers with versatile compositions and structures. Afterward, we summarize the applications of such “controlled” nanofibers, including their uses as “smart” wearables, energy harvesting/conversion/storage components, and biomedical scaffolds. In the end, we discuss the opportunities and the development directions for this promising area.

Introduction

Due to the rapid development of computer science and artificial intelligence (AI) technology, additive manufacturing (AM) is emerging as a revolutionary and commercial manufacturing technology in the past decade [1]. However, the resolution of the conventional AM is restricted to about 50 μm, which limits its further step toward high-precision application scenarios. The material in fiber-shape has great potential in the process of multi-dimension structure because of its high length-diameter ratio. When the diameter decreases down to tens of nanometers, intriguing new characteristics will emerge. The so-called surface effects, size effects, and quantum effects, [2] make these ultra-thin nanofibers attractive and functional for a wide range of applications, including tissue engineering, smart wearables, and flexible electronics [3]. Electrospinning is the most widely used technique to fabricate ultrathin fibers with various precursor materials, from biopolymers to ceramics [4]. However, the patterning of nanofibers by the conventional electrospinning method is still challenging because of the bending instability caused by repulsive electrical force [5]. With the advancement of applications involving nanofibers, further requirements include high alignment degree, precise patterning, and multi-dimension ultrafine structures that are essential for advanced applications. To this end, great efforts have been devoted to developing the electro-hydrodynamic printing (EHDP) technology, built upon the general electrospinning setup.

EHDP integrated conventional electrospinning technique with the layer-by-layer stacking principle of additive manufacturing [6] to be able to print two-dimensional (2D) micropatterns and three-dimensional (3D) microstructures. A typical EHDP system contains a three-axis (XYZ) precise motion system, a high-voltage system, a pneumatic dispensing system, and a thermal control system (Fig. 1a). In the EHDP process, the ink is pumped out through the spinneret to generate a spherical droplet that is deformed into conical shape due to the competition between surface tension and electrical forces. With the increase of charges, a jet will emanate from the apex of the cone [7,8]. Subsequently, the jet will break up into fine droplets or remain intact to form fibers or threads after solution evaporation and melt solidification on the collector [9]. EHDP primarily includes three types (Table 1) of printing modes [8], namely EHD jet (E-jet) printing (dots formation, Fig. 1b), EHD direct-writing (fiber formation, Fig. 1c, d), and electrospray [10] (particle formation, Fig. 1e). The three types of printing modes can be achieved by setting up appropriate parameters, including ink flux, offset height, employed voltage, etc.

EHD direct-writing (EHDDW) belongs to electric field-based printing, which can be dated back to the electrostatic siphon recorder in 1867 (Fig. 2). In the year of 1999, the first direct-write “dip-pen” nanolithography (DPN) [13] was developed to deposit materials of better than 30 nm line width, which is an advantage in fiber deposition over conventional electrospinning. After that, inkjet printing [24] achieved a large area and fast deposition with discrete droplets with micrometer-scale size in 2000, and further improved the production controllability.

At the end of the twentieth century, tremendous efforts start to explore the methods of fiber alignment, including using dynamic collectors [25], auxiliary parallel electrodes [[20], [21]], and magnetized polymeric solution. However, there still exists the limitation in directly, continuously and individually controlling isolated fibers to fabricate high precise patterns and devices. The length of the stable and straight segment of the EHDDW at the initial part of the jet ranges from 500 μm to 3 mm. Therefore, a rather small nozzle-to-collector distance is adopted to obtain high-resolution micro/nanopatterns with directly writing and relatively low voltage, which is the so-called Near-Field Electrospinning (NFES) technique [16]. NFES was reported to succeed in providing the feasibility of controllable electrospinning to realize the direct-writing of straight micro/nanofibers firstly in 2006 [16]. The temporal research focus is largely on increasing the precision of NFES systems and patterning and exploring complex collector substrates [17,22].

Electrohydrodynamic lithography (EHL) is a significant process of EHDDW in 2010. It is a single-step and cost-effective approach for directly patterning of conjugated polymers on solid substrates with high fidelity. Some researchers prepared user-specific micropatterns with parallel lines as well as lattices with a line width of about 2 μm on a flexible substrate [26,27]. For the first time, Rickard et al. fabricated well-defined patterning of conductive polymer structures through combining EHL with tuning the dimensions of architectures, which opened up many opportunities for applications in nano and bio-technology related fields and devices [28].

To overcome the electric breakdown in the NFES system, a mechano-electrospinning (MES) [29] method was presented to deposit fibers in 2012. The fiber diameters were tuned from 400 to 200 nm continuously in a linear relationship by stretching the fibers though the mechanical drawing of a moving substrate while the Taylor cone was kept stable by the lower voltage [19]. Subsequently, the bead-on-string structures were fabricated successfully [[30], [31], [32]]. The investigation of the fabrication mechanism indicates that the force balance between mechanical drawing force and the capillary force leads to the switch of structures. Although MES could fabricate highly aligned nanofiber arrays and complex patterns, it was restricted to the flat area. Additionally, the straight fiber-based structures enable devices to be bendable but not stretchable.

Nonpannal direct-writing offers opportunities for manufacturing multi-dimension curved devices (2014), even on highly curved surfaces (the radius of curvature: ≈ 50–65 μm) [33]. It’s feasible to implement 3D movements of the nozzle to maintain a uniform electric field via modifying the EHD printing system [34]. Liu et al. digitally printed large-scale high-resolution photoresist micro/nano-pattern on ultrathin/curved substrates based on metal-network electrodes (MNEs) which are prefabricated via programmable electrohydrodynamic (EHD) lithography [35]. The wavy direct-writing (WDW) method [20] in 2015 and helix-electrohydrodynamic printing (HE-Printing) [36] in 2017 was developed to directly write high-resolution continuously serpentine patterns to meet the requirements for the manufacture of stretchable micro/nanodevices.

The nozzle also plays a critical role in EHDDW through affecting the efficiency, the process parameters, and the fiber morphology. Researchers explored novel nozzles to improve the direct-writing precision, reduce process steps, and increase the efficiency of nanomanufacturing. Laminar sheath gas nozzles [37] were applied to promote the position precision of the direct-written pattern. To improve the printing efficiency and devices with different materials, multi-nozzle printing including parallel nozzles [38], addressable nozzles [39], and revolver nozzles were appeared. Similarly, tip-in-nozzle (conductive or non-conductive), co-axial nozzle, and multi-hole nozzle (always in parallel) printing have also been viable with the vary of nanofiber morphology.

In the meanwhile, multiple voltages and various inks (inorganic inks, organic inks, and composite inks) [36] were also explored to improve printing resolution and efficiency in the EHDDW system. Moreover, other endeavors were also taken to improve the property of the EHDDW system. Suspension Near-Field Electrospinning (SNFES) [22] technique, which implements an automated platform to maneuver the pillar electrodes around the emitter to suspend fibers in the free space between the electrode support structures, was developed to surpass the restriction of the layer-height-limit in 2019. Recently, the uniform field electrospinning (UFES) [23] was proposed as an easy-handling strategy by inserting the electrospinning nozzle into the center of an aided metal plate to create complex geometries.

Therefore, depending on the fabrication characteristics, the devolvement history of EHDDW as shown in Fig. 2 can be classified roughly into original electrospinning (nonwoven fabric), conventional direct writing (part-oriented fiber assembly, including DPN [14] and ink printing [18]), and custom-made direct-writing (high-precision micro/nanostructures, such as scanning tip electrospinning (STES) [37], NFES, and MES). Nowadays, EHDDW has been one of the most popular techniques applied to deposit nanofibers in a large-scale, direct, continuous, and controllable manner.

The attractive capabilities of EHDDW, as well as the related research advances (materials[1,8,61], mechanical properties [3], structure design [6,15,35], producing parameters [12,13,16,39,44,49,58], and applications [41]), have been examined in several reviews before 2014. This review provides a comprehensive overview of the EHDDW technology, including its history, fundamental, innovative modules, and novel applications, especially those developed in the recent five years. We first start with the basic principle of EHDDW technology with emphasis on the modus to achieve the direct writing of nanofibers. Then, we introduce the applied materials and related controlling factors. Additional parts highlight the latest advances of EHDDW applications in fabricating smart materials, electronics, and biomedicals. Last but not least, the review concludes with an overview of key remaining challenges and a summary of opportunities where advances in EHDDW will be critically important for continued progress.

Section snippets

Electro-hydrodynamic direct-writing technology

An EHDDW system includes a voltage supply, an ink supply, a spinning unit, a collector, and a 3D motion system (Fig. 3a, b). Generally, a DC power supply is used to generate an electric field between the collector and the spinning nozzle. The addition of an electric field allows the spinning solution to overcome the surface tension to form a Taylor cone, which in turn forms the nanofiber structure. Different nozzle forms, collector shapes and materials have a significant impact on the spinning

Applications

As the EHDDW technique exhibits great superiority in the controllable deposition of a single fiber in precise, continuous, non-contact, high-efficient, and low-cost manner, extensive explorations have been devoted to making breakthroughs for applications in various fields, such as wearables, electronics, biomedicals, etc.

Conclusion and outlook

In the past decade, EHDDW has become one of the most promising technology in nanofiber control and the related direct device molding. It has a significant advantage over conventional electrospinning in continuously controlling the morphology, inter- and intra-porosities, dimension, and direction of the nanofiber deposition. Besides, it has wider compatibility with viscous inks to realize high-resolution patterns and high-aspect-ratio 3D micro/nanostructures. Furthermore, its characteristics of

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgements

This work is supported by NUS Hybrid-Integrated Flexible (Stretchable) Electronic Systems Program Seed Fund (Grant No. R265000628133), Lloyd's Register Foundation, UK (Grant No: R265000553597), and NUS COVID-19 Research Seed Funding (Reference No: NUSCOVID19RG-11). Wanlin Fu thanks the support from the China Scholarship Council (File No. 201806290052).

Zhenfang Zhang received his Master`s degree at Xi`an Polytechnic University. He is currently working with Prof. Seeram Ramakrishna and Dr. Dongxiao Ji at National University of Singapore. His research includes wearables, electrospinning and 3D printing.

References (147)

  • Y. Han et al.

    Procedia Manufacturing

    (2017)
  • E. Tan et al.

    Composites Science and Technology

    (2006)
  • G.C. Rutledge et al.

    Advanced drug delivery reviews

    (2007)
  • C.J. Hogan et al.

    Colloids and Surfaces A: Physicochemical and Engineering Aspects

    (2007)
  • L.S. Carnell et al.

    Scripta Materialia

    (2009)
  • Z.-M. Huang et al.

    Composites science and technology

    (2003)
  • D.H. Reneker et al.

    Polymer

    (2008)
  • Y. Wu et al.

    Materials Letters

    (2008)
  • Y. Huang et al.

    Nano Energy

    (2017)
  • L. Persano et al.

    Progress in polymer science

    (2015)
  • K.I. Lee et al.

    international conference on micro electro mechanical systems

    (2013)
  • Z.-C. Yao et al.

    Journal of Drug Delivery Science and Technology

    (2019)
  • K. Barton et al.

    Mechatronics

    (2010)
  • R. González et al.

    Synthetic Metals

    (2005)
  • I.S. Chronakis et al.

    Polymer

    (2006)
  • Y.-C. Huang et al.

    Sensors and Actuators B: Chemical

    (2015)
  • S. Hong et al.

    Carbohydrate polymers

    (2011)
  • M. Kim et al.

    Chemical Engineering Journal

    (2015)
  • W.E. Frazier

    Journal of Materials Engineering and Performance

    (2014)
  • D.H. Reneker et al.

    Journal of Applied physics

    (2000)
  • B. Zhang et al.

    Nanoscale

    (2016)
  • J. Choi et al.

    Applied Physics Letters

    (2008)
  • D. Ye et al.

    Small

    (2018)
  • J. Zeleny

    Physical Review

    (1914)
  • S.-Y. Min et al.

    Nature communications

    (2013)
  • D. Ye et al.

    Small

    (2018)
  • R.D. Piner et al.

    science

    (1999)
  • D. Li et al.

    Nano letters

    (2003)
  • B. Sundaray et al.

    Applied physics letters

    (2004)
  • D. Sun et al.

    Nano letters

    (2006)
  • J. Xie et al.

    ACS nano

    (2010)
  • N. Bu et al.

    Journal of nanoscience and nanotechnology

    (2014)
  • F. Fang et al.

    Polymers

    (2015)
  • Y. Duan et al.

    Polymers

    (2017)
  • A.R. Nagle et al.

    Nanotechnology

    (2019)
  • Q. Liu et al.

    Nanotechnology

    (2019)
  • H. Sirringhaus et al.

    Science

    (2000)
  • Z. Yin et al.

    Nozzles for EHD Printing

    Electrohydrodynamic Direct-Writing for Flexible Electronic Manufacturing

    (2018)
  • J. He et al.

    Applied Physics Letters

    (2014)
  • P. Goldberg-Oppenheimer et al.

    Small

    (2010)
  • J.J.S. Rickard et al.

    Acs Nano

    (2016)
  • N. Bu et al.

    Materials and Manufacturing Processes

    (2012)
  • N. Bu et al.

    Journal of Physics D: Applied Physics

    (2012)
  • W. Zuo et al.

    Polymer Engineering & Science

    (2005)
  • W. Zuo et al.

    Science

    (2005)
  • W.A. Byeong et al.

    Small

    (2015)
  • B. Seong et al.

    Journal of Micromechanics and Microengineering

    (2014)
  • J. Liu et al.

    Advanced Materials Technologies

    (2018)
  • Z. Yin et al.

    Electrohydrodynamic Direct-Writing for Flexible Electronic Manufacturing

    (2018)
  • J. Zheng et al.

    AIP Advances

    (2016)
  • Cited by (28)

    • A novel method and printhead for 3D printing combined nano-/microfiber solid structures

      2023, Additive Manufacturing
      Citation Excerpt :

      In the last two decades, many research activities have been done to develop strategies for FFF printing textile structures like nonwovens and nanofibers [14,15]. In this regard, novel methods like melt electrospinning writing (or melt electrowriting) (MEW) were developed using the fundamentals of AM combined with electrospinning [16,17]. It is rooted in traditional electrospinning, which is a technique that involves stretching a polymer solution or melt into nanofibers under a high electric field and accumulating it on the counter electrode in the form of a randomly oriented fiber web [18,19].

    • 4D bioprinting of smart polymers for biomedical applications: recent progress, challenges, and future perspectives

      2022, Reactive and Functional Polymers
      Citation Excerpt :

      To date, 4D printing technology has shown tremendous progress in different dynamic micro-environment TE areas, namely tissue cardiac, vascularization, muscle, neural, bone tissue engineering (BTE), and production of vascular stents [80–82]. The major focus of the tissue engineering and regenerative medicine (TERM) approaches is to fully repair or regenerate the traumatized tissues by delivering biocompatible materials to promote the healing mechanism [83–85]. The healing process is attained by combining human cells, biochemical, dynamic, and mechanical signals, and mechanical stabilization which can be achieved through the employment of scaffolds [86–88].

    • Recent advances in 3D-printed polylactide and polycaprolactone-based biomaterials for tissue engineering applications

      2022, International Journal of Biological Macromolecules
      Citation Excerpt :

      Nowadays, electrohydrodynamic printing (EHDP), an emerging IJP technology, gains immense interest in the manufacturing of tailored micro-/nano-scaled scaffolds. This process uses ejection of droplets driven through a high voltage electric field, that is developed between the substrate and the nozzle [200–202]. However, the printed scaffolds usually possess low mechanical properties, biodegradability, and biocompatibility.

    • Fabrication of multi-material electronic components applying non-contact printing technologies: A review

      2022, Results in Engineering
      Citation Excerpt :

      Therefore, limitless enhancement in resolution is only possible by decreasing the diameter of the nozzle. Electro hydrodynamic droplet jetting (Fig. 5(c)) is an alternative to other jetting techniques, enabling it to form droplets of significantly smaller dimensions than the nozzle and achieve high resolution without reducing the nozzle diameter, thus avoiding potential orifice clogging, as well as enabling the deposition of highly viscous fluids [122]. Even though it is driven by an electric field, it was reported that the process can be applied for dielectrics printing as well.

    View all citing articles on Scopus

    Zhenfang Zhang received his Master`s degree at Xi`an Polytechnic University. He is currently working with Prof. Seeram Ramakrishna and Dr. Dongxiao Ji at National University of Singapore. His research includes wearables, electrospinning and 3D printing.

    Haijun He is currently a Ph.D. student at the Department of Polymer Engineering, Budapest University of Technology and Economics in Hungary. He obtained his B.Sc. and M.Sc. degree in Textile Engineering and Textile Materials at Xi’an Polytechnic University (in China) in 2014 and 2017, respectively. He started his Ph.D. study with the Stipendium Hungaricum Scholarship funded by the Tempus Public Foundation (in Hungary) and China Scholarship Council (in China) in 2017. His research interests are in the development of new electrospinning methods for the scale-up of the nanofiber productivity, polymeric composites reinforced with nanofibers, 3D printing, and smart/functional nanomaterials.

    Wanlin Fu obtained her B.Sc. of Chemical Engineering at Southeast University in 2015. She is pursuing her Ph.D. at Southeast University. In the fall of 2019, she joined the Seeram group at National University of Singapore as a visiting Ph.D. student supported by the China Scholarship Council. Her research interests include new synthesis and modification approaches for functional nanomaterials based on electrospinning towards a cleaner environment and sustainability.

    Dongxiao Ji is a research fellow in department of mechanical engineering at National University of Singapore. He received his M.S. and Ph.D. degrees in textile science and engineering from Donghua University in 2012 and 2018, respectively. Dr. Ji is experienced in experimental and analytical research work on heterogeneous catalysis, functional nanofibers, and large-scale electrospinng. His research interests include smart/electronic textiles, functional electrospinning membranes, heterogeneous catalysis, and surface modification for environmental and energy applications.

    Seeram Ramakrishna is a professor in department of mechanical engineering at National University of Singapore. He is a Highly Cited Researcher in Materials Science (Clarivate Analytics, 2014; 2015; 2016; 2017; 2018), and in ‘Cross-Field’ category (2019). A European study placed him among the only 500 researchers in the world with H-index above 150 in the history of science and technology. He is the world’s foremost scientist on nanomaterials by electrospinning for uses in diverse fields such as healthcare, energy, water, and environment. His research work over the past three decades led to seminal contributions in novel processing and mechanistic understanding of functional behavior of composite materials, nanofibers, and nanoparticles. He co-authored ∼ 1,400 SCI peer reviewed papers which received over 100,000 citations and 150 H-index.

    View full text