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A flexible additive manufacturing technology to deposit 2D or 3D material ranging from nano- to mesoscales has developed huge interest in researchers, scientists and engineers. This technology is known as direct writing and can deposit materials in liquid, paste or gel state as well as tissues or cells onto a substrate. Substrate materials include all type of glasses and metals and their alloys, plastics and ceramics. This write up focuses on the applications of this technology in a large scale. The aim is to deposit or develop products as small as possible, leading to product miniaturization, functional and improved performance. Four categories (ink base, energy beam, flow or filament base and tip base direct writing) of this technology give the capability and potential to achieve this task. This technology is being applied in the food and drugs industries, manufacturing, biomedical, aerospace, electronic industries and institutions for the training of young engineers who have interest in this specialization.
Table of Contents
Literature Review of Direct Writing Application
The need for improving products to perform better in our modern society in order to satisfy customers and meet competitive demand have always bring about the research for new technology for product development. Numerous technologies and processes have been discovered within the past few decades and more, yet to be revealed. Technological advances keep emerging and one of these advances is direct writing (DW) technology. This technology, described by Piqué A. and Chrisey B. (2002) as a subset of a layer by layer process of building 3D models, has been in existence for quite some time and currently undergoing more research for improvement. Direct write technology involves a wide range of different technologies and they differ in terms of process parameters such as critical dimension, exposure, writing speed, materials deposited, resolution, operational environment (pressure, gas and temperature). Temperature is a key factor in direct writing processes and most material deposited are at temperature between 200 to 400oC.
In order to describe this process in a simple phase, various definitions have been developed but these definitions did not uniquely differentiate the termed process from rapid prototyping processes. Two suitable and precise definitions of direct writing were adopted in this review. The first one, defined by Hon K. et. al. (2008), "as a group of processes which are used to precisely deposit functional and/or structural material on to a substrate in digitally defined locations". The term "substrate" indicates that materials are deposited on another material. Unlike rapid prototyping materials are deposited on a platform or powder bed as in the case of 3-dimensional printing. Substrate material that are used in direct writing process include all types of glasses and metals, alloys, crystals, ceramics and synthetic materials such as plastics, natural organic materials and biological materials e.g. a cells.
While the second definition was defined by Zhang, Y., et. al. (2009) "as an additive techniques enabling the deposition of electronic components and functional or structural patterns, out of different kinds of materials, directly following a preset layout in a data driven way without utilizing masks or subsequent etching processes". This definition was derived from a combination of other definitions and the "preset layout" described in this definition not only gives the deposited material a unique aesthetic feature but also deposit the required amount of material in a confined space on the substrate, Martin, L et. al. (2009) and Piqué A. & Chrisey B. (2002).
Having given brief description of this technology, this write-up focuses more on the application of direct writing processes. This technology is not new and numerous papers have been published on the working principles of direct writing. Hon, K. et. al. (2008) compiled a list of journals of additive manufacturing processes such as direct writing, rapid prototyping and ink jet between the years 1990 and 2007, figure 1. A further compilation of journal was made on only direct writing between the years 2007 till date to know the number of publications published on this topic, figure 2.
Figure 1: Publication of 'direct writing', 'ink jet' and 'rapid prototyping' between 1990 and 2007 .
Figure 2: Publication of 'direct writing' between 2007 and 2010.
From figure 1 & 2, researchers and engineers are developing massive interest in the discovery of new application areas of direct writing. Mortara, L. et. al. (2009) proposed that direct writing technology have the potential to transform not only the manufacturing sector but cuts across different sections in modern development such as automation, ICT, biotechnologies, military & aerospace electronics (Mil-Aero), chemical & petrochemical, construction, medicals devices, ceramics and leading to many other benefits which includes:
Lower development costs
Faster time to market
Better product design and performance
Improve inventory control
Supply chain impact result in direct cost savings.
Direct writing, regarded a rapid prototyping process as mentioned earlier, involves lots of processes which are classified into four categories. The four categories includes
Droplet based direct writing
Energy beam direct writing (EBDW)
Flow or filament based direct writing
Tip direct writing.
Droplet based direct writing
This is one of the oldest methods of direct writing. The direct deposition of ink or lead onto a paper from a pen or pencil was later revolutionised into this method. In droplet based direct writing, ink is ejected as droplets from a single or multiple nozzles, and it is categorised into ink jet and aerosol jet. The ink jet DW, is further sub-categorised into drop-on-demand (DoD) and continuous ink jet (CIJ) method of direct writing. Piezoelectric and thermal inkjet nozzles are further explained by Zhang Y. et. al. (2009) as the two main types of actuation methods in DoD, while Hon K. et. al. (2008) expresses his opinion on the classification of continuous ink jet by Lee H.P. (1998), in the following manner. CIJ "can be further sub-divided into binary deflection, multiple deflection, Hertz and microdot." The diameter and discharge rate of the ink for single droplet on demand ranges from 15-200µm and 0-25,000 per seconds. These parameters increase the adaptability of the droplet-base direct writing and have application areas in small and large scale printers as well as making purpose for industrial and specialty use.
The application of ink jet is used for the printing of expiring date and other information on different products from their manufacturers. Bar codes, printed on products and packs of products with this technology, allow the smooth transfer of products from one location to another. Food industries are not the only one benefitting from this technology. Electronics manufacturers and medical diagnostics have also shared some benefits. Piqué A. & Chrisey B. (2002) proposed recent development in photonics, micro electromechanical system (MEMS), wireless communication and portable electronics are driving forces through the use of inkjet printing technology for product development. Other areas or products where this technology play a significant role is in the manufacturing of sensors and photonics elements, refractive microlenses, optical lenses, light emitting polymers, adhesives, resistors, capacitors, inductors, batteries, DNA and peptide arrays and antibody-antigen interaction of bacteria materials.
Figure 3: 3D Silver interconnects (150µm linewidth) Figure 4: Aerosol system printing antenna directly written over an aluminium cube.  on a curved surface. 
King B. & Renn M. (2009) suggested that the unique ability of the aerosol jet system to print on non-planar surfaces makes it an ideal solution for printing sensors, antennae, embedded components that can be integrated into military applications such as Mil-Aero electronics.
Energy Beam Direct Write (EBDW)
Energy beam direct writing is one of the versatile direct writing technology and several journals have been published based on classification and principles, discharge parameter, improvement and materials, and the commercial and economical aspect of EBDW. Hon K. et. al.(2008), classified EBDW into nine categories which are: laser chemical vapour deposition (LCVD), laser enhanced electroless plating (LEEP), laser enhanced or activated electroplating, laser consolidation of the solid films, laser induced forward transfer (Lift), matrix assisted pulsed laser direct write (MAPLE DW), laser induced backward transfer, focussed ion base (FIB) and laser contact-free trapping and transferring of particles in solution for 3-D direct write. Each of these sub-topics was explicitly dealt with in theory and application. Glasser L. (2007), described EBDW as the "...technology of the future..." with each technique gaining a vast interest and has a long history in the development and improvement of semiconductors manufacturing.
Undoubtedly, Martin L. (2009) actually identifies the multi-beam solution as one potential technique for the next generation lithography technique. EBDW, have much interest in laboratories and R&D, also have a shortfall of proximity effect that limits the resolution. A feature common with all classification is the use of laser which is a unique material processing tool. The useful parameters that are considered when using this technology are processing speed, precision, patterning accuracy, resolution capability and critical dimension.
Application areas of EBDW are 3D laser ablation micromachining to process glass and other hard materials like ceramics, Sl3N4, Al2O and ZrO2. Piqué A. & Chrisey B. (2002), also disclose a host of industrial application such molecular electronics, optical communication, micro photonics, magnetic information storage, quantum effect electronics, biological research and MEMS.
Another application area of laser direct writing undergoing research is the direct writing of electronic material on living biological substrate or specimen. Chrisey D. et. al. (2000), explained that the need not to only electrically interface animal kingdom for better understanding but also to manipulate it. A typical example is to train and use an adult worker honeybee to trace and detect extremely low toxic chemical. To do this, scientist must attach an extremely small antenna to each animal for communication.
Figure 5: 35-GHz fractal antenna design (left) and MAPLE-DW printed antenna on the abdomen of a dead drone honeybee (right), .
Flow or Filament Base Direct Writing (FBDW)
This method of dispensing of material is seen as a much easier method of material deposition. The two main technologies under FBDW are microPen and nScrypt. They basically differ from ink jet direct writing, where in this case the material flows continuously from a syringe rather than in drops or jet. Hon K. et. al. (2008), explains further that the material is discharged by means of a pump, air pressure or other mechanical means and the material used include ink, paste or slurries. Main advantage of this printing tool is its ability to dispense materials with wide range of viscosity forming 2D and 3D structures, and plays a significant role in biotechnology and tissue engineering. The emerging ability is to produce highly spherical particles with control size between 0.3-1.5µm, purity and crystalline structure, Piqué A. & Chrisey B. (2002), as shown in the figure blow.
Figure 6: (A) Synthesis direct writing of three dimension polymer scaffolds using colloidal gels. (B) Magnification of (A), .
Tip Base Direct Writing
There are two techniques involve in tip based DW which are dip pen nanolithography (DPN) and nanofountain pen (NFP). Zhang Y. et al. (2009) stated the discovery of DPN technique by Piner et. al, that has left several researchers confused in the study of atomic force microscope (AFM) for a long time, where he tried to transport condense water either from a substrate to the AFM tip or vice verse depending on the relative humidity and substrate wetting properties. The capillary effect of the AFM makes it possible to dispense the ink onto the substrate, which have affinity for the ink.
Hon. K. et. al.(200), mentioned that DPN technology is a scalable, high-throughput, flexible and versatile method of precision pattern formation. This shows its ability of creating fine features as small as 12nm in linewidth and 5nm spatial resolution. Also, NFP is quite similar to DPN but the AFM is replaced by a cantilever nanopipette.
There are vast numbers of application for this probe base lithography method and DPN is fast becoming a mainstream technique for the expanding field of nanomaterial discovery. The use of nanomaterial discoveries, such as the selection of biomolecules in the creation of nanoarrays, alignment of single walled carbon nanotubes with the use of DPN templates and the development of nanoarrays spotted anti-bodies for the early detection of HIV relative to other methods, are employed in clinical diagnostics.
It was recently discovered, at the Texas A&M University by Dr. Debjyoti et. al., that DPN can be used as personal safety device by sensing explosives. This can be used in the airport, military or even a scanning device before entering a musical concert. Both this methods are used for the deposition of material on a nanoscale feature, either pure or mixed with biological material such as molecule, dye, protein or peptide.
Schematic of the transformation of the deposited polymer into a nanoscale three dimensional hydrogel network. Helmet with Direct Write antenna applied to surface
Figure 7: Schematic of the transformation of the Figure 8: Helmet with direct write antenna applied to surface  deposited polymer into a nanoscale three dimensional
Direct writing technology, the ability to deposit functional or structural material onto a substrate with any topology, is increasing gaining tremendous application in different and wider area of life. The potential capability to deposit material as small as nanometers has given this branch of additive manufacturing an added advantage over the conventional rapid prototyping. Many industries such as food, manufacturing, electronics, aerospace, biological, medical and military are exploiting this potential technology to improve their product base on miniaturization and increased performance. Armed forces do not need to go to battle with heavy radio transmitter, when such transmitter is integrated with their helmet. In biotechnology, researchers have developed means of using deposited biological material such as antigen-base for the early detection of HIV and other harmful diseases and also to detect explosive material.
The energy based DW; one of the most widely used categories is employed for the deposition of metal (like antenna) onto living tissue for communication and tracking harm devices. This also, has potential capabilities in the mil-aero application where IC, semiconductors and chips have developed through this technology, to reduce the weight of military aircraft, reduce unnecessary parts and increase efficiency.
The well established DW, the droplet based, is used in private and commercial larger and small scale printers and also used for the deposition of metals. This technique have been use to create mini electronic components such as capacitor, antenna, resistors, inductors and sensors e.t.c.
In the future and from the technical and commercial view, based on its capability to deposit materials in nanoscale and high resolution, is to see improved reliability of DW technology, higher fabrication accuracy, easier operation mode and the fall in cost and volume of direct writing technology in order to be used for large scale production as shown in figure 9, Matsuoka G. & Tawa T. (2003). Two constrains which characterises the high cost of DW is the cost of design and the cost of reticles, Glasser L. (2007). With the cost of reticles declining at the rate of 30% each year as shown in figure 10, "virtual reticles set" have been developed as a substitute to further reduce the high price of the physical reticles.
Figure 9: Comparison of cost of lithography Figure 10: Physical reticles declining per year .
with and without using mask for writing .