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Surfaces and interfaces

Surfaces and interfaces

Ink-jet technology moves beyond paper

04 Jan 2006

The technology behind ink-jet printers has developed so fast that they can now be snapped up for bargain prices on the high street. But manufacturers are keen to take ink-jet technology to other markets, including electronics, medicine and energy, as Chris Williams explains

An eye for detail

At a Glance: Ink-jet printing

  • Ink-jet printers have helped to drive the IT revolution and can now be bought on the high street for as little as $100
  • The printers mostly work by ejecting tiny droplets of coloured ink from nozzles using pressure or heat
  • Although tens of millions of printers are sold each year, the physics of ink-jet technology remains something of a black art: questions include how the droplets split when leaving the nozzle and how they bounce off a substrate
  • Companies are now trying to extend ink-jet technology to other markets, such as pharmaceuticals, electronics and renewable energy

Every time you buy a new computer, you know that within months it will be superseded by a machine that is faster, cheaper and boasts even more memory. However, it is easy to forget that a similar revolution has quietly been taking place in the world of printing. Desktop printers, which now cost as little as $100, are marvels of scientific and technological achievement.

From lowly beginnings just 30 years ago, ink-jet printers have developed at a staggering rate and have consigned the humble dot-matrix printer to the technological dustbin (see “A brief history of ink-jet printing”). More than 100 million ink-jet printers are manufactured each year – cleverly marketed so that the printers themselves are priced cheaply but the ink that they use sells for up to $1500 a litre.

Given that ink-jet printing is such a profitable business, you might think that the underlying technology is now fully understood. But despite the billions of dollars that have been spent on research into ink-jet technology – about 10 ink-jet patents are registered every day around the world – there is still a frustrating sense of alchemy and magic to the ink-jet process.

For instance, a printer that works well with ink A may simply not print ink B, even though both inks appear to share the same characteristics. Similarly, an ink that prints perfectly onto substrate X will smudge and blur on substrate Y. Determining why is rarely easy. It can also be hard to measure key parameters such as the amount of ink discharged by a printer’s nozzle or where a particular droplet will land.

But ink-jet technology has done more than just underpin the “direct-writing” revolution, which has enabled us all to print extremely high-quality text, images and photos on paper under computer control. It can also, for example, create patterns on textiles or make furniture or floor tiles look like wood, stone or ceramics at a fraction of the cost of the real thing.

Most interestingly of all for physicists, ink-jet technology is a great way of depositing tiny quantities of “smart” materials in the form of minute liquid droplets. Researchers are therefore using it to develop new ways of making everything from solar cells and medical sensors to electronic circuits and pharmaceuticals. Some of the applications may sound far-fetched, but they are entirely within the bounds of reason.

Creating images with ink-jet printers

All ink-jet printers work by ejecting a series of droplets of ink from nozzles in a printhead onto a substrate. These droplets are deposited line by line under computer control, forming the dots that together make up an image. The number of dots that are deposited at any one time depends on the total number of nozzles in the printhead.

Most desktop ink-jet printers have several hundred nozzles, each of which is 20-30 μm in diameter. However, the speed of printing does not necessarily increase in direct proportion to the number of nozzles. After all, if you have four times as many nozzles, there will be four times as many electrical control lines to monitor and manage.

In the case of colour printing, every dot in the image is formed from several different droplets, each with a different colour. When viewed from a distance, these droplets appear as a single colour to the eye. Most printed colour images are built up from just four separate colours – cyan, magenta, yellow and black – that are deposited in different amounts to simulate most colours that the eye can discern.

The tone and hue of each dot (i.e. each pixel) depend on the exact volume and position of each coloured droplet. However, it can be hard to mimic human flesh tones with just four colours. Spurred on by a desire to make printed images as realistic as possible, many ink-jet printers now use five, six and sometimes even eight colours.

To create top-quality images, the printed dots need to be about 50 μm in diameter and vary in size by no more than ±2%. They need to be placed with an accuracy of better than 20 μm, while the volume of each droplet of ink is as little as 3 picolitres (3 × 10-12l) and will weigh only about 10 nanograms, depending on the constituents of individual inks. If the droplets are the wrong size or are placed inaccurately, the eye can notice unwanted patterns, such as “Moiré images” – alternating bands of lighter and darker fuzzy stripes that are most noticeable when looking at blocks of solid colour. Most printers use various electronic and software tricks to fool the eye into overlooking these defects, although cheaper devices omit them to save money.

If used continuously, most desktop printers would run for about 24 hours before the printhead’s nozzles break or get blocked and blur the printed image. This is equivalent to printing several thousand sheets of paper, which is fine for home users but would be totally unacceptable for industrial printers – those used to print, say, posters for billboards (figure 1). These machines have more than one printhead and can contain as many as 30,000 nozzles in total, with each nozzle capable of firing up to 40,000 droplets of ink per second.

The number and size of the colour droplets determines the resolution and clarity of the image. But as ink droplets have become smaller, we can now print images with resolutions that are so high that they exceed the ability of the human eye to resolve them. In other words, pictures can now be printed with more actual detail than we can possibly see at a normal viewing distance!

But beware advertisements claiming resolutions of many thousands of dots per inch. If a printer has six coloured inks and can print 2400 dots per inch, the actual resolution is only 400 pixels per inch because each coloured dot has six differently coloured “sub-pixels”. In any case, a resolution of 400-600 pixels per inch is more than satisfactory when printing documents. Going beyond this simply adds unnecessary detail, although it can assist in removing some of the banding artefacts mentioned earlier.

The basics of ink-jet printing

There are two types of ink-jet technology: “continuous” and “drop-on-demand”. In continuous ink-jet printing, a mechanical pump forces ink at high pressure through a nozzle on a printhead, while adding an electrostatic charge to each droplet (figure 2). This process creates a steady stream of charged ink particles that are ejected in a forward direction.

Each droplet can be steered to where it is needed on a substrate by applying a variable voltage to horizontal and vertical electrodes. Where no ink is required, the voltage is applied so as to direct the continuous flow of ink downwards into a “gutter”, where it is collected, recirculated and reused. This technology is widely used to stamp date-codes onto drinks cans, but is limited to printing low-viscosity and relatively volatile inks.

Drop-on-demand printing includes both “bubble-jet” and “piezoelectric” printers, which are widely used in homes and offices. Both consist of an array of chambers, each of which is linked to an individual nozzle. An electrical signal sent to an actuator then generates a pressure pulse that ejects a droplet of ink onto the substrate. (These devices should not be confused with “laser” printers, which do not even use lasers but are based on light-emitting diodes and are a different technology altogether.)

In the case of bubble-jet printers, tiny resistive heaters are used to create small bubbles of steam inside the ink, which is usually water based. As these bubbles expand, they produce the pressure pulse that is needed to eject the ink droplet. The bubble then collapses, the pressure falls, and fresh ink is sucked in from a reservoir. This mechanism is ideal for simple, small devices, which is why bubble-jet printers are so cheap. Manufactures recoup their investment costs by charging customers high prices for the replacement ink cartridges, even though ink is relatively cheap. (Note that the new cartridge may also include a replacement ink-jet printhead as well.)

Piezoelectric ink-jet printers, in contrast, use a class of ceramics based on lead zirconium titanate. These materials can be processed so that they change shape slightly if subjected to an electric field. When incorporated into an ink chamber, the ceramics apply a pressure to the fluid, which causes the ink to be expelled. Piezoelectric printer mechanisms cost more to make than bubble-jet systems, but they can use a wider range of inks and can eject far more drops before they fail.

The main reason for this is that the ink in a piezoelectric-based printer does not heat up, which means that no steam or other vapour is created that might disrupt the chemistry of the ink. The ink can therefore include highly volatile solvents, which allows companies to try different inks for novel applications. Of particular interest are smart, or “functional” inks, which consist of a core substance that will perform some electrical, chemical, optical or mechanical function when deposited onto a substrate. By dissolving this substance in the appropriate solvent, the fluid can then be passed through an ink-jet printer.

For example, many companies are using electrically conducting inks to make electronic circuits. These inks typically consist of nano-sized flakes or particles of silver, carbon or other conducting materials dissolved in a suitable solvent. This results in a low-viscosity fluid that can be “jetted” from a printhead to form the conducting strips of an electrical circuit. As a result, this process is cheaper and much better for the environment than conventional manufacturing techniques because it only prints material where needed and does not require toxic chemicals to etch away unwanted metal regions.

Unfortunately, functional inks are often chemically aggressive and can rot the materials in the printhead. The mixture of a core functional material dissolved in a solvent can also create unwanted lumps of material that block the printhead’s nozzles and create gaps in the pattern of the printed material. While a clogged nozzle in a desktop printer is not critical, a missing dot in an electrical circuit could make it fail. Solving such problems is critical if ink-jet printing is to enter new market areas, and it will require sustained research into the physics of the inks in the ink-jet process.

Technical challenges

The goal of all ink-jet researchers is to create a controlled and consistent flow of ink from the printhead onto the substrate – be it paper, plastic or another material – and to prevent the printhead’s nozzles from becoming clogged. Of particular interest to physicists is the question of how the droplets of ink leave a nozzle, during which the drops are subject to a shear force that is so large that it cannot be measured using any existing techniques. This force can literally tear apart the complex ink molecules as they pass through the nozzle, which can totally destroy the functional part of the droplets.

Another problem is how to accurately predict the trajectory of the droplets, which are becoming ever lighter and smaller as manufacturers seek to make printers with ever higher resolutions. Controlling the placement of such droplets, which have a volume of just a few picolitres, is far from easy. The slightest perturbation – even an air current between the printhead and the substrate – can make the droplet veer off course and land in the wrong place.

Moreover, when a droplet leaves a nozzle, it usually consists of one main drop and a series of smaller “satellite” drops (figure 3). These drops are formed when the nozzle sucks back the last part of the droplet while drawing new ink in from the reservoir. It is essential that these satellite droplets coalesce with the main droplet before it lands on the substrate, otherwise you end up with a series of smaller dots around the main dot and hence a blurred image. This can be achieved by tweaking the electrical signal that creates the droplets in the printhead. Industrial ink-jet printers are also fitted with high-speed video cameras to verify that the droplets have successfully coalesced.

But the problems do not stop at the printhead. When a droplet lands on a surface, it may bounce and can even fragment. This must be minimized to ensure a sharp image. Furthermore, if the droplet lands on a rough surface, such as “untreated” paper, it may spread unevenly as it dries – particularly if there are fibres or other microscopic structures that “wick” the ink in different directions. Indeed, the goal of paper science is to make paper with “filled-in” surfaces so that ink can dry evenly on it with minimal spread.

Taking ink-jet printing beyond paper

Printing colour images is fairly simple because the human eye is very tolerant of error. But in applications where the goal is to place functional fluids very accurately, rather than to please the eye, printing errors can be catastrophic. This is a particular problem for companies trying to create electrical circuits by ink-jetting functional fluids onto plastic substrates, rather than conventional fibreglass, which is nice and rigid.

The problem is that a sheet of plastic lying on a surface – even if stationary and subject to no external force – will wobble slightly. If an array of dots is printed on this surface, the actual position of each dot will move over time. Although this movement is small – typically less than 5 μm – it can be disastrous in an electronic component made from a sequence of inks that need to be applied in precisely aligned layers. Researchers therefore have to monitor the position of the substrate in real time and compensate for any slight movement.

Ink-jet technology may also have applications in the displays industry, where it could be used to create the transparent electrodes that criss-cross the front and rear surfaces of computer displays. The substrates for most displays are currently sputter-coated with tin-doped indium oxide (ITO) and patterned using photolithography. However, this requires several manufacturing steps and unpleasant chemicals. It is also wasteful of indium, which costs about $1000 a kilogram and is becoming increasingly difficult to obtain.

An alternative material is antinomy-doped tin oxide, or ATO, which has similar optical and conductive properties to ITO but is cheaper and much more abundant. Unfortunately, it has never been widely used in the electronics industry to make transparent conductive tracks because it is hard to etch into high-resolution patterns. Now, however, Steve Lipiec from Keeling and Walker, together with researchers from Nottingham Trent University and Patterning Technologies, have joined forces under the ELJET project of the UK’s Department of Trade and Industry (DTI) to develop ATO inks made from nanopowders for ink-jet printing. The partners are currently seeking to optimize the conductivity of the inks while ensuring they remain optically transparent.

There are also moves to develop transparent conducting polymers as an alternative to metal oxides. For example, scientists at Merck, TWI and the universities of Cambridge and Manchester are developing conducting polymers that can be processed in solution and hence be ink-jet printed. These materials have a much higher resistivity than metals because the electrons can move freely along the molecules of each polymer chain when a voltage is applied but have to jump between chains to progress further in space.

Ink-jet technology could even be used to make solar cells, which convert sunlight into electricity. At the moment, these devices have to be made in steps using different pieces of equipment; ink-jet technology would allow all parts of the device to be created at once, which would be cheaper. The ink-jet process could also be used to lay down the materials that harden to form the walls of fuel cells, which generate electricity by reacting hydrogen with oxygen. These walls have to be mechanically sound because fuel cells store such a large amount of energy in a small volume.

Such power sources could be integrated into a variety of electronic display systems, such as road signs, hand-held equipment, advertising posters or e-books, where the underlying electronics, the wiring and the display itself are all ink-jet printed onto a suitable substrate. These devices would be “energy neutral”, generating the energy that they need to run.

Moving into medicine and biology

Ink-jet technology is also starting to find applications in the biological sciences. For example, Brian Derby from Manchester University, Tim Claypole at Swansea University and co-workers are using it to make sensors that can test for, say, pregnancy or diabetes. The sensors consist of a strip of paper or plastic onto which droplets of one or more enzymes have been ink-jet printed. The enzymes, which are surrounded by a suitable buffer material so that they are not exposed to airborne contaminants, can monitor the properties of saliva, urine or other liquids. The advantage over conventional pregnancy test kits is that an electrical circuit could be ink-jet printed onto the sensor, which could be used to give the user a clear yes or no on a bulb or small display. If this technology takes off, such sensors could become so cheap that patients could print their own strips at home using dedicated ink-jet machines.

Amazingly, Derby’s team has also shown that it is possible to pass live animal cells through an ink-jet printer – and that the cells remain alive when they reach the substrate (figure 4). Based on this finding, work has begun to build materials consisting of live skin cells within “scaffolds” of bio-compatible fibres. Nutrients are then jetted into the structure to allow the cells’ DNA to control the process of cell multiplication and joining. Once the required material is ready, the scaffold fibres are dissolved, leaving a patch of “skin” that can be applied to burns or ulcers. As this skin would be realistic, the chance of the body rejecting it is greatly reduced.

Claypole and Derby are also investigating whether ink-jet technology can be used to make drugs. It may sound like science fiction, but one day you could take your prescription to your local pharmacist, who would dial up the required formulation into an ink-jet printer that will print your pills on the spot. This process would involve ink-jetting micrograms of chemicals into picolitres of fluid to create the active drug. This drug would then be injected into the body of the pill in a series of layers. If you take medication that needs to be released into your body over a certain period of time or that must target specific areas of the body, then these active components could be built into soluble capsules within the main pill to give the required release characteristics.

Such techniques would let companies make small to medium quantities of a product at a relatively low price because of minimal set-up and tooling costs. Furthermore, they would not have to store pills that are never used, which would help to lower the cost of the medication still further. However, this technology is at a very early stage and will take at least another five years or more before it reaches the market. Unfortunately, few details of this work can be released to the public for commercial reasons.

Another commercially sensitive area of research involves using ink-jet technology to bring together tiny quantities of liquids to carry out the combinatorial tests that are used, for example, in DNA testing. This would involve printing a series of columns of tiny droplets onto a substrate, with each column consisting of a different reagent. A second set of chemicals could then be printed in rows so that each point in the 2D array represents a unique combination of reagents. The results of the reaction could then be analysed optically, for instance by measuring the light emitted by the reaction. As each droplet would only need to have a volume of a few picolitres, the reactions would be quick and efficient.

The future

The colour-printing industry is estimated to be worth over $300bn worldwide, and ink-jet technology is likely to continue to eat into this market by taking over from traditional, analogue printing techniques. It is also showing great promise as a way of enabling tiny quantities of smart materials to be deposited on surfaces, thereby making possible a range of products and devices – from solar cells to pharmaceutical pills – that are not available using any other means.

Given the success of desktop ink-jet printers, ink-jet devices could one day be as commonplace in other applications, notably in the electronics industry. However, we need to be realistic and remember that the best ink-jet printers can currently achieve a maximum resolution of 25 μm, whereas the components on electronic circuits need to be less than 10 μm in size.

In spite of these tough challenges, ink-jet printing remains commercially attractive because it is environmentally friendly, uses relatively cheap components, and does not require expensive manufacturing facilities. Provided we have an ongoing programme of research into the physics of why and how this technology works, every walk of life will one day be affected by this technology – limited only by the imagination of scientists and engineers.

A brief history of ink-jet printing

1867
William Thomson (later Lord Kelvin) is granted a patent for his proposal to use electrostatic forces to control the release of ink drops onto paper. Without computers, however, he has no way of controlling the pattern of the droplets.
1951
Siemens produces the first commercial “continuous” ink-jet printers. These are mainly utilized in the food and packaging industries, where they are used to print sell-by dates and bar codes on products.
1973
The desktop printer is born with the invention by Cambridge Consultants of the “drop-on-demand” ink-jet process. It allows the production of droplets to be controlled, which is vital for printing documents.
1979
Hewlet-Packard and Canon independently invent their own variations of this initial bubble-jet printing phenomenon.
1987
Cambridge Consultants patent an alternative form of drop-on-demand printing using piezoelectric crystals.
1993
Epson enters the market with the first commercial piezoelectric desktop printers.
1994
Epson, Canon and HP introduce colour ink-jet printers.
c.2000
Ink-jet printing displaces conventional “screen printing”, in which coloured images are laboriously created by squeezing individual coloured inks through a series of carefully patterned masks onto a substrate, usually paper.

More about: Ink-jet printing

Inca Digital: www.incadigital.com

UK Displays Network: www.ukdisplay.net

Xaar: www.xaar.co.uk

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