µPlasmaPrint Technology Basics

Electrical discharges – or plasmas – have fascinated mankind for centuries, because of their aesthetical beauty and uncontrollable behaviour. For a few decades now plasmas are being used abundantly in industry for a myriad of applications,...

mostly in the controlled environment of vacuum chambers. In recent years it has become possible to operate plasmas in atmospheric pressure in a controlled manner, opening up the possibility of plasma treatment of surfaces that cannot withstand heat and/or are difficult to use in vacuum. InnoPhysics has developed an atmospheric plasma source that can deliver plasma in a patterned manner, such that the benefits of digital printing now apply to plasma treatment as well. Instead of ink droplets, dots of activated gas molecules are being printed: µPlasmaPrint.

PlasmaViewThe technology behind µPlasmaPrint can be most easily explained by referring to traditional impact printing techniques in which a needle is mechanically displaced, impacts an ink ribbon and thereby transfers ink to paper. When moving a needle electrode down towards the substrate, at a specific distance to the substrates, the plasma discharge is ignited. By using a print head with 24 electrodes that are independently controllable the plasma discharges can be generated on-demand, based on a predefined digital pattern. The photo on the right shows a side view of a single row (12 needles) and bottom view of the dual row (24 needle) plasma print head simultaneously in action.

By means of plasmas, we have a way of creating specific molecules efficiently, or to deliver chemical energy to surfaces without heating the substrate. This was realised a long time ago and plasmas found their applications in deposition of layers and in etching. Both are essential steps in the production of semiconductors nowadays. By being able to create controlled plasmas at atmospheric pressure, the versatility of plasmas can be applied to materials that cannot stand vacuum (such as biological tissues) or to materials that are unwieldy for vacuum equipment (such as big rolls of plastic sheets). As such, plasmas are finding applications in the emerging field of biomedical devices and plasma medicine [5], in which they are tested for disinfection and stimulation of wound healing. Already more established is their capability of functionalisation of surfaces: the wettability of substrates can be altered substantially and surfaces can for example be prepared before they receive a layer of inks or glue that would otherwise not hold. In the textile industry plasmas are used as a pre-treatment before dyeing, or, using different chemistries, to render textiles water repellent. 

Surface modifications can be categorised in three types: activation, etching and deposition. The type of modification realised with a plasma depends largely on the gas environment in which the plasma is ignited. It is therefore possible to make tools that can do any of the three of the operations by changing the gas supply and choosing the right settings for the power supply. In the following we will focus predominantly on activating the surface, although the equipment described is indeed not limited to this application.

 Changing, activating and functionalising surfaces are terms that are typically used when one talks about changing the surface energy of materials, or adding molecular groups to surfaces in order to change the adhesive properties of specific chemical compounds to the surface. Plasmas achieve this via the reactive atoms and molecules that are created in the collisions that happen between electrons, ions and neutral atoms and molecules. If oxygen and nitrogen are present, many radical species can be formed, such as O3, OH radicals and ions, H3O+, singlet oxygen, NO species, and many, many more [4]. The chemically energetic particles are capable of opening bonds of surface molecules, establishing new bonds and assembling molecules at the surface of the material.

fig03_wetting_difference

 In general, all these activation effects add to the surface energy of the material. This surface energy is important in the way the surface interacts with other, solid or liquid surfaces. If little energy is available, creating a joint interface with another material is generally not energetically favourable: the individual components are energetically better off on their own and this shows for example by liquids forming beads on the low energy surface, rather than wetting it. If a surface is treated, bonds are broken and the resulting surface energy is larger, it might become favourable for liquids to create a common interface with the treated surface, thereby wetting it. In the photo on the right hand side this is illustrated: in the middle one can see a trace where the plastic has been plasma treated and where the fluid spreads. Around it, the liquid forms beads. Surface energy and wettability are quantified by so called water contact angle (WCA) measurements: the angle between the solid surface and the meniscus of the droplet are measured and if one knows the surface energy of the test fluid, one can calculate the surface energy of the solid via Young’s law. High surface energies lead to low contact angles and vice versa.

Contact angles do not tell much about the specific surface chemistry though. If one wants to know this, one can use ellipsometry to determine the energy of molecular bonds at the surface and x-ray photoelectron spectroscopy (XPS) for the stoichiometrical composition of the surface. Atomic force microscopy (AFM) can be used to quantify the texture, which is another property that can have its effect on the contact angle. Such more detailed data about the surface can become important if one has certain specific applications in mind. For example, the presence of –NH groups at a surface is very interesting from the viewpoint of living cell adhesion to the surface and therefore their density an important quantity in bio-engineering.

Most atmospheric plasma treatment systems that are available on the market are either meant to homogeneously treat surfaces (such as is common for textiles) or generate plasmas of a few millimeteres up to centimetres in diameter (for example for wound healing). InnoPhysics has developed a plasma printing technique that combines plasma treatment with digital printing and that allows for feature sizes of typically between 100 µm up to 1 millimeter. Instead of the droplets that are for example delivered at a specific place on a substrate by an inkjet printer, a plasma is briefly ignited at the desired spot. 

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Electrical discharges – or plasmas – have fascinated mankind for centuries, because of their aesthetical beauty and uncontrollable behaviour. For a few decades now plasmas are being used abundantly in
µPlasmaPrint Technology Basics

Plasma activated molecules can be used to print nano surface modification. The plasma enables the molecules to react with the surface without the need for elevated surface temperatures, hence even at room temperature the surfaces can be modified. In addition only a few nanometers of the surface is modified by the process.

Printing nano surface modification with molecules

µPlasmaPrint is an enabler for subsequent processing:

Process flow
Section 1: PlasmaPrinter in action Section 2: inline camera capture of the plasmas while printing a pattern Section 3: visualization of the wetting/dewetting result on the treated teflon-like polymer foil
μPlasmaPrint Demo Movie

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InnoPhysics B.V.
Fransebaan 592a
5627 JM Eindhoven, the Netherlands
phone +31 (0) 40 248 4774
Chamber of commerce 17 19 4772

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InnoPhysics B.V.
Fransebaan 592a, 5627 JM Eindhoven, the Netherlands
phone +31 (0) 40 248 4774
Chamber of commerce 17 19 4772
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