The present invention relates to drop on demand printing, and to mainatenance of drop on demand printheads. The present invention relates particularly to the sterilisation of drop on demand printheads.

It is known to use drop-on-demand printheads to deposit biological and pharmacological substances. The present inventors have identified that for such processes to be viable, the printheads themselves must, in many instances, be sterile. In particular, it is desirable for any given printhead to be repeatably sterilizable to allow repeated use, because the lifetime of such fluids can be relatively short (typically less than 10 hours). At present, the need for such sterilisation does not appear to have been recognised, and there are no published methods by which the sterilisation may be accomplished.

It is well known to use heat in sterilization processes. The present inventors have identified that, where a printhead employs a piezoelectric actuating mechanism, elevated temperatures are unsuitable since heating above a threshold temperature results in depolarisation of the piezoelectric material. Heating may give rise to further problems, particularly in the presence of water, such as rapid corrosion within the printhead, or deterioration of adhesives used in the printhead. A limit is therefore imposed which effectively prevents sterilisation at more than 100 ⁰ C. Furthermore, heat alone will not remove residues from former fluids - rather it will tend to leave them as dried deposits on the internal surfaces.

Alternatively, sterilising fluids (such as alcohols - "sterilants") may be used, but in this case it may be difficult to ensure that all parts of the internal passages of the printhead are flushed by the sterilant. This is particularly difficult in the case of drop on demand printheads, where the nozzle plate typically forms the end of a much larger channel or cavity, and wherein flushing through the nozzle cannot be guaranteed to purge all the internal surfaces.

According to a first aspect of the present invention, there is provided a method of sterilization of a droplet deposition head, the method comprising the steps of

introducing into the droplet deposition head a liquid sterilant, and reducing pressure so as to boil the sterilant within the head.

In this way, bubbles of vapour of the sterilant are then formed internally and act as a purging means to displace trace fluids and dissolve them in the sterilant. ensuring that internal purging is complete. The vapour phase of the sterilant is advantageous in permeating all parts of the printhead, which a liquid phase might otherwise not reach.

The process of vaporisation of the sterilant at cavitation sites within the printhead produces a continuous scavenging action. This relatively aggressive physical process is effective at penetrating a meniscus of a contaminant which would be less affected by a simple liquid or vapour flow.

Additional advantage may be provided by introducing the sterilant using an existing fluid supply system. For example, the fluid supply described in co- pending WO 2006/030235, could be used to control the ingress of sterilant to the printhead.

Preferably a volatile sterilant is used, and preferably the sterilant is boiled at a temperature less than or equal to 100 degrees Celsius, more preferably less than 75 degrees, and still more preferably less than 50 degrees Celsius. It is particularly advantageous for the sterilant to be vaporised at or around room temperature.

Example of suitable sterilants include Ethanol and Hydrogen Peroxide.

The inventors have identified that the problem of ensuring that 100% of the internal surfaces are purged by the sterilant is much simpler in the case of printheads made with the nozzle as a side exit to an open channel (see, for example, WOOO/38928).

According to a second aspect therefore, the invention provides a method of sterilizing a droplet deposition head including a plurality of ejection chambers

having an inlet, an outlet and at least one nozzle opening, and arranged such that fluid can flow past the nozzle opening from the inlet to the outlet, the method comprising flowing a sterilant through the printhead, the sterilant passing through each ejection chamber, from inlet to outlet.

In this type of design, known as a "Through Flow Actuator", careful design of the ink manifolds and supply system can ensure that all internal surfaces experience a positive flow during flushing.

The nozzles themselves can be flushed by ensuring a positive pressure behind them (in normal operation, the pressure behind the nozzles is slightly negative). This can be accomplished by changing the balance of flow from the positive (input) side to the negative (exhaust) side - generally by introducing a slightly higher resistance to flow on the negative side.

In addition, it may be desirable to seal cavities not normally accessed by the ink in the printhead structure in such a manner that they offer bacteria and the like no refuge from the sterilization process. In one embodiment, cavities in the printhead are filled with foam. Typically this would be accomplished after assembly of the printhead by injecting foaming materials such as "Espanex" or self-foaming polyurethane compounds into such cavities. As an additional precaution against deterioration of the foam, the whole assembly may be Parylene coated so as to seal all surfaces against chemical attack.

Figure 1 illustrates a through flow actuator. Channels 28 are closed by a cover member 26, having apertures 29. A nozzle plate is attached to the cover member with nozzles 30 communicating with apertures 29. In this arrangement it is known to have a double ended channel, and ink is supplied from a manifold region 34 and ejected from nozzles 30 located midway between along channels 28. In this way fluid flows past the nozzle opening and is ejected from the side of the channel. Region 50 represents contaminant material held behind a meniscus in a corner of the manifold region. It is known that such a deposit can remain after flowing a fluid such as sterilant through the printhead. This results in the risk that only the surface of the deposit is

exposed to the sterilant. According to the present invention, the boiling action of the sterilant acts to break the meniscus down, resulting in an improved degree of sterilization. It is noted that even if a small deposit remains after boiling, sterilization can still be effective if the sterilant has successfully acted on the remaining deposit.

Alternatively of additionally, a region of faming material 50 can be inserted into corner region 50 of the manifold area, in order to eleiminate a possible dead spot where ink flow is low, and where print fluid residue might otherwise accumulate.