Introduction

MEMS-based technologies are already commonplace in inkjet print heads for many years. The actuation techniques used can be divided into three main categories:

Thermal
Piezo
Capacitive (or electrostatic).

Whilst all three have a long history, only the first two are being used in commercially available heads. In this article:

We will explain why the capacitive actuation technique is an excellent alternative to the more mature thermal and piezo actuation techniques;
We will demonstrate how the technology has been matured in a completely different application field (medical ultrasound imaging), and show its feasibility for inkjet applications.

Capacitive actuator basics

The basic operating principle of a capacitive actuator is fairly straightforward: a flexible freestanding membrane is displaced by a voltage between one electrode on the membrane and one on the underlying substrate. This displacement can be used to move fluid.

The main advantages of capacitive actuation compared to piezo-based are:

1. Manufacturing

Easier integration with electronics (“CMOS compatible processing”)
Better reproducibility
Environmentally friendly (no Pb)
Lower production cost

2. Performance

Higher output pressure
Larger bandwidth
Lower power consumption

CMUT for medical applications

The capacitive actuation (and detection) technology has been developed and matured inside Philips first for the application in medical ultrasound products, so-called CMUT (Capacitive Machined Ultrasound Transducer). Next to the potential for replacing the piezo-based transducers in conventional ultrasound probes, the ability to miniaturize the transducer allows it to be used on top of catheters, for in-body ultrasound imaging.

Using the medical imaging application domain, we have been able to develop CMUT technology into a mature, manufacturable process.

CMUT technology for inkjet

The inherent advantages of the CMUT technology mentioned above are also relevant for inkjet applications. We have therefore conducted a simulation study to assess the ability of our capacitive actuation technology to generate droplets from an aqueous solution. The picture below is a snapshot of this, showing the feasibility.