A novel electrostatic inchworm micro-actuator with integrated microprobe is described. Using the principle of hydrophobia, the device is made waterproof, which makes it suitable for a range of in vivo applications, including for accurate position control of micro needles in brain applications.
By: M. Erişmiş,
H. Pereira Neves, P. De Moor and
M. Van Bavel, IMEC, Leuven, Belgium
Micro-actuator opportunities
Micro actuators use microelectromechanical system (MEMS) technology to convert energy into micro movements that allow elements to be positioned or controlled with a high precision, and with steps of a few micrometers or nanometers. They are useful in biomedical applications when biological objects or their environment need to be controlled at a microscopic scale. Today, micro-actuators are being used as, or integrated with, micro manipulators, micro surgery tools, micro pumps, micro valves and micro needles. One particular application is to integrate them with micro probes for brain applications. This integration increases their potential for use in applications such as three-dimensional scanning of the brain. Today, actuators for brain implants are already used during brain research, but they are placed outside the body. If they could be placed inside, it would enable long-term patient treatment. Work is underway to make this aspiration a reality.
Demanding in vivo requirements
Using micro actuators for in vivo biomedical applications is challenging. The actuators have to provide large ranges with sufficient forces, because they are going to be used for position control. They should use low operating voltages and consume low power, because battery life is an important design criterion for these applications. The actuators will operate in aqueous environments, therefore proper packaging is mandatory. All these requirements call for a suitable actuator architecture. Micro actuators today can be classified in different categories: electrostatic actuators, thermal actuators, conjugated polymer actuators, electrochemical actuators and so on. From close investigation of their advantages and disadvantages, it has been found that electrostatic actuators with an inchworm topology are most suitable for meeting the above challenges. Micromachined inchworm actuators can achieve large ranges (a few millimeters are possible) without compromising the output force. Moreover, nanometer range resolution has already been demonstrated. This article describes an innovative electrostatic inchworm micro actuator topology with watertight encapsulation, and shows that it is suitable for in vivo biomedical applications.
Concept and fabrication
The term “inchworm” was originally given to a specific type of linear stepwise piezoelectric actuators that could grab and release a shuttle and “inch” the shuttle towards its destination. Now, inchworm has become a general name to indicate a family of actuators that add small deflections step by step and achieve larger displacements. Different inchworm actuation mechanisms exist such as piezoelectric, electrostatic and thermal. An electrostatic inchworm topology has been selected that is simple to model, easy to integrate with MEMS and capable of providing a large output force.
| FIGURE 1: Schematic of the actuator (a, left) and its six-step operation principle (b, right). |
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The inchworm actuator has four main blocks: a driving unit, two latching units and a shuttle. The driving unit creates the step displacement along the shuttle, and the latching units maintain this motion one by one. Figure 1 a and b shows the six-step operation cycle of the actuator. In the initial state both Latch A and Latch B are activated, grabbing the shuttle arms (Step 0). In Step 1, Latch A releases the left arm. In Step 2, the Drive creates the step displacement by pulling in. As the trusses of the Drive are pulled towards the electrodes, in-plane-angular conversion creates a right displacement step of the free shuttle arm. In Step 3, this displacement is maintained with Latch A. In Step 4 and Step 5, Latch B and the Drive are released, respectively. As the Drive is released, it transfers the displacement step that is obtained on the right arm to the left arm. Lastly, Latch B fixes its shuttle arm, preserving this displacement step and completing the cycle. Reverse displacement can be achieved by
activating Latch B instead of Latch A in Step 1.
| FIGURE 2: Optical microscope image of the fabricated actuator. |
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The goal of this work is to achieve low-voltage actuators that provide large ranges and large output forces. For this objective, the topology introduced in this study does better than the existing topologies in the literature, because it includes a mechanical gain adjustment stage. The bent-beam angle formed within the hexagonal shape (see Figure 1a) creates an in-plane angular deflection conversion, which provides a tunable mechanical gain. This tunable mechanical gain helps to achieve a force-displacement trade-off during the step generation. This angle is the critical design parameter so that inchworm designs can be optimised for low-voltage and low-power applications.
The actuator was fabricated using a silicon-on-insulator (SOI) based multi-user process, called SOI Multi-User MEMS Processes (SOIMUMPs). SOIMUMPs is a commercially available process that allows a high quality, high aspect ratio silicon structural layer, without virtually any stress-related bending, which makes it a preferred technique. It is a commercial multi-user process that allows designs to be fabricated extremely fast. Figure 2 shows an optical microscope image of a fabricated actuator.
Watertight encapsulation
Integrating electrostatic micro actuators in biomedical applications is a big challenge for two main reasons. First, the environment can be too harsh for the electrostatic actuator to work properly. The body liquid, which is ionic and contains biological particles, can cause corrosion, particulate contamination and stiction (the force required to cause one body in contact with another to begin to move). The second reason results from the nature of the electrostatic actuation: electrolysis and polarisation can hinder the operation of the device. Watertight encapsulation of the actuator is essential for these applications. This should be done in such a way that water cannot penetrate the comb fingers and so that actuator movement is not hindered by the encapsulation.
| FIGURE 3: The encapsulated actuator. |
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However, this poses a problem: How do you allow an external probe, controlled by an actuator, to move back and forth while still maintaining the hermeticity of the package, which is required for in vivo applications? In this work, the actuator has been packaged by using a flip-chip mounted glass cap that forms a small clearance to allow movement of the actuator. Water ingress via the clearance is further prevented by a hydrophobic surface treatment, that is, Teflon deposition in an inductively coupled plasma system and surface roughness formed by dummy comb fingers render the walls of the clearance hydrophobic (Figure 3). The biggest advantage of this concept is that the characteristics of the actuator in air are similar to the ones in water. Hence, there is no large loss in output force and small voltage operation is possible. For this reason, this encapsulation method is preferred rather than completely encapsulating the actuator in elastic material, which would restrict the output force.
Testing the actuators
By using these fabrication techniques, actuators with different characteristics have been created and successfully tested. The performance of the actuators has been evaluated using a probe station equipped with a camera with 100x magnification. Digital image correlation was used to measure the small displacements. The performances of the prototypes are impressive compared with the results of other inchworm actuators reported in the literature. The actuator with the largest range (±50 µm) works at 11 V; it has an output force of up to ±195 µN and an average step size of 1.1 µm. The lowest operating voltage electrostatic inchworm actuator works at 6 V and provides ±18 µm range and ±25 µN. The lifetime of these actuators has been tested up to 20 million steps.
| FIGURE 4: Encapsulated actuators successfully operating in water. |
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The actuators could successfully operate in water after Teflon deposition. Operation was not significantly affected by the aqueous environment. The actuators operated in water for one week without a significant problem, which demonstrates the feasibility of the encapsulation technique (Figure 4).
Potential of the inchworm actuators
The large-range actuators are believed to be state-of-the-art for low-voltage, low-power, large-range and large-force applications. They consume only dynamic power because of their electrostatic nature. Moreover, the concept of the actuators allows the possibility of designing different step size actuators for different applications. The in-plane angular deflection conversion provides a force–displacement trade-off and allows step sizes to be set that vary from a few nanometers to a few micrometers with only a minor change in design.
However, the inchworm actuators described here combine small size, low-power and large-range operation with water tightness and a long autonomy. This innovative combination of characteristics makes the actuators particularly suitable for use in in vivo applications, and, in general, for all applications that need to combine a long autonomy with small batteries. With this device, long-term patient treatment can become reality. In particular, the actuator could be used to accurately control the position of micro needles used in brain applications. This is necessary to reach the correct groups of neurons for the specific disorder and to be near the neurons for a better signal-to-noise ratio. For these applications to become reality, a biocompatible coating is being developed that enables full compatibility with the human body.
The technology extended
Large-range, low-voltage, low-power, large-force electrostatic inchworm micro actuators that are integrated with a micro probe and fabricated using SOI-based micromachining operate below 10 V, consume below 100n W, provide output forces larger than 50 µN and strokes larger than 20 µm. The operating voltage is three times lower than that of currently available actuators. An innovative encapsulation method, which includes a flip-chip mounted glass cap and hydrophobic surface treatment, allows the microprobe to operate in an aqueous environment. All these characteristics make the devices especially suitable for biomedical uses, including brain applications.
Mehmet Akif Erişmiş¸
is a BioMEMS researcher,
Hercules Pereira Neves
is Principle Scientist and Programme Manager,
Piet De Moor
is Group Leader Heterogeneous Component Integration, and
Mieke Van Bavel
is Scientific Editor at IMEC
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