All currently available microgrippers use stiff, fragile materials. We will use ionic polymer-metal composites (IPMC) that are electroactive and move significantly when excited with relatively low voltages. UNM has patented the method to manufacture these devices. IPMCs can be either used as actuators or sensors.
Artificial Muscle Actuators
IPMCs are highly active actuators that show very large deformation in the presence of low applied voltage. They have been modeled as both capacitive and resistive element actuators that behave like biological muscles and provide an attractive means of actuation as artificial muscles for biomechanics and biomimetic applications. Grodzinsky, Grodzinsky and Melcher , and Yannas, Grodzinsky and Melcher were the first to present a plausible continuum model for electrochemistry of deformation of charged polyelectrolyte membranes such as collagen or fibrous protein and were among the first to perform the same type of experiments on animal collagen fibers essentially made of charged natural ionic polymers and were able to describe the results through electro-osmosis phenomenon. Kuhn and Katchalsky, Kuhn, Kunzle, and Katchalsky, Kuhn, Hargitay, and Katchalsky, Kuhn, and Hargitay, however, should be credited as the first investigators to report the ionic chemomechanical deformation of polyelectrolytes such as polyacrylic acid (PAA), polyvinyl chloride (PVA) systems. Kent, Hamlen and Shafer were also the first to report the electrochemical transduction of PVA-PAA polyelectrolyte system. Recently revived interest in this area concentrates on artificial muscles which can be traced to Shahinpoor and co-workers ,. More recently, Osada, Kishi, Brock, and Bar-Cohen have looked at various applications for artificial muscles. Shahinpoor et al. , have been experimenting with various chemically active as well as electrically active ionic polymers and their metal composites as artificial muscle actuators applied to robotic and microrobotic applications.
Polyelectrolytes possess many ionizable groups on their molecular chain. These ionizable groups have the property of dissociating and attaining a net charge in a variety of solvent mediums. According to Alexanderowicz and Katchalsky these net charge groups, which are attached to networks of macromolecules, are called polyions and give rise to intense electric fields of the order of 10 10 V/m. Thus, the essence of electromechanical deformation of such polyelectrolyte systems is their susceptibility to interactions with externally applied fields as well as their own internal field structure. In particular if the interstitial space of a polyelectrolyte network is filled with liquid containing ions, then the electrophoretic migration of such ions inside the structure due to an imposed electric field can also cause the macromolecular network to deform accordingly. Shahinpoor , , and co-workers , have recently presented a number of plausible models for micro-electro-mechanics of ionic polymeric gels as electrically controllable artificial muscles in different dynamic environments.
These muscles are manufactured by a unique chemical process in which a noble metal is deposited within the molecular network of the base ionic polymer. One of the interesting properties of IPMC artificial muscles is its ability to absorb large amounts of polar solvents, e.g., water. Platinum metal ions, which are dispersed through out the hydrophilic regions of the polymer, are subsequently chemically reduced to the corresponding metal atoms. This results in the formation of dendritic type electrodes. When an external voltage of 1 to 2 volts is applied on an IPMC film, it bends towards the anode. An increase in voltage level (up to 6 or 7 volts) causes a larger bending displacement. When an AC voltage is applied, the film undergoes swinging movement and the displacement level depends not only on the voltage magnitude but also on the frequency. Lower frequencies (down to 0.1 or 0.01 Hz) lead to higher displacement. Thus, the movement of the muscle is fully controllable by the applied electrical source. The frequency dependence of the ionomer deflection as a function of the applied voltage is shown in Fig. 1. With only a few volts of excitation, significant actuation is possible. Fig. 2 shows the difference between actuation/no actuation of an IPMC strip (5mm x 20mm x 0.2mm).
Artificial Muscle Sensors
Investigations of the use of ion-exchange-membrane materials as sensors can be traced to Sadeghipour, Salomon, and Neogi where they used such membranes as a pressure sensor/damper in a small chamber that constituted a prototype accelerometer. However, it was Shahinpoor who first discussed the phenomenon of flexogelectric effect in connection with dynamic sensing of ionic polymeric gels. To get a better understanding of the mechanism of sensing, more explanation must be given about the nature of the ionic polymers in general.
As shown in Fig. 3, IPMC strips generally bend towards the anode and if the voltage signal is reversed they also reverse their direction of bending. Conversely by bending the material, shifting of mobile charges becomes possible due to imposed stresses. Consider Fig. 4, where a rectangular strip of the composite sensor is placed between two electrodes. When the composite is bent a stress gradient is built on the outer fibers relative to the neutral axis. The mobile ions therefore will shift toward the favored region where opposite charges are available. The deficit in one charge and excess in the other can be translated into a voltage gradient that is easily sensed by an instrumentation amplifier.
Experimental results show that a linear relationship exists between the voltage output and the imposed quasi-static displacement of the tip of the IPMC sensor. The experimental set up was such that the tip of the cantilevered IPMC strip as shown in Fig.4 was mechanically moved and the corresponding output voltage was recorded. The results are shown in Fig 5.
When strips of IPMC are dynamically disturbed by means of a dynamic impact or shock loading, a damped electrical response is observed as shown in Fig. 6. The dynamic response was observed to be highly repeatable with a bandwidth of over 100 Hz. Based on our knowledge of IPMC properties at the macro scale, we believe that if IPMC were used to create microgrippers in the range of 10 to 200 microns, sensing and actuation in the micro-world for biological and non-biological objects becomes both possible and inexpensive.