Conducting polymers as simultaneous sensor-actuators
Biological organs combine technological elegance and efficiency in transforming chemical energy, at constant temperature, into functions. Living cells are made of reactive, soft, and wet materials. Actuation (movement) of natural organs such as muscles involves a chemical reaction (adenosine triphosphate hydrolysis) and simultaneous sensing, providing living creatures with a perfect awareness of both the characteristics of the mechanical movements and their interactions with the environment: they are intelligent devices. By contrast, artificial machines are constructed from dry materials whose composition during work remains unchanged.
The chemical and physical processes linked to the electrochemistry of conducting polymers have much in common with the composition, properties, and functions of biological tissues.1,2 Reactions promote changes. Double bonds are broken or joined, radical ions (polarons) are stored, and charges (bipolarons) associated with molecule chains, solvent, and ions become interchanged (see Figures 1 and 2). Using these principles, we have generated conducting polymer films on metals or transparent electrodes by current flow through a solution including a monomer and a salt. The film thickness is controlled by the consumed charge.
In one typical polymer reaction (which we will call Reaction 1), a polypyrrole chain is oxidized (i.e., loses electrons) from the neutral state. In a second reaction (Reaction 2), polymers based on thiophene, a common industrial solvent, can also be reduced (i.e., gain electrons) from the neutral state to store polarons and bipolarons. For charge and osmotic-pressure balance, counterions (neutrality-promoting ions) and solvent penetrate from the solution into the polymer, which consequently swells. The material composition (polymer-ions-water) resembles that of biological organs.3,4
During chemical reactions the counterion/polymer ratio changes continually, resulting in a giant nonstoichiometric material (i.e., it can exist over a range of composition). Similarly, we have found that even physical properties that are composition dependent—i.e., conductivity, film volume and color (polaronic and bipolaronic states are chromophores that absorb or reflect light in both the visible and IR regions), charge and chemical storage, and porosity—can be changed under reaction control over a large range of magnitude. This amenability to tuning makes these properties eminently suitable for biomimetic devices: actuators and artificial muscles (volume changes); smart windows, smart mirrors, and color filters in both the visible and IR regions (polarons and bipolaron population control); supercapacitors and polymeric batteries (charge storage); smart membranes (interchain free-volume control); drug delivery, nerve interfaces, and electron-ion transducers (counterion storage and delivery).
A single reaction—multifunctionality—drives different biomimetic tasks, which suggests versatile technologies. For example, we have observed experimentally that during actuation, smart windows and artificial muscles store charges (energy) that can be recovered during reverse actuation (i.e., movement in the opposite direction for muscles or reverse color change in electrochromic devices): see Figure 2. Any property—e.g., light absorbance or reflectance, or muscle position—can have infinite intermediate and stable values, each corresponding to an intermediate and stable nonstoichiometric composition. The flow of current (anodic or cathodic) controls the magnitude of change of a property. When the current is switched off, both the material composition and the corresponding magnitude of the property are maintained.
Moreover, thanks to the control of properties and devices, any physical or chemical variable acting on either the reaction rate (under a flow of constant current) or the chemical equilibrium (under a stationary state) will influence the energy of the electrons at the connecting metal. In Reaction 1, electrons are reactants, and in Reaction 2, they are products. Thus, when we subjected self-supported (i.e., freestanding) films of conducting polymers to oxidation and reduction by a flow of constant anodic or cathodic current, we found that the evolution of the material potential is influenced by experimental variables such as current, temperature, salt concentration, and mechanical conditions (see Figure 3). The electrical energy consumed during actuation becomes a sensor of the variable being investigated. This is a general property of the material. Accordingly, we expect that all of the electrochemical devices described here should be able to work both as actuators and sensors, at the same time.
For example, the change in volume of reactive materials acts as a mechanical sensor. We have developed both artificial muscles that detect trailed weight and tactile muscles that sense the presence of an obstacle and supply information about its mechanical resistance (see Figure 4). The muscle consists of a bilayer conducting polymer/tape or a three-layer conducting polymer/tape/conducting polymer that bends in response to current.1 Actuating responses (evolution of the free muscle potential, i.e., with no obstacle in its way) and sensing signals (step in potential at contact time) have the same order of magnitude, from a few millivolts to several volts. Driving current (actuation signal) and muscle potential (sensing signal) are simultaneously supplied by the same two connecting wires.
In summary, a number of sensing tools—windows, mirrors, light filters, batteries, membranes, glands, and nerve interfaces—will be fabricated in the near future using conducting polymers, carbon nanotubes, and other organic reactive materials. We expect that they will enable a new generation of intelligent robots and devices that can sense working and ambient conditions.5,6 Most of the intelligence will shift from the software, a familiar component of current conventional technologies, to materials. Among the challenges we face are a lack of theoretical models for reactive, wet, and dense materials (i.e., that behave like biological cells) and still-to-be-developed industrial methods for using reactive materials and devices. In addition to smart windows and polymeric batteries, both of which sense ambient and working conditions, we are currently working on nerve interfaces for computer-neuron interactions.
Financial support from both the Spanish and regional governments is acknowledged.
Toribio Fernández Otero received his PhD in chemistry in 1978 from the Universidad Complutense (Madrid). He was appointed as professor of physical chemistry and macromolecules at the Universty of the Basque Country in 1989, and has been at the Technical University of Cartagena since 2002. His work focuses on the electrochemistry of conducting polymers: synthesis, properties, applications, and models. He has been an invited lecturer at a variety of conferences, and has served on the SPIE Electroactive Polymer Actuators and Devices program committee.
