This work is part of a research project aimed at realising conducting polymer matrices for interfacing with cultured neurons. The polymer matrix has a dual function, one as a medium for recording electrical activity; the other is chemical stimulation through the release of bioactive molecules. In this work we use poly-3-hexylthiophene as a conducting polymer matrix. To test the polymer’s ability to release molecules upon the application of a potential it was doped with glutamate (GA). GA is an important neurotransmitter, and its controlled release can be important in several medical and tissue engineering applications. Diffusional and controlled release of GA from the polymer were assessed. Biocompatibility of the samples was evaluated at each stage using neuroblastoma cell cultures.
This paper presents preliminary results on the characterisation of the actuating performances, never explored before, of an elastomeric material (Dr Scholl’s, Canada, Gelactiv tubing), to realise dielectric elastomer actuators. Strain and stress performances of this material were compared to those of the most currently used acrylic elastomer (3M, U.S.A., VHB 4910). Planar actuators were realised and tested, using films of the two materials, coated with compliant electrodes made of carbon grease. Following the application of a two-second high-voltage impulse, the isotonic transverse displacement and the isometric transverse force were separately measured along a prestrained direction. Actuators made of the new elastomer showed, respect to those made of the acrylic polymer, a lower dielectric strength, a transverse strain more than twice greater for the same applied electric field (e.g. 1.8 % against 0.7 % @ 27 V/μm) and a transverse stress less than twice smaller (e.g. 3.7 KPa against 5.3 KPa @ 24 V/μm).
This paper presents a strategy of design, realization and control of pseudomuscular actuator controllable in position and compliance. The actuator was designed as a bundle of electromechanical actuating elements, made by dielectric elastomers. The control strategy was inspired to the Feldman's biological muscle model. Presented simulations show that opportune recruitments of the bundle active units enable a satisfying approximation of the quadratic length-force characteristic of the biological muscle.
Following the biological paradigm, artificial polymeric systems considered as candidates for actuation applied to biomedical devices and systems were tested taking into account longitudinal strain as a result of energy conversion from external sources. Among them, dielectric elastomers show good mechanical performances, but they require very high voltages for the driving, on the order of kilovolts, which are not suitable for devices that are in contact with biological systems. Conducting polymers work in a voltage range much more reasonable, but they show only few percents of longitudinal strain. On the other hand, it is known that, for instance, in a planar configuration of DBS-doped polypyrrole, the longest dimension undergoes a dimensional change of 0.5% up to 4% while the shortest one has a strain of roughly 35%. In this work, we discuss the latest advances concerning conducting polymer based devices and assess the worth of exploiting the interesting properties characterizing the radial strain of conducting polymer fibers rather than the axial strain. We also describe a possible method able to convert radial to longitudinal strain via a braided mesh acting as a merely mechanical transducer or even as a strain amplifier. The described technical improvements and observations, together with a voltage drop range acceptable for biomedical applications, give conducting polymers a new appeal for this kind of utilization and promise new interesting applications.
A variety of microfabrication techniques have been developed at the University of Pisa. They are based either on pressure or piston actuated microsyringes or modified ink-jet printers. This work present the results of a study aimed at fabricating carbon nanotube (NT) actuators using micro-syringes. In order to prevent the nanotubes from aggregating into clumps, they were enclosed in a partially cross-linked polyvinylalcohol - polyallylamine matrix. After sonication the solution remained homogenously dispersed for about 40 minutes, which was sufficient time for deposition. Small strips of NT, about 5 mm across and 15 mm long were deposited. Following deposition, the films were baked at 80 degree(s)C and their thickness, impedance and mechanical resistance measured. The results indicate that 50 minutes of baking time is sufficient to give a constant resistivity of 1.12 x 10-2 (Omega) m per layer similar to a typical semiconductor, and each layer has a thickness of about 6 micrometers .
The demand for actuators featuring biomimetic properties such as direct drive, high power density and intrinsic compliance is growing in robotics and bioengineering. Our work is aimed to increase the performance of a class of actuators utilizing active polymer components which are characterized under several different electrical stimulation conditions. In order to increase the active strain of the system we have considered a configuration inspired to McKibben muscle. In this configuration each active element is covered by a braid mesh shell (made with flexible, but not extensible, threads), which contracts when the element increases its volume. This technical solution amplifies the strain up to 50 times and can be utilized to reach tangible shortening of the actuator.
Our previous work has shown that elastic textiles covered with an epitaxial layer of conducting polymer show piezoresistive properties. They can be used to fabricate sensorized garments such as gloves, leotards, socks and seat covers as man-machine interfaces. A purposely designed screen printing process has been implemented to realize sensors/tracks patterns. Polypyrrole/lycra fabrics were prepared using the method developed by Milliken Co. (Spartanburg, USA). The epitaxial deposition is obtained controlling the concentrations of monomer and the temperature of the reaction medium. Investigation on mechanoelectric transduction properties (static and dynamic) of the fabrics, calibration of wearable sensing devices and ongoing R&D efforts in multimedia, sport and rehabilitation fields are reported.
Commercial steerable catheters and catheter prototypes actuated by active materials still present limitations in terms of self-sustaining capability and miniaturization. Specifications for the intravascular catheter we are developing are: bending angle up to 20°, bending stiffness of a few N/m, response time of the order of seconds. Simulations with finite element method (FEM) showed these specifications can be satisfied using a polymer with active strain of 1 percent and elastic modulus E=4.5 GPa and a solid polymer electrolyte (SPE) matrix with E=MPa. The actuator is thought to be made of a composite structure which includes polyaniline fibers, a copper wire electrode and SPE matrix. Its measured characteristics are: active strain 0.2%, active stress 2 MPa, fiber elastic modulus 1.5 GPa, SPE elastic modulus 1-2 MPa. The major problem to realize the catheter is the stiffness of SPE, which has to be considerably augmented. Fiber active strain is below the required value, but it can be increased by proper drive. The production of fibers with a diameter of 10 microns will reduce the response time to the required value.
There is considerable need for light, low-volume actuators having long-cycle-life that can generate displacements and high forces when low voltages are applied. Electroactive polymers possess some of these characteristics, but improvements are needed. We describe work on a promising new type of actuator that is based on non-faradaic electrochemical charge injection in carbon nanotube sheets. While large actuator strokes combined with giant stress generation capabilities are predicted for optimized materials, the present stage of actuator development is embryonic and major materials advances are required to realize these features. The present work describes recent advances in increasing the actuator stroke and stress generation capabilities well above our initially obtained values for carbon nanotube actuators. Operating these actuators in 1M NaCl at low voltages (-0.5 to 1.5 V vs. SCE) we obtained actuator strain of up to 1%. Although the generated stresses are much higher than those of natural muscles, they are many orders of magnitude lower than predicted for nanotube sheets that fully utilize the mechanical properties of the individual nanotubes.
In the last period, the interest in the development of devices that emulate the properties of the 'par excellence' biological actuator, the human muscle, is considerably grown. The recent advances in the field of conducting polymers open new interesting prospects in this direction: from this point of view polyaniline (PANi), since it is easily produced in fiber form, represents an interesting material. In this conference we report the development of a linear actuator prototype that makes use of PANi fiber. All fabrication steps (fiber extrusion, solid polymer electrolyte preparation, compound realization) and experimental set-up for the electromechanical characterization are described. Quantitative measurements of isotonic length changes and isometric stress generation during electrochemical stimulation are reported. An overall assessment of PANi fibers actuative properties in wet and dry conditions is reported and possible future developments are proposed. Finally, continuum and lumped parameter models formulated to describe passive and active contractile properties of conducting polymer actuators are briefly outlined.
Several porous material systems (e.g.. hydrogels, conducting polymers, electrorheologic fluids) make possible a
control of their properties in response to an appropriate stimulus (and viceversa) and therefore. they belong to
the class of intelligent materials.
In the present paper we propose a first classification of intelligent porous systems dividing them in three main
classes: porous material as semi-permeable media, as reservoir and delivery systems and as biphasic composites
with large interfacial area between solid and fluid phases.
Then we present a continuos model to describe the passive mechanical behaviour of a generic porous
conducting polymer saturated by a fluid. The model is solved for a stress-relaxation test and it is verified in the
specific case of a doped polypyrrole porous matrix saturated by an electrolytic solution. The goodness of fit
between experimental date and theoretical data confirms the validity of the model.