Electro active polymers (EAP) are actuation materials that are used to drive mechanisms and are fastly replacing conventional methods. Several investigations are in its way to utilize the excellent properties of the polymer. These materials are now applied in various fields including robotics, medicine, defense etc:-and are effective alternatives for conventional sensors and actuators such as motors, gears, piezoelectric crystals, bearings, screws etc: - These are unique materials capable of soft actuation under low applied voltages. They have been called by some researchers ‘artificial muscles’ due to their large strain characteristics and electro-mechanical-chemical muscle- like behavior.
They have been shown to be quite capable of low temperature actuation as well as being quite durable when compared to other actuators in their class. This leads to the belief that there is great potential for use in space applications. EAP’s can change all the paradigm of design and they show great potential as soft robotic actuators, artificial muscles and dynamic sensors in macro-to-micro size range.
MICROSTRUCTURE AND COMPOSITION
The bending actuator is composed mainly of perflourinated ion exchange membrane metallic composite backbone called ionic polymer metallic composite or IPMC (0.18µm). IPMC have commercial name Nafion®. The ionomer background or matrix is coated on both sides with metallic electrodes made of noble metals such as Pt, Au or Pt/Au (5-10µm). It is then neutralized with a certain amount of counter-ions such as monovalent cations of alkali metals such as Li+, Na+, K+ and Rb+. A finishing layer of gold is provided to increase surface conductivity. It is then fully solvated. The most common solvent used is water but we can also use organic solvents like Ethylene glycol or Glycerol. An IPMC has to be kept moist continuously for long working (4 months) and it is done by providing a polysilicon coating.
The preparation of ionic polymer metallic composites (IPMC’s) requires involved laboratory work. The current state of the art of IPMC manufacturing technique incorporates two distinct preparation processes:
1. Initial compositing process
2. Surface electroding process
Rectangular silicone wells with size 1cm by 3cm by 2-5mm is used as containers for
IPMC fabrication. It is deposited onto glass microscopic slides. We use a multiple material freeform fabrication system with Cartesian gantry positioning system and multiple changeable deposition tools. A low vapour pressure solvent (N-N- dimethyl formamide,DMF) is added to the ionomer dispersion to help control cracking on drying.
Initial Compositing process
It is to metallize the inner surface of polymer by a chemical reduction process. Ionic polymer is soaked in salt solution of a complex salt Pt (NH3)4HCl to allow platinum containing cations to diffuse through via the ion exchange process. A proper reducing agent such as LiBH4 or NaBH4 is introduced to metallize the polymer by a chemical reduction means. The metallic platinum particles are not homogeneously formed across the membrane but concentrate predominantly near the interface boundaries. It has been experimentally observed that the platinum particulate layer is buried microns deep (typically 1–20 µm) within the IPMC surface and is highly dispersed.
The primary reaction for platinum composites is:
LiBH4 + 4[Pt (NH3)4]2+ + 8OH- ==> 4Pt + 16NH3 + LiBO2 + 6H2O
• Soft and flexible, hence find wide application in bio-medical field
• EAP’s can be mass produced. Hence it results in low cost.
• EAP’s can be easily fabricated in various shapes.
• Inherent vibration damping.
• Lighter compared to other actuators and sensors.
• Response speed is significantly higher.
• Superior fatigue characteristics
• Large actuation strains
• Can withstand extreme conditions esp. up to -1400C.This suits EAP in planetary applications.
• No effective and robust EAP material is currently commercially available.
• Selection of suitable and satisfying materials poses a problem as new and new materials emerge.
• A compromise between stress and strain needed