| Mammalian Skeletal Muscle |
- Large strains (20%)
- Moderate stress (350 kPa blocking)
- Variable stiffness
- High energy fuel (20-40 MJ/kg)
- Efficient (~40%)
- Good work density (<40 kJ/kg)
- High cycle life (by regeneration)
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- Not yet engineering material
- Narrow temperature range of operation
- No catch state (expends energy to maintain force w/o moving unlike mollusk muscle)
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- Incredibly elegant mechanism that is a challenge to emulate. Muscle is a 3D nanofabricated system with integrated sensors, energy delivery, waste/heat removal, local energy supply, and repair mechanisms.
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| Dielectric Elastomers |
- Large strains (20% - 380%)
- Moderate stress (several MPa peak)
- Large work density (10k to 3.4MJ/m3)
- Moderate to high bandwidth (10 Hz to > 1kHz)
- Low cost
- Low current
- Good electromechanical coupling & efficiency (>15% typical, 90% max)
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- High voltages (> 1kV) and fields (~150 MV/m)
- Typically requireds DC-DC converters
- Compliant (E ~ 1MPa)
- Pre-stretching mechanisms currently add substantial mass and volume, reducing actual work density and stress
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- Potential to lower fields using high dielectric materials
- Small devices are favored for high frequency operations eg. MEMS (due to the more efficient heat transfer which prevents thermal degradation, and the higher resonant frequencies)
- Starting materials are readily available
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| Relaxor Ferroelectric Polymers |
- Moderate strain (<7%)
- High stress (45 MPa blocking)
- Very high work density (up to 1MJ/m3 internal strain)
- Stiff (400 MPa)
- Strong coupling (0.4) & efficiency
- Low current
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- High voltages (> 1kV) and fields (~150 MV/m)
- Typically requireds DC-DC converters
- Synthesis of typical materials involves environmentally regulated substances
- Cycle life is unclear & may be limited by electrode fatigue and dissipation
- Limited temperature range
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- Lower voltages and fields are being achieved using new high dielectric composites.
- Small devices are favored for high frequency operations eg. MEMS
- Unique combinations of high stiffness, moderate strain & reasonable efficiency
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| Liquid Crystal Elastomers |
- Large strains in thermally induced materials (45%)
- Moderate strains in field induced materials (2-4%)
- High coupling (75%) in electric materials
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- Subject to creep
- Thermal versions are slow unless very thing or photoactivated
- High fields (1-25MV/m)
- Low efficiency in thermal materials
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- New material with much promise and much characterization to be done. Photo-activation has been achieved.
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| Conducting Polymers |
- High stress (34 MPa max, 5MPa typical)
- Moderate strains (~2%)
- Low voltage (~2V)
- High work density (100kJ/m3)
- Stiff polymers (~1GPa)
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- Low electromechanical coupling
- Currently slow (several hertz maximum to obtain full strain)
- Typically needs encapsulation
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- Promising for low voltage applications. Speed and power will improve dramically at small scales.
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| Molecular Actuators |
- Large strain (20%)
- Moderate to high stress (>1MPa)
- Low voltage (2V)
- High work density (>100kJ/m3)
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- Currently slow
- Needs encapsulation
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- Great promise of overcoming may of the shortcomings of conducting polymer actuators, but still very early in development
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| Carbon Nanotubes |
- High stress (>1MPa)
- Low voltage (2V)
- Very large operating temperatures
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- Small strain (0.2%)
- Currently has low coupling
- Materials are presently expensive
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- Great potential as bulk materials approach properties of individual nanotubes
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| Ionic Polymer Metal Composite (IPMC) |
- Low voltage (<10V)
- Large displacement (mechanical amplification built into the structure)
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- Low coupling and efficiency
- Usually no catch state (consumes energy in holding position)
- Requires encapsulation
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- IPMC driven toys and demonstration kits available
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| Thermally Activated Shape Memory Alloy |
- Very high stress (200 MPa)
- Unmatched specific power (> 100kW/kg)
- Moderate to large strain (1-8%)
- Low voltage (actual voltage depends on wiring)
- Great work density (>1MJ/m3)
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- Difficult to control (usually run between fully contracted and fully extended but not between)
- Large current and low efficiencies (<5%)
- Cycle life is very short at large stess amplitudes
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- Readily available. Geerally thought of as slow, but can achieve millisecond response times using short high current pulses and water cooling
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| Ferromagnetic Shape Memory Alloys |
- High stress (<9 MPa)
- High frequency (>100Hz)
- Moderate strain (up to 10%)
- High coupling (75%)
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- Bulky magnets are required which greatly reduce the work density
- Costly single crystal materials
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- Operates in compression and thus needs a restoring force
- Displacement is typically all or nothing, as intermediate states are difficult to reach reproducibly
- Commercially available
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