Actuating Materials and Biomedical Applications

Learning Objectives

• Alternative actuating material awareness, including:
  • Thermoactivated materials
  • Shape Memory Alloys/Polymers
  • Fluid-Driven systems
  • Elastic and Inelastic shortening mechanisms
  • McKibben actuators
  • Bending actuators
  • Vacuum/Air driven actuators
  • Electric actuation methods

Challenges in Biomedical Applications

• Size constraints
• Power consumption limitations
• Force requirements
• Displacement precision
• Bandwidth considerations
• Control demands
• Stiffness needs
• Biocompatibility imperatives
• Efficiency expectations

Thermoactivated Actuators

• Thermal energy is converted into motion

Thermal Expansion

• Change in length (ΔL) = αLΔT, where:
  • α is the coefficient of linear thermal expansion
  • L is the original length
  • ΔT is the change in temperature

Bimorph Actuator

• Uses two metals with different coefficients of thermal expansion

Shape Memory Alloys

• A typical alloy is Nitinol (NiTi)
• Nitinol is highly biocompatible

Shape-Memory Polymer

• 3D printable PLA exhibits shape-memory effect

Fluid-Driven Actuators

• These actuators use a network of fluid channels for actuation
• The channels are filled with a working fluid
  • Compressed gas, such as air for pneumatic systems
  • Liquid, like water for hydraulic systems
• A need of pumps and valves exists
• Construction employs elastic or inextensible plastic films

Elastic Shortening Actuators

• McKibben actuators consist of an elastic tube, a braided mesh-like sleeve, and end fittings
• The braid angle in the initial state defines:
  • Motion direction
  • Displacement value
  • Generated force

McKibben Actuators – Biomedical Case Study

• Seven McKibben actuators, fixed in a helical orientation mimic the twisting motion of the heart during contraction
• An optimized device could eject 92 ml per cycle
  • Exceeding typical stroke volumes of a heart (70 ml)

Inelastic Shortening Actuators

• Pleated structure involves inelastic material constrained by special end fittings to create equal radial pleats at the actuator's ends
• Bubble structures incorporate stiff metal rings around inextensible plastic tubing
  • The metal rings constrains it to form a tight folded shape within the ring radius

Bending Actuators

• Three internal chambers act independently
• Elastic tubes are reinforced with fiber in the circular direction

Pre-Programmed Elastic Bending Actuators

• Soft fiber-reinforced bending actuators use a silicone rubber tube with an inextensible layer
• The tubes are wrapped with a reinforcing fiber

PneuNet

• Series of chambers are embedded in a layer of extensible elastomer
• PneuNet is bonded to an inextensible layer
• The bending motion is tuned via the chamber's:
  • orientation
  • size
  • number

Inextensible Bending Actuators

• Pouch motors use the mechanical work of the deformation of inflatable pouches

Vacuum Muscle

• Constructed with elastic beams
• Interconnected cavities sealed within a thin elastomeric membrane

Growing Robots for Minimally Invasive Surgery

• Made of a soft, flexible tube that undergoes eversion and grows
• Surgical implantation of large-area surface electrodes on the cortex through small burr holes

Air Sources

• Compressed gas chambers store gas at high pressure
  • The gas creates the capacity to drive several actuation cycles within fluidic elastomer networks
• Chemical fuels for chemical reactions include:
  • Combustion of methane or butane
  • Decomposition of hydrogen peroxide -Controlling chemical reaction (and hence generated pressure) can be challenging
• Liquid-gas phase change uses embedded heating elements with low boiling point liquids
  • During the cooling cycle, the condensed fluid must return to the heating element.

Syringe Pumps

• Syringe pumps deliver precise amounts of fluid
• Syringe pumps are useful for:
  • Delivery of drugs
  • Microfluid biology research
• Syringe pumps work from pico-litres to hundreds of millilitres/minute

Emerging Electric Actuation Technologies

• Utilizes ionic actuators and electromechanical coupling due to diffusion, or conduction, of ions within the polymer network

Ionic Polymer/Metal Composites (IPMCs)

• IPMCs are composed of an ionic polymer (Nafion or Flemion)
• Conductive surfaces such as platinum or gold coat the surfaces
• Applying voltage across strips causes ion migration and redistribution, thus bending

Electric Field (E)

• Key component

Electronic Dipole Moment

• Is equal to p=qd, where:
  • q = charge
  • d = distance
• Measures the effects of charge separation

Dielectric Elastomer Actuators

• The Electostatic Maxwell stress is Pz = ε0 εr E^2 = ε0 εr (V/d)^2, where:
• ε0 is the permittivity of free space
• εr is the relative permittivity of the medium
• E is the electric field intensity (V/m)
• V is the applied voltage
• Pz is the power density (energy stored within the electric field per unit volume)
• d distance or separation between the plates

Electro-Ribbon Actuators

• Minute quantities of oil (<0.1 ml) can amplify the electrostatic force of a material due to its impact on:
  • The electric field
  • Charge distribution
• Most oils have a higher relative permittivity (dielectric constant, εr) compared to air
• The electrostatic force between two charged objects in a medium involves permittivity equal to:
  • F = 1/(4πε0εr) * (q1q2/r^2)
• Where:
  • ε0 is the permittivity of free space
  • εr is the relative permittivity of the medium
  • q1,q2 are charges
  • r is the separation distance
• Since oil has a higher permittivity than air, it increases system capacitance and leads to stronger electrostatic interactions