Artificial muscles, also known as soft robotics or robotic muscle fibers, are materials and devices that are designed to mimic the functionality of natural muscles but do not necessarily resemble biological muscles in appearance or structure. They are typically made from synthetic polymers that can repeatedly contract, relax, and generate force like natural muscles do in response to various stimuli like electrical signals, temperature change, or moisture. It offer potential advantages over traditional electric motors through their soft and flexible nature, high power-to-weight ratio, and ability to perform multifunctional motion.
Types of Artificial Muscles
There are three main types of muscles currently being developed:
Piezoelectric polymers - These use materials like nylon that generate a small electric charge when bent or stressed. Applying a voltage causes the material to contract or expand mimicking muscle function. Piezoelectric polymers offer high strength and can precisely control contraction but generally have low stroke and force.
Electroactive polymers - Also known as ionic polymer-metal composites (IPMCs), these use polymers embedded with metals that bend when a low voltage is applied due to migration of ions in the polymer. EAPs are lightweight, low power and highly tuneable but have low force output and stroke.
Shape memory alloys - Alloys like nitinol exhibit the shape memory effect where they ‘remember’ an original cold-forged shape and revert to it when heated. They contract and relax smoothly providing high force, stiffness and fatigue resistance suitable for prosthetics and orthotics. However, shape memory alloys require higher voltages than polymers.
Potential Applications
With their inherent compliance, scalability and ability to mimic human movement, artificial muscles open up many potential application areas:
Exoskeletons & Prosthetics - Soft artificial muscles can enhance human strength in industrial exoskeletons for lifting or extend the capabilities of prosthetic limbs and orthoses beyond what is possible with conventional motors. Their compliance makes for a safer and more natural fit.
Robotics - Soft robotic muscles could enable entirely new classes of robots ranging from minimally invasive medical robots to soft humanoid robots. They may allow robots to operate in uncertain, complex environments safer than traditional rigid robots.
Wearables - These muscles integrated into smart fabrics and garments could power wearable devices for assistance, rehabilitation or just enhanced functionality. Possibilities include powered gloves, assistive costumes and morphing clothing.
Aerospace - In the future, these Global Artificial Muscles might replace hydraulic, electric or mechanical actuators in applications like morphing wings or shape-changing aerostructures for increased aerodynamic efficiency.
Biomedical Devices - Artificial muscles could act as pumps in medical devices, power catheters and endoscopes inserted into the body or assist circulation in damaged tissue replacing damaged muscles.
Progress and Challenges
Over the past few decades, researchers worldwide have made steady progress in developing new varieties of artificial muscle materials, actuators and prototypes. Key milestones include:
- Early development of nitinol shape memory alloys in the 1960s for use as self-expanding vascular stents.
- Introduction of conducting polymers as artificial muscles in the 1980s able to contract on application of electric current.
- Commercialization of McKibben pneumatic artificial muscles in the 1990s utilising a braided sleeved to contract on filling with compressed air or fluid.
- Demonstration of advanced IPMC and conducting polymer actuators in the 2000s with multi-degree of freedom movements and swimmer robots.
- Recent advances include 3D-printed ionic polymer truss structures and thread-based artificial muscles that can lift over 1000 times their own weight.
While lots of progress has been made, challenges still remain around low energy density, durability over cycles of use, control complexity, and integration into practical mechatronic systems and devices. Improving these characteristics through new materials development, fabrication techniques and mechanisms design will determine the future success and adoption of artificial muscles.
Future Outlook
Looking ahead, these muscle technology is expected to continue advancing and achieving new levels of functionality inspired by nature. Key future trends may include:
- Multifunctional materials - New polymers and composites that can sense as well as actuate, self-heal damage, or modulate their properties in response to multiple stimuli.
- Miniaturization - Micro and even nanoscale muscles able to access cellular spaces for medical applications like targeted drug delivery.
- Biomimicry - Closer replication of muscle fibre ultrastructure, vasculature and control through multiscale hierarchical designs.
- Integrated systems - Tighter integration of these muscles, electronics and power sources into complete mechatronic or robotic systems.
- Manufacturing scalability - Roll-to-roll polymer fabrication techniques or 3D printing enabling mass manufacturing.
- Commercial products - Widespread adoption in applications like prosthetics, exoskeletons, soft robots, medical devices as performance and cost barriers are overcome.
As a critical enabling technology, artificial muscles have the potential to profoundly impact a wide range of future technologies from human augmentation to soft robotics if material properties and control challenges are successfully addressed. Their ability to turn energy inputs into graceful, life-like motion at human-scale opens tremendous opportunities.
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