Along the straight and narrow

A linear actuator is anything that converts power into movement along a straight line. Often, that means turning the rotation generated by a motor into linear movement, but not always: see the simple spring-powered linear actuator in an autoinjector that pushes the needle forward, sets a piston in motion to inject the drug and covers the spent needle with a safety cap. Variations appear in everything from hospital beds and dentist chairs to syringe pumps, ventilators, probes, and micro and nano devices, right down to the cellular and molecular levels.

For most applications, the choice of actuator comes down to a decision between hydraulic, pneumatic and electronic technology. Hydraulics are generally used for heavy loads like beds and patient positioning systems; they tend to be smooth and quiet, but they can be expensive with a lot of moving parts to maintain and there is always the risk of leakage.

Pneumatic actuators are cheaper and less complex; they work well for high-speed applications, but slowing their movement is difficult and they tend to be less accurate. For accuracy at high speeds, electronic actuators are more reliable; their speed and stroke length are fully programmable, but with that comes a higher upfront cost and a complicated installation process.

A matter of precision

What medical devices tend to need most is precision. In a ventilator, a linear actuator needs to be capable of exactly delivering airflow or risk either bursting a lung or under-oxygenating the patient; the growth in laser surgery requires actuators that can position a beam exactly where it needs to be and hold it stable.

As the miniaturisation trend progresses, so too does the need for smaller actuators that can run efficiently on tiny power sources and deliver finely calibrated movements. That’s one reason for the current boom in piezoelectric actuators – where high voltages applied to certain materials, often ceramics, causes them to expand in tiny increments.

Until recently, piezoelectric actuators struggled with an image problem. They are typically associated with high power consumption, poor reliability and other challenges because of the higher voltages required.

In the past few years, new low-voltage drivers have made piezoelectric actuation more feasible. This is reflected in the global market figures for piezoelectric devices, including medical devices, is projected to reach $34.7 billion by 2025.

Piezo devices aren’t affected by electromagnets, which makes them one of the front runners for MRI-compatible surgical devices. Deep brain surgery patients currently have to be kept awake and responsive because without the ability to scan and operate at the same time, it’s the only way a neurosurgeon can be sure they’ve targeted the right area of the brain.

Printed to fit

Plastic devices using pneumatic or hydraulic actuation are also in the pipeline. In 2019, Germany’s Fraunhofer Institute for Manufacturing Engineering and Automation IPA announced a 3D-printed robot using hydraulic linear actuators to help position biopsy or thermotherapy needles while the patient is inside an MRI scanner. It’s operated externally, which addresses the other practical issue posed by MRI surgery: the surgeon can’t always reach the patient.

The project aimed to develop a design that could be generated in a single stage by a PolyJet printer, but that at the same time comprised fully functional components, such as rotary joints with hydraulic actuators.

“The movement is provided by a hydraulic drive system developed by PAMB researchers using tiny tubes with diameters of just 4mm, seals and pistons,” explained Marius Siegfarth of the Fraunhofer IPA project group for Automation in Medicine and Biotechnology (PAMB) at the Medical Faculty Mannheim at Heidelberg University. “What is particularly special here is that the design of the piston produced by 3D printing technology exerts hydraulic pressure on the seal and thereby enhances its effectiveness.”

Also making its mark is voice coil actuator technology. The name comes from its original purpose, which was vibrating the paper cone in a loudspeaker. It uses a coil and a permanent magnetic field to generate perpendicular force; the most common type involves a moving magnet attached to a shaft. Reversing the current changes the direction of the force, and the force is proportional to the size of the actuator. They’re simple to build, but they weren’t efficient for medical devices until micro-controllers and more precise drivers were developed. They’re now used to focus smartphone cameras and for devices that need repeatable, reliable and controllable motion, such as ventilator valves and drug dispensers.

In the prosthetics field, assistive and rehabilitative devices are freeing physical therapists from carrying out repetitive motion sessions by hand. After strokeinduced paralysis, the exercises needed to stop the shoulder from dislocating and drooping can be rough on both patient and therapist.

$34.7 billion

The projected global market ?gures for piezoelectric devices, including medical devices, by 2025.

MarketsandMarkets

Put into action

The Harmony device, launched by Texas’ Harmonic Bionics, is a shoulder-mounted exoskeleton that rotates the arm and shoulder through its natural range of motion. It allows therapists to focus on engaging with the patient and delivers a full range of movement for faster healing.

Development of high-power, robust and compact actuators capable of closed loop impedance control at high refresh rates is a huge challenge. However, the design team addressed this by building out all of the motor control and communication electronics in-house, enabling a high level of performance.

The customised actuators in the Hero Arm bionic prosthesis have also given amputees the chance to use their hands again. Developed by Open Bionics, the arm-length 3D-printed glove uses electrodes to translate muscular signals into movement for four fingers controlled by custom screw-and-nut linear actuators.

In February, the UK NHS announced the introduction of CMR’s Versius surgical robot to carry out keyhole colorectal procedures, the first use of the system in Europe. However, at the forefront of current surgical robot studies is soft robotics, which replaces steel or hard plastic with flexible materials. It’s especially useful in minimal access (keyhole) surgery because it dovetails better with the unstructured, malleable environment inside the human body. Soft robots tend to be less frightening and painful for patients, and flexible endoscopes can allow a close-up view of something that otherwise would require open surgery to examine. However, soft robots still need to reliably move in a straight line sometimes.

Soft robotics have the potential to enable a number of new technologies in which humans and robots physically interact. However, the necessary highperformance soft actuators still do not yet exist. There have been a variety of attempts at soft linear actuators, including hydrogels, polymers, this study’s use of liquid crystal and dielectric elastomers, and pneumatic actuators powered by gas-filled pouches. So far, none have emerged as a front runner.

A look ahead

Looking to the future, traditional linear actuators stand a chance of becoming obsolete on a nano scale as research into smart nanomaterials advances. In 2016, researchers at EPFL, Lausanne and ETH Zurich announced they had developed a smart, highly flexible drug delivery microrobot that could swim through fluids and blood vessels to target diseased tissue. They used hydrogel nanocomposites and magnetic nanoparticles to create a bot whose shape could be preprogrammed via electromagnets, depending on the fluid it needed to move through – no sensors or actuators required.

“Nature has evolved a multitude of microorganisms that change shape as their environmental conditions change,” said project co-lead Bradley Nelson of the ETH Zurich team. “This basic principle inspired our microrobot design.”

A 2019 study by a team at Georgia Tech’s School of Electrical and Computer Engineering used vibrations from piezoelectric actuators to power 2mm-long ‘micro-bristle-bots’. The polymer bots had legs that moved up and down, which vibrated in response to a tiny actuator glued to their bodies to move up to 8mm a second. The researchers envisioned them working as a team to heal internal wounds – although first they need to work out how to steer.