Active vs. passive exoskeletons explained


The proper functioning of the limbs in our human body plays a fundamental role in health. Significant motor difficulties appear when these limbs are temporarily or permanently affected. A person’s functional autonomy and interaction with the environment are harmed by any form of disability, whether mild, moderate, or severe.

Currently, disabilities-related diseases are on the rise all over the world. According to the World Report on Disability, about 15% of the world’s population suffers from some form of motor disability, with 4% of those suffering from diseases related to motor or neuromotor dysfunction.

Stroke is recurrent in all populations when it comes to the most common problems that trigger neuromotor impairment. Hemiparesis (hemiplegia), cerebral palsy, and traumatic brain injury constitute some of the main causes of disabilities in the upper limbs in the medium and long term.

Notably, the effectiveness of traditional treatments in rehabilitation is somehow determined by therapists’ abilities, prior experience treating similar cases, and ability to formulate successful rehabilitation plans. Typically, patients’ assessments and progress are not quantified in a timely, adequate, and objective manner, limiting the ability to determine the impact of rehabilitation.

Although the application of therapeutic systems based on robotic exoskeletons started in the past two decades, exoskeletons offer new opportunities to develop robust and reliable methods, enabling the recovery of lost motor control due to accidental injury or illness.

Exoskeletons have shown encouraging results in the rehabilitation of upper limbs. The use of these devices in rehabilitation is feasible with direct benefits. These benefits are limited to patients with motor or neuromotor injuries and other areas where movement is critical in terms of optimizing healthcare resources.

Many studies show that various artificial neural network (ANN) architectures are on the rise, as is the mixing of traditional control techniques with intelligent or adaptive optimizers to create robust or hybrid systems. Artificial intelligence (AI)-based processing and control systems have improved mobile robotic exoskeletons used in upper-limb motor rehabilitation.

The use of AI-based techniques can enhance rehabilitation by providing a comprehensive assessment of a patient’s performance and increasing the confidence of both patients and healthcare professionals when interacting with rehabilitation robots. As far as mobility is concerned, the current trend is towards reductions in weight and dimensions of exoskeletons to promote performance in activities of daily living (ADLs) and therefore increase independence. Still, it was necessary in some cases to reduce the degrees of freedom (DoF) to have more compact systems.

Now, exoskeletons are generally classified as active and passive.

Active exoskeletons

Active exoskeletons use the drive systems such as motors, hydraulic systems, or pneumatic systems to enhance human strength or reduce the body’s energy consumption. They are comprised of one or more actuators (e.g., electrical motors) that actively augment power to the human body. The active control mode provides all necessary movement to the limb by the robotic exoskeleton.

In 2005, an electric motor-assisted device to assist trunk flexion was developed to reduce the load on the waist. However, the weight of the second-generation prototype of this wearable power assist device was still 6.5 kg, which is too heavy for users in a bent position.

Later, a smart suit developed by Takayuki could reduce about 14% of muscle fatigue in the bending process. A 24V DC motor drove their device. However, because the motor was cumbersome to carry around, it was too difficult to integrate into the workplace. Moreover, several other well-known active exoskeletons, such as HAL, Muscle Suit, and BLEEX, were too large and expensive for workers.

At present, the industry has not recognized the price, stability, and versatility of active exoskeletons. Some active exoskeletons for industrial applications are still being developed in the lab.

Passive exoskeletons

Passive exoskeletons use elastic materials to store and release energy during lifting works. They do not use a power source but use materials, springs, or dampers to store energy and release it when required.

Some passive exoskeletons, such as Happyback, Personal Lifting Assist Device (PLAD), Laevo, and Bendezy, have entered the marketing stage. They have been shown to reduce the muscle activity of the lower back significantly.

Happyback is made up of fiberglass rods connected by a chest harness, waist belt, and leg units. When bending down, PLAD is made up of elastic elements that support a portion of the upper body’s weight. Laevo is a flexible exoskeleton that supports the chest and back and transfers some load to the chest and legs. Bendezy is made up of a back unit and straps around the shoulders, back, and legs, with springs supporting some of the weight.

Most of these passive exoskeletons focus only on protecting the lower back muscles and ignore the fatigue of the arm muscles in the lifting task. Unfortunately, there is little research on the local discomfort caused by the exoskeleton.

Exoskeletons with a passive operation mode are appropriate to support the upper limbs of the patients as they eliminate the gravitational load and provide the user with the opportunity to perform the necessary rehabilitation tasks.