New instrument unveils how humans grasp slippery objects
For the first time, scientists at TU Delft have uncovered how the human grip adapts to sudden changes in friction and load, using a novel device that alters friction on demand with ultrasonic vibrations. This deeper understanding of the neurological processes behind grip control could pave the way for innovative rehabilitation tools for stroke survivors and enhance the development of tactile robotics. Their findings have been published in the Journal of Physiology.
Every day, we handle dozens of objects with different textures and surfaces, rarely thinking about the precise adjustments our fingers make. ‘Despite vastly different surfaces, our grip strength adjusts automatically, even when the slipperiness of the object changes unexpectedly,’ explains postdoc Laurence Willemet. ‘Whether it's sweaty palms, peeling a mango, or handling a slippery bar of soap, our nervous system works behind the scenes, fine-tuning our grip without any conscious effort.’
‘This is the first time we’ve been able to study how grip responds to friction changes during object manipulation,’ says associate professor Michaël Wiertlewski. The team’s device, developed with the help of master’s student Felix Roël, can alter friction instantly without changing the surface texture by using ultrasonic vibrations. Unlike traditional methods, this approach mimics real-life scenarios where friction changes dynamically and subtly, such as those caused by sweating or condensation.
Adjusting grip automatically
During the study, participants were asked to hold and lift the device between two fingers while watching a documentary, unaware that the device could change its surface friction. Willemet: ‘When friction was lowered—making the surface more slippery—most participants didn’t consciously notice the change but instinctively adjusted their grip strength.’ This automatic response mirrors how we naturally stop a slippery glass from sliding out of our hands. Interestingly, when friction increased, participants neither noticed the change nor adjusted their grip.
Improving robots and rehabilitation
The team plans to develop an advanced version of the friction-modulating device, incorporating a camera to investigate skin contact more closely. ‘If we can understand how humans regulate grip, we can design robots that mimic this behaviour,’ says Wiertlewski. Such advancements could lead to lifelike haptic feedback systems, enabling users to "feel" friction and texture from a distance.
The team also envisions applications in clinical settings. By applying these insights, they hope to develop tools to assist stroke survivors and patients with motor impairments, providing tailored solutions for rehabilitation and recovery.
Laurence Willemet
