Can You See How I Feel?

Reproducing the tactile sensing capabilities of human skin is extremely difficult, and is by no means a solved problem. Robots often rely on technologies like piezoelectric or capacitive pressure sensors to understand the world around them, but the data they produce is very coarse. Higher-end robots are increasingly relying on vision-based tactile sensors (VBTSs), which offer far greater resolution, and a much better understanding of the world, than conventional options.

These VBTSs have some issues of their own, however. The fabrication processes required to produce them are considerably more complex, which drives up the cost of VBTSs. Furthermore, the design and production phases are generally treated as separate processes, which means that lots of back and forth has to take place before a suitable solution can be found, and that slows forward progress.

A group of researchers at the University of Bristol and Imperial College London have an idea that might make VBTSs less expensive, easier to produce, and that might increase the speed of innovation in this area.

Their proposal is called CrystalTac, which is a family of VBTSs manufactured using rapid monolithic 3D printing. Using their approach, the team can fabricate a complete, integrated sensor in a single print job using multimaterial 3D printing, eliminating many of the complex, multi-step assembly processes that have historically made VBTSs costly and slow to develop.

Traditional VBTSs convert physical interaction into optical data using multiple layers and components — such as flexible skins, embedded markers, lenses, and coatings — each often made using different fabrication methods. The production workflow is split between a design phase, where engineers plan the sensor’s tactile response, and a creation phase, where the sensor is built and assembled. This disconnect causes friction, since each new design may not translate well into manufacturable hardware.

The CrystalTac approach addresses this challenge by unifying design and creation through a single-pass printing process. This allows researchers to rapidly test and iterate new sensor architectures without worrying about whether they can be manufactured cost-effectively.

The CrystalTac family includes five sensor types — C-Tac, C-Sight, C-SighTac, Vi-C-Tac, and Vi-C-Sight — each demonstrating a different tactile sensing mechanism or a combination of them. These mechanisms include intensity mapping, where light levels change based on contact pressure; marker displacement, which tracks the movement of internal patterns; and modality fusion, where multiple sensing types are combined for richer data.

For instance, C-Sight uses pixel brightness variations to detect pressure depth, while C-Tac tracks specially designed embedded markers to analyze contact force and direction. More advanced versions like Vi-C-Tac and Vi-C-Sight combine multiple sensing modes with transparent elastomer layers to detect both visual and tactile cues simultaneously.

During testing, the CrystalTac sensors showed excellent performance in sensing resolution, responsiveness, and adaptability. Importantly, the rapid monolithic manufacturing method significantly reduced production costs and enabled easy customization, making it viable for scalable deployment in robotics.

The CrystalTac designs are not meant to be final products, but rather a framework and proof-of-concept. The researchers’ intention is to offer the robotics community a flexible, modular platform that can be extended or modified for specific applications — whether it is giving a robot hand the sensitivity of a fingertip, or helping machines interact more safely and intuitively with humans.