LUMITAC IS A CAMERA-FREE SIX-AXIS TACTILE SENSOR, 12 MM WIDE AND ABOUT $7 IN PARTS

Summary from the TLDR Hardware newsletter

LumiTac pairs five photoemitters with five RGBW photoreceivers on opposing circuit boards joined by translucent silicone, so load-driven motion between the layers shifts the internal light field under compression, shear, bending, and torsion. The head is 12 mm across, 4.75 mm thick, under 2 g, and made with about $7 in components. Independent testing gave R² of 0.966 to 0.994, with mean absolute errors of 0.054 to 0.190 N in force and 0.304 to 1.419 mN·m in moment.