• Human cells with increased ultrasound sensitivity were shown to respond to other types of mechanical stimulation, such as shear stress and touch.
• Touch responsive cells were used as a canvas in our sci-art touch painting implementation.

While ultrasound was used to stimulate mechanosensors in our project, since it is most appropriate for the stimulation of deep tissue, other mechanical stimuli could also trigger activation of mechanosensors. Mechanical stimulation appears to be a critical modulator for many aspects of biology, both of living tissue and cells Seriani2016 . Mechanosensing can cell proliferation and differentiation Shah2014 . In recent years, many methods of activation of mechanosensitive channels have been implemented. Among the activators of mechanosensors, hyperosmolarity has been shown to trigger the activation of certain genes Veltmann . Additional mechanical stimuli such as fluid shear stress causes activation of embryonic endothelial cells Ranade2014 .

Therefore our system might be also activated by other mechanical stimuli. One of them was stimulation by direct contact that underlies the sense of touch. Our designed mechano-responsive system is composed of two modules. First we included modules to provide and increase the sensitivity to mechanical stress by mechanoresponsive ion channels and gas vesicles and secondly, the influx of calcium is visualized by the dimerization of calmodulin (attached to the C terminus of split luciferase) and M13 (attached to the N terminus of split luciferase) which results in bioluminescence after the reconstitution of split luciferase. This means that this system is able to convert the mechanical stimulus into light signal, or to put it differently, with our system we are able to see when cells are touched.

Shear flow also exerts mechanical force on cells, which also has relevance for the endothelial cells within blood vessels, therefore also the flow of the liquid medium around cells might be sensed by cells. The experiment to test this ability was performed by agitating the petri dish with attached cells. In order to identify if the engineered cells are able to respond to the sheaf flow we included control cells that either constitutively express the luciferase and cells that harbour only the calcium-dependent reporter without constructs to increase the mechanosensing along with cells expressing the gas vesicles and reporters.

Results show that the light was emitted only in cells with the constitutive luciferase if the plate has not been moved ( 1 ). On the other hand shaking triggered activation of the luciferase, with significantly higher level in cells that harboured gas vesicles in addition to the reporter, demonstrating that activation does not require high pressure on cells.

Next we wondered if cells could respond to a gentle touch. We were excited to see that already in the first experiment we could clearly see where cells have been touched by a glass rod. The best paintbush to stimulate cells without removing them from the plate has yet to be identified but we found that glass rod functions quite well. We realized that we could use touch to illuminate cells, effectively painting or drawing on cells by recording the trace with emitted light, which we called Touchpaint. At that time the Argentinian visual artist Laura Olalde, PhD, interested in arts and science symbiosis, and interested in connections between art and science visited our lab. She was immediately eager to test Touchpaint as the new artistic medium, which enables us to transfer the sense of touch into light.

The immediate results were visible from drawing of symbols of our project on cells that were immediately (within 5 min) visualized by imager.

Light emitted from cells was recorded by an imager by capturing light for with 30 sec. We observed that the lines that were drawn first were more fain that the those drawn later, which suggested that the response is quite fast. This was tested by sequentially drawing separate letters and recording the image after draing each letter.

Laura prepared several stamps and we experimented with different methods of cell immobilization on the plate, however due to the time constraints of the iGEM project we have yet to explore the intricacies of Touchpaint techniques. Response of the system is quite fast as the sequential drawing of letters of the iGEM acronym already started to fade by drawing the text letter few minutes after the first one, therefore real time monitoring of cell drawing is a very realistic proposition. While Touchpaint is just one artistic implementation of the technology, there are many other scientific and applied uses of this technology, which are discussed in the Impact section.