My discipline does not fracture

This past fortnight was a tough one, for I managed to fracture my right foot two weeks ago. I was immobilized for 4 days before I couldn't stand for anymore home arrest and went back to the laboratory anyway. Indeed, it was a hard time for everything from financial impact to simply getting somewhere. I deeply thank all my friends who helped me survive. I shall get back on my feet very soon.

Well, I am not about to continue this unfortunate story although that was part of what I had to deal with last two week. Since I spent more time at home due to the injury, I started to work on that pile of papers that I "was" supposed to read... and guess what? These were papers about the kind of skeleton that doesn't fracture: hydrostatic skeleton!!

According to my ground reaction forces analysis of caterpillar crawling, hydrostatic skeleton was not active during normal locomotion (manuscript still under review at J Expt Biol). However, we know that Manduca caterpillar can pressurize itself substantially and cast about with precise control. Then where is that switch that "turns on" the hydrostatic skeleton?
Well, I thought I would look into discussions on body pressure of all the major soft-bodied animal systems. In annelids [leeches, earthworms, sandworms...etcc], the cylindrical body is covered by very tough layers of circumferential and longitudinal muscles. Segments are separated by septum muscles which constrict the flow of incompressible coelomic fluid. This type of hydrostat has been modeled and studied for a long time. Deformation can be mapped given that the body volume and pressure are known. Nematodes, on the other hand, does not have circumferential musculature. But they tend to have much higher internal pressure and some evidence suggests that their body wall has very high residual stress. In this model, longitudinal muscles are working against the highly stressed elastic cuticle through the incompressible body fluid. Now, what is the case in caterpillars?
Unlike worms, caterpillar cannot breath through the skin; it has to ventilate via an extensive trachea system. Morphologically, these are significant air cavities open to the exterior. The body cannot be "incompressible" even if the body fluid is. In fact, air bubbles are expelled when a caterpillar crawls underwater. In addition, caterpillars don't have circumferential muscles. To pressurize the body, a caterpillar can do two things: close the spiracles, and compress the body fluid. Indeed, when we try to tear an caterpillar off its substrate, the animal gets shorter and stiffer. However, what can we say about other caterpillar behaviors where the body seems to extend beyond resting length and still maintain body pressure. I suggest a flattening action via the oblique muscles that cause the animals to compress body fluid. Some EMG data might support it, but there is no sufficient proof at this moment. How much compression is require to pressurize the hydrostatic skeleton is totally unclear?

There will be more robotics coming up in the next posting in two weeks I promise! I've been working my leg off (... almost) on the robot radio control system and other robot behavioral integration.

InchBot-VII starts to climb!!

After two weeks of endeavor (sleeping in the lab etc...), I finally convinced the new InchBot to climb up a steep incline. According to the animal locomotion literature, climbing is defined as moving up a incline over 45 degree. Well, currently this InchBot-VII can handle just over 45 degree... so it's a climbing robot for sure!!
This 124mm long robot has two batteries in the head capsule and the rear capsule respectively. So it does not need to be tethered really. It weighed less than 4g if we include the R/C control circuits. It climbs with three sticky pads and several morphological features. The climbing gait was adopted directly from the principles of motion in Manduca caterpillar. Stay tune to my upcoming JEB paper titled: Substrate as skeleton: ground reaction forces from a soft-bodied legged animal for details.
Oh...this robot also got some pink sparkles in the body which made it kind of cute!
Go InchBot!!

Sticky Pads... for climbing robots

In order to improve crawling efficiency, we started to develop controllable grippers for the caterpillar robot. There are three common mechanisms for gripping: Hooking, Adhesion, and Suction. For the scale of our robot, micro-hooks array and adhesion pads seemed most probable. Manduca caterpillars use the former(crochets) while most insects employ both. However, there are great challenges in both systems.

To effectively dig into substrate of non-uniform stiffness, the micro-hooks array has to vary in hook size and compliance. Up to date, we have very little knowledge about these hooks arrangement and properties. Let alone the task of manufacturing such an micro-array for load bearing.
On the other hand, sticky pads are easier to produce. There are hundreds if not thousands of different adhesive materials to choose from. However, release of sticky pads may be quite a challenge for small robots.

After evaluating the situation, I decided to design a membrane with sticky substance on it. Once I master how to control such a membrane, I could control the sticky pads. Then, micro-hooks array can be incorporated into the system.