Chemnitz University of Technology has reported a steerable jet-powered micro-submarine, built using wafer processing techniques.

Curious, Electronic Weekly got in touch with the team to find out more.

A note on the diagram (right) – this is the best diagram of the basic structure we can get our hands on, although it is of a future concept rather than the proof-of-concept discussed here. Right now, there are two hollow tubes connected by a flat central chassis, with no ‘battery’ or ‘controller’, and the antenna is an inductive planar coil where ‘controller’ is marked. Also, ignore the studded structure at the front.

The 0.8 x 0.8 x 0.14mm vehicle starts off life in an array of similar structures on a silicon wafer.

As deposited, they are considerably wider and flatter, with the extra width later self-rolling into the two engine tubes.

A single continuous 200nm-thick polyimide layer forms the central chassis and the two engine tubes, initially deposited as a wide rectangle on a silicon wafer.

But before this is deposited, rectangles of cross-linked hydrogel are laid down where the polyimide is required to roll up into engines.

The release agent later used to separate the submarines from the wafer does not affect the polyimide, but it does permanently swell the hydrogel, causing differential strain between the hydrogel and polyimide that rolls the motors into tubes – while leaving the central polyimide ‘body’ flat because it had no hydrogel under-layer.

Before this release process, while everything is still flat, metal layers are deposited to form:

• a spiral inductive antenna on the body section, covered by a water-proofing layer
• catalytic platinum patches where the engines will be, at the rear of the engines
• a titanium heater for one of the engines, powered by the inductive antenna

Once released, the submarine is dropped into a solution of hydrogen peroxide, which immediately starts to break down into oxygen and water where it touches the catalyst, causing the oxygen to bubble out of the rear of the motors, providing thrust.

Fabrication process control is close, forming symmetrical motors that produce symmetrical thrust in a high proportion of finished submarines.

The catalytic reaction is temperature dependent, so when power is applied to the motor that has a heater, that motor increases thrust and the submarine turns.

A second version of the submarine has been built with a reduced area of catalyst in the heated engine. This version naturally swims in circles with no inductive field, then can be made to swim straight, then circle the other way, by applying, then increasing, the inductive field.

Expanding on the self-rolling principle, the same team has created a proportionally-sized controllable gripper for the front of the submarine – not shown in diagram: visualise it as a broad tongue the full width of the front of the body – rolled up across the front, but capable of un-rolling on command, then re-rolling – hooking object in the process.

This is another layered structure through which the main polyimide layer extends, but in this case the companion layer is not hydrogel but a polymer known as ‘PNIPAM’ that expands and contracts depending on its temperature – causing the tongue to roll and un-roll with temperature.

Heating in this case is provided by an iron-based layer deposited on the PNIPAM, energised by an external inductive field.

In comparison, the inductive field that actuates the gripper needs to be much more powerful than the field needed to power the steering – the latter of which is provided by a simple one-transistor self-oscillating ‘Joule thief’.

This work is described in full in the Nature Electronics paper ‘A flexible microsystem capable of controlled motion and actuation by wireless power transfer‘, only the abstract of which is available free. The free supplementary information also makes good reading.

Chemnitz University of Technology worked with the Leibniz IFW Dresden.