Root mechanics tip balance of power

An ingenious device – inspired by growing plant root tips – is opening up a new type of hydraulic fluid power. Tom Shelley reports

Hydraulic actuators – which are powered by osmosis and inspired by growing plant root tips – are able to deliver very large forces at low power.
They have been developed as soil exploration probes, and for driving in anchorages for robotic systems on other planets. But they could well find similar applications on earth, both on land and under the sea. Because they act very slowly, they could never replace conventional hydraulic applications such as earth moving equipment. But potential applications include: an alternative way to break up rocks or concrete – offering potential in the mining or construction industries; and as a way of keeping steel ropes in tension – such as on a suspension bridge.
Barbara Mazzolai, from the Scuola Superiore Sant’Anna in Pisa, Italy, described the design and development of the ‘Robotic Root Apex’ at a recent conference on Biological Approaches for Engineering – organised by the Institute of Sound and Vibration at the University of Southampton.
Root tips in nature deliver massive forces slowly – sufficient to split concrete, road surfaces and solid rocks – so there has long been interest in harnessing the effects industrially.
The main driving force is osmosis, which can generate huge pressure. A membrane is used to separate two aqueous solutions, one more concentrated than the other. Water molecules in the more dilute solution pass through the membrane more often than those from the more concentrated solution. This generates an increased hydrostatic pressure in the more concentrated solution. In nature, this is a one-way process. But, in the world of engineering, it is necessary to be able to retract actuators reversibly, as well as extend them. So while the actuators devised at Pisa use water diffusion as the primary mechanism, the barriers between chambers also include ion-selective membranes that allow the passage of suitable, negatively charged ions.
On each side of the membrane is a metal electrode in a solution of its own positively charged ions. To maintain electrical neutrality, the number of positively charged metal cations has to be balanced by negatively charged anions. If a metal electrode in one compartment is charged positive, while another metal electrode is charged negative in a second compartment, the metal electrode in the first compartment dissolves in solution, while metal ions in the second compartment come out of solution and plate on the electrode. To maintain charge neutrality, negatively charged anions then have to diffuse from the second compartment to the first. This means there are then more water molecules per unit volume of solution in the second compartment, which thus tend to pass through the membrane in the same direction as the negatively charged ions, to a greater extend than the smaller number of water molecules passing in the opposite direction, generating an increased hydrostatic pressure in the first compartment.
Mazzolai said that her team’s experiments used lead electrodes and solutions of lead chlorate, nitrate and acetate – though most of the experiments seem to have been with lead acetate. Membranes are ‘Ionac’ anion exchange membranes made by Sybron Chemicals, a US company owned by Lanxess.
“The redox reaction must be reversible,” stressed Mazzolai.
The development is still at an early stage: Mazzolai talked about experiments producing pressures of 0.3 to 0.5MPa – 3 to 5 bar – but Eureka is aware of osmotic pressures of 10 times that. Practical limits are normally decided by the mechanical strength of membranes, but membrane tubes for reverse osmotic purification of sea water have no trouble withstanding 50 bar.
The work at Pisa has progressed as far as designing and prototyping a practical, osmotic-driven device that can slowly force its way into soil and steer itself. It is made in two parts. The front is connected to the rear by three actuator pistons, spaced at 120º intervals, with a spherical joint pivot in the middle. The length of the whole device is 62mm and it is 22mm in diameter. The osmotic module is 22mm long and the control module 20mm. The tip makes up the remaining length. The pistons are pressurised by a system in which the three osmotic chambers are separated by gratings, each of which contains ion selective and osmotic membranes. A head block compresses a deformable membrane over the osmotic chambers and also functions as a guide for the pistons. By applying current to the appropriate electrodes in a suitable manner, the concentration of ions in each chamber can be regulated and so drive the water into a selected chamber in order to make the membrane locally expand into the piston guide and push the piston. The device can bend itself in the middle by up to 13º.
In the control module is a microcontroller: it acquires information from sensors, and controls current to the electrodes to drive the actuators. Two sensors have been integrated so far: an accelerometer to sense gravity; and a soil moisture sensor, which works by measuring soil resistivity. Signals pass from the control module to the osmotic chambers through the screws that lock the osmotic modules.
There was some discussion at the conference about how the module could drive itself in with nothing behind it to push against – although, if the front and rear sections were barbed, there would appear to be no problem – since the front of the device could be made to move forward slightly, draw its rear part after it, then push the front forward again – in the manner of an earthworm. Cycle speed would appear to be very slow. Mazzolai mentioned times of around three hours – rather than minutes – but many soil-type materials are visco-elastic, so slow movements can be accomplished at much lower forces than fast movements. This means that its method of progression is potentially very energy efficient.
Mazzolai believes the devices could be used to seek out and find pollution underground, in a manner that would not risk contaminating the surrounding environment – as would happen by digging. One could envisage such probes being deployed to survey radioactive contamination in piles of rubble, such as at Chernobyl. One of their virtues is that they should be quite cheap to manufacture and so completely expendable.
The development has been partly supported by the Italian Institute of Technology Network.


* Osmotic actuators can be made reversible by employing a reversible redox reaction to vary dissolved ion concentration

* Movements are very slow, but force is several bar at the moment – and could be extended to tens of bar

* Energy consumption is very low and the prototype soil-penetrating soil exploration probes could be made cheaply enough to be expendable in hazardous environment explorations

Tom Shelley

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