Soft tourniquet: three designs of pneumatic silicone rings
A conventional tourniquet applies static pressure around the limb. If it is too low it does not stop the bleeding, and if it is excessive or prolonged it damages tissue. This project explores whether a silicone structure with pneumatic chambers can generate that pressure in an adjustable way. Three concepts were designed, simulated and manufactured. The one that worked best does not inflate: it contracts when vacuum is applied.
Context
Arterial occlusion pressure is the minimum pressure that stops arterial flow in a limb, and it depends on limb circumference and on each person's blood pressure. A tourniquet that could be set close to that value would reduce the risk of compression injury. Soft actuators are natural candidates because they conform to the limb and distribute the load.
Three concepts
SerpentSoft. A spiral with zigzag folds that wraps around the limb, inspired by snake constriction. It was expected to shorten under vacuum and reduce its diameter.
SerpentSoft model around a limb and mesh of its internal cavity
M-Ring. A closed ring with the same folded profile, tested under both positive and negative pressure.
M-Ring model and mesh of its cavity
Tardigradum. A ring made of modules with chambers that collapse under negative pressure, so that the perimeter shrinks.
Tardigradum model and mesh of its cavities
Simulation
Each geometry was meshed in Gmsh and simulated in SOFA Framework under positive and negative pressure, to verify the direction of deformation before making molds.
Tardigradum in SOFA: initial state and under negative pressure
Manufacturing
All three models were cast in silicone in 3D-printed molds, in at least two pours: one for the body with the cavity and one to close it. Each model went through three versions, and the thesis records what failed in each.
Cavity, contour and base molds of the M-Ring, and the resulting part
Some lessons from the process:
- Rigid contour molds in PLA and PETG broke on demolding. A flexible TPU mold solved the problem.
- A soluble core printed in PVA was used for the closed cavity of the Tardigradum ring. Dissolving it is slow and, when the core is printed in two halves, silicone leaks through the joint and leaves plugs inside the cavity.
- Two silicones of different hardness were used to control which walls deform.
Portable device
To drive the models, a unit was built with an air pump and a vacuum pump, two three-way valves, two differential pressure sensors and an ESP32 with a display.
Pneumatic circuit of the device, with a pressure branch and a vacuum branch
Device assembled in its 3D-printed enclosure
Results
Tests were qualitative. The venipuncture training arm provided by the medical skills laboratory had no instruments for measuring pressure, so ease of placement and deformation were assessed, and the models were then tried on real upper limbs.
- SerpentSoft. The reinforcement mesh added in the third version changed the model's orientation and cancelled its operation under positive pressure. Under vacuum it produced compression, not enough to occlude.
- M-Ring. The second version generated considerable pressure on the test bench, but the third, with mesh, did not work.
- Tardigradum. The second and third versions generated notable pressure, with tingling in the fingertips a few seconds after the vacuum was connected.
Tardigradum in its initial state and under negative pressure
The Tardigradum also showed two problems: an oval deformation caused by the internal ring that connects the chambers, and collapse of the air channel at the points where the fittings are inserted, which blocks the cavity.
The pumps in the portable device did not deliver enough pressure. The results described were obtained with larger pumps.
What is missing
There is no measurement of the pressure applied on the limb, and without it one cannot claim that any model reaches occlusion pressure or regulates it. The next step is to instrument the interface, for example with a reference cuff or an instrumented phantom, and to solve the manufacture of the soluble core as a single piece.
How it fits in Robiolab
The project applies the lab's soft-actuator line to a medical device and relies on two tools that also appear in other work by the group: simulation in SOFA and vacuum actuation, which is inherently safer than positive pressure because a material failure does not produce a burst. It shares with the finger rehabilitation actuator the same pending problem, namely measuring contact pressure between a soft actuator and the body.