Lecture 12
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what do artificial muscles need to do ? what is the simplest muscle from a technical point of view ?
Take over the natural function : generate force and cause motion (stroke) Simplest : sphincter
artificial sphincter AMS 800 : how does it work ? what is the focus of new research ?
Purely mechanically driven pump : pumps full or empty a fluid filled cuff around the urethra -> constant pressure acting on urethra. Focus : optimization of cuff geometry and alternative concept with variable external force
what apparatus can be used for static in vitro measurements ?
Hydrostatic pressure that simulates the bladder pressure, connected to a urethra with a metal sphincter around it (controllable length)
urethra compression model : what pressure are we interested in finding out ?
the leak point pressure : bladder pressure (external urinary) when the first drop can go through the urethra
how does the necessary bladder pressure for opening / closing the urethra ? Relationship between pressure and sphincter length?
open : lower pressures close : higher pressures Small sphincter -> high pressures needed. Too long : not advantageous anymore. Middle length are the best.
is a second cuff helpful ?
If urethra is soft (it can inflate) then a second cuff right next to the first one is recommended.
how can cough be simulated ?
• compressible air above water in cylinder, connected to urethra with artificial sphincter • dynamic testing machine (electromagnetic) -> better cough profile
how do you fix the urethra to determine stress/strain ? and compression ? in which range is the modulus ?
No clamping (because viscoelastic) so use holders. For compression, a laser is used to determine the deformation and weight pulls the sphincter down. Modulus : 5-20 kPa
what is the aspiration method ? why was it used as well ?
see the curvature of the tissue as a function of applied pressure by aspiration -> we can find modulus. This was used because it could be applied in vivo as well (in clinic). But harder to do than expected.
4 different principles for artificial sphincters
1) fluid filled cuff 2) shape memory alloy operating close to body T 3) electromechanical clamp 4) elastic scaling cuff (connected to motor)
what is the problem when giving nitroglycerine to people with a blocked vessel (stenosis) ? what do we want to do ?
Opens ALL the blood vessels (lowers blood pressure), so we are limited with the dosage. We want to TARGET the delivery to the stenosis
what could be used as a physical trigger for the release of the drug ?
Mass conservation means higher velocity within constrictions -> increase in wall shear stress as trigger
what can be used as containers to deliver the drug ? why do we need to make artificial ones ?
Liposomes are naturally spheres -> they are too stable and don’t react to the small difference in shear stress. Non-spherical liposomes : artificially made because interdigitation is needed, leads to stiff bilayers and facetted liposomes
how is the wall shear stress at constrictions and bifurcations ? which is higher ?
highest values at constrictions, not at bifurcations
what can temperature increase cause ?
loss of interdigitation