Current treatments for underactive bladder (UAB) have negative side effects, such as urinary tract infections, mainly because devices used for bladder emptying are in direct contact with urine (e.g. permanent or intermittent catheterisation). Moreover, most of the current solutions are invasive [1]. In the present study, we are evaluating the feasibility of a new approach to help empty the bladder, consisting of a non-invasive, urine-contactless device. If this proves to be successful, it could improve the quality of life in patients suffering from UAB. This system is based on the impedance pump principle. An impedance pump is a valve-less pump which is able to drive flow by means of an impedance mismatch: travelling waves are generated by compressing an elastic tube at a specific frequency and location. Reflections of the waves at the points of impedance mismatch create a complex pattern of nonlinear wave interference. The result of these wave-interactions can be a directed flow [2]. 
This study tests the hypotheses that an impedance pump externally compressing a porcine urethra is able to: i) increase the urinary flow and ii) lead to complete bladder emptying.

Ex-vivo tests were conducted on 14 fresh female pig bladders and urethras placed on a specifically designed test bench (pig species: Sus scrofa domesticus. Average age and weight: 4 months and 110kg, respectively). All bladders were harvested from a local slaughterhouse. A linear motor was mounted such that its tip could externally compress and release the urethra at a given location, acting as impedance pump (Fig. 1). Before each experiment, bladders were filled with 122ml of water via a Foley catheter sutured through the bladder wall (at the most distant location from the urethral outlet, Fig. 1). The choice of 122ml as initial volume derived from preliminary investigations aiming at determining an initial passive flow (natural flow coming out of the urethra <1.5 ml/s) that would mimic UAB conditions. 
We measured: i) output flow rates, ii) intravesical pressure, and iii) total time to empty a bladder. 
Output flow rates: The initial passive flow rate was recorded for 5 seconds before starting the impedance pumping. The impedance pump was then activated for 20 seconds, and finally, 5 seconds of passive flow were recorded after stopping the pumping (final passive flow). Three compression positions (A=+4cm, B=+6cm, and C=+8cm, measured from the bladder neck, Fig. 1) and four compression frequencies (4Hz, 8Hz, 12Hz, and 16Hz) were investigated. The amount of water expelled from the bladder was continuously recorded using a scale connected to a computer. 
Intravesical pressure: The pressure inside the bladder was continuously monitored using a pressure sensor which was connected to the Foley catheter.
Total time to empty a bladder: For one bladder, the impedance pump remained activated continuously until the whole volume of water (122ml) was expelled out of the bladder. The corresponding total emptying time was recorded. This experiment was repeated three times on the same bladder.

Output flow rates: A typical flow-waveform curve is shown in Figure 2. The activation of the impedance pump always markedly increased the output flow rate from the initial passive flow, for every compression position and compression frequency. Due to geometrical and mechanical differences among bladders, each bladder had its own optimal compression position and frequency that would allow for a maximum flow rate. It was not possible to identify a unique combination of best frequency and best position to maximise output flow rate in all bladders. The best average flow rate (calculated as total volume expelled in 20 seconds divided by the corresponding time) was 3.5ml/s, with a peak flow rate of 5.2ml/s (frequency: 8Hz, position +4cm), and the worst configurations created an average flow rate of 0.1ml/s, with a peak flow rate of 0.15ml/s.
Intravesical pressure: An oscillating pressure signal was measured inside the bladder while the impedance pump was activated. This signal was characterized by small pressure oscillations with maximum amplitude of 0.3cmH2O. A general decay of the pressure signal (~2cmH2O) was also visible as the bladder progressively emptied.
Total time to empty a bladder: All three attempts to completely empty a bladder using the impedance pump were successful. The average time needed to completely empty the bladder was 618 seconds (n=3, V0=122ml). Longest and shortest time recorded were 793 seconds, and 519 seconds respectively. Note that this bladder would never have emptied itself without the impedance pump, as its initial passive flow was 0ml/s.

Output flow rates: Our results show, for the first time, that the impedance pump principle: i) can be used to generate/increase the “urinary” flow rate and ii) leads to bladder emptying. 
Intravesical pressure: When the impedance pump was activated, the generated pressure signals were far below the limit of 40cmH2O (considered as the maximum safe pressure for a human bladder).
Total time to empty a bladder: All three attempts aimed at reaching complete bladder emptying were successful. It is expected that the emptying time can be significantly reduced by optimizing the compression mechanism. Moreover, the designed test bench only allowed a horizontal configuration (Fig. 1) while a filled bladder placed in vertical position (such as in standing or sitting patients) could also help improve the flow rate due to the hydrostatic pressure.

The results are very promising and warrant further studies to explore the full potential of developing a non-invasive device, based on the impedance pump principle, which can help empty UAB patients. Existing solutions for these patients are based on catheterisation which, very often, causes infections, bleeding and discomfort.  The possibility for UAB patients of having a portable, external and urine-contactless device to help them empty their bladder would represent a big improvement in their quality of life.

Chapple C., Osman N, Crystallizing the Definition of Underactive Bladder Syndrome, a Common but Under-recognized Clinical Entity. Low. Urin. Tract. Symptoms. 2015; 7(2), 71-6
Hickerson AI, Rinderknecht D, Gharib M. Experimental study of the behavior of a valveless impedance pump. Experiments in Fluids 2005; 38:534-540.