Disc Pump for Less Obtrusive Ambulatory Blood Pressure Systems

TTP Ventus, the UK-based developer of the Disc Pump, has recently begun working on a new application for this technology – ambulatory blood pressure systems. The company claims that the Disc Pump, which is a miniaturized, quiet, and highly efficient pump, could revolutionize ambulatory blood pressure measurements.

At present, 24-hour blood pressure monitors are commonly used to monitor blood pressure at regular intervals throughout the day and night. Such devices typically use motor-driven diaphragm pumps to inflate a cuff on the upper arm. However, these pumps are bulky and noisy, and affect airflow pulsation within the blood pressure monitor. This means that blood pressure measurements can only be taken after the cuff has been fully inflated and begins to deflate.

Taking measurements as the cuff inflates would be preferable, since the pressure the cuff would exert would not have to exceed systolic pressure by much, improving patient comfort and making night-time measurements less disruptive for sleeping users.

Conventional ambulatory blood pressure monitors also
typically require a belt- or strap-worn pump unit, which is connected to a
brachial cuff through a long hose. This is cumbersome, and the hose can become
kinked and obstructed during sleep.

To address these issues, TTP Ventus has proposed that the Disc Pump could offer an alternative to motor-drive diaphragm pumps. The pump operates at over 20,000 cycles per second and moves less than a microliter of air during each cycle. This means that cuff pulsation is negligible during inflation, and blood pressure measurements can be obtained during this process. Moreover, because the pump is very small, it can be directly integrated into a brachial cuff, avoiding the need for cumbersome pump housing and a connecting hose.

See a video about the pump below.

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Medgadget previously
covered the Disc Pump in an interview with Tom Harrison, Business Development Manager of TTP Ventus
about the role of the pump in the aerSleep system for obstructive sleep apnea.
In this follow-up interview, we asked Tom about the potential of the Disc Pump
in blood pressure monitoring systems.

Conn Hastings, Medgadget: Please give us an overview of current blood pressure monitoring systems, and their disadvantages.

Tom Harrison, TTP Ventus: Most current systems use a
conventional, motor-driven pump to inflate a cuff worn on the arm. As the cuff
squeezes the arm, the system makes careful measurements of the pressure oscillations
resulting from the expansion and contraction of an underlying artery. Because
conventional pumps tend to introduce pulsation whilst running, the pump needs
to be turned off before the measurement process starts; therefore, current
systems inflate the cuff to an upper pressure, before disengaging the pump and
bleeding the pressure out with a valve.

This approach has drawbacks for user comfort and
measurement integrity. First, the cuff must be inflated to well above systolic
pressure (the higher of the two blood pressure measurements), applying
significant forces to the arm. With the flow capacity of the pumps typically
used, it can also take a little while to inflate the cuff meaning the arm is
compressed for longer than you’d like. Both issues affect user comfort.

Furthermore, because conventional pumps tend to be
large, noisy and to vibrate, manufacturers tend to design the pump to be worn
on the belt or around the neck on a lanyard. Carrying around a bulky pump in
this way is itself a drawback, particularly given that these systems are used
for 24 hours at a time. Over that time, some measurements will necessarily
happen whilst the patient is sleeping; clearly a noisy pump is a disadvantage
here.

Another less obvious drawback is that the pump must
be connected to the cuff via a hose. Any inadvertent pressure applied to this
hose can interfere with the measurement; this is particularly relevant for
measurements made during the night, where the hose is at greater risk of
becoming kinked during sleep, or where the user may roll onto it.

Medgadget: How would the inclusion of the Disc Pump in such systems help to overcome these shortcomings?

Tom Harrison: Disc Pump has a number of unique features that, taken together, offer a substantial improvement for ambulatory blood pressure systems. First, unlike a conventional pump there is no pulsation in the output. This allows measurements to be made whilst the pump is running – and therefore, whilst the cuff is inflating. In turn, that means that the measurement can be completed as soon as the systolic condition is detected, i.e. the cuff needn’t be inflated any higher than necessary, minimizing the compression forces applied to the patient and therefore improving comfort.

Once
the measurement has concluded, the cuff pressure can be rapidly vented with a
valve. In principle this can be done much more quickly than the time taken to
inflate the cuff with the conventional ‘measure on deflation’ approach. Therefore,
the measurement duration can be minimized too.

Second,
Disc pump is small and lightweight – it weighs around 5 g (1/4 oz) and is
roughly coin-sized. Being this compact allows the pump (and electronics, battery,
etc.) to be integrated into a small module mounted on the side of the cuff.
That eliminates external tubing and the risks associated with obstructions. It
also dispenses with the belt or lanyard-worn pump unit, improving the
experience of the patient.

Third,
the pump doesn’t emit any noticeable sound or vibration when operating,
minimizing disruption to the patient – particularly during measurements at
night.

Finally,
the pump is precisely controllable, meaning that constant cuff inflation rates
(either mmHg per second, or mmHg per heartbeat) can be easy maintained to
ensure measurement accuracy.

Medgadget: What makes the pump different from traditional motor-driven diaphragm pumps?

Tom Harrison: Diaphragm pumps rely on the compression and expansion of gas within a confined space, thereby increasing altering its pressure. The pressure changes cause one-way valves to open, creating pumped flow. The motors (or in some cases, piezoelectric elements) and valves used by these pumps are limited in the frequency they operate at – typically no more than a few hundred cycles per second. At these frequencies, traditional pumps can generate substantial noise, vibration and a pulsatile flow of gas.

In contrast, Disc Pump
does not rely on compression. Instead, it creates a high frequency, high
amplitude standing wave and then rectifies that wave with an ultra-fast valve. Because
Disc Pump operates at an ultrasonic frequency, it is completely silent and
creates negligible pulsation and vibration.

What’s more, because Disc
Pump doesn’t have a heavy motor (the whole device weighs only 5 grams), there
is very little inertia – this means it can be turned on or off in a matter of
milliseconds.

Finally, unlike a conventional
pump, Disc Pump has no ‘stall’ speed. This means it can be driven at the very
lowest levels without stopping.

Medgadget: Was it difficult to design and fabricate such a small pump, with such fine control?

Tom Harrison: One
of the biggest challenges has been the design of the valve, to offer high
performance and efficiency and yet robustness over the course of the pump’s
operational life. On the one hand, the valve needs to respond in a few
microseconds, which drives you towards using lightweight materials; on the
other, over the course of its life operating 20,000 times per second, the valve
will endure several hundred billion cycles. To put that in context, that’s
roughly 20 cycles for every year the universe has existed. And that drives you
towards looking at stronger materials and minimizing wear on them.

Certain aspects of the
pump assembly require very precise tolerances, with some parts being aligned to
a few microns’ accuracy – that’s around a tenth of the width of a human hair. We’ve
combined largely conventional assembly processes as far as possible, customized
only as necessary to meet our requirements.

Medgadget: Has the pump been integrated into a blood pressure monitor yet? If not, when do you expect that this will happen, and when might such devices be available for patients?

Tom Harrison: Not yet, although we have a number of customers
working on it. Whilst the technical side is relatively straightforward, product
development cycles like this often take several years and so I suspect such
devices won’t be available to patients for a little while yet. 

Medgadget: What other medical applications has the pump been used for to date, and do you have any future plans for the pump?

Tom Harrison: All manner of interesting applications, from sleep
apnea therapy, to breath-based disease diagnostics, compression therapy,
microfluidics for point-of-care diagnostics and elevated vacuum prosthetics.

Our plans for the pump are focused on continuing to build on its success in the market. We’re working hard to reach new OEM customers across the medical and life science sectors, whilst keeping an eye on the emerging applications of tomorrow where the Disc Pump often adds significant value over conventional pumps. Our product pipeline is packed with a range of new models planned for release over the next year or so: these will push out the performance, efficiency and lifetime substantially, allowing us to support an ever-growing set of applications. The first of these new designs will be launched and on display at this year’s Compamed tradeshow in Düsseldorf.

Link: TTP Ventus homepage…

Flashback: aerSleep System for Obstructive Sleep Apnea (Interview)