Page 442 - Handbook of Biomechatronics

436 Graham Brooker

recharging (or battery replacement) once a week. These form factor and
power consumption requirements drive component selection.

4.2.1 Portability
The size and shape of insulin pumps is determined largely by the reservoir.
However, that presupposes that the electronics can be highly integrated
and low power, leading to a small battery pack. To achieve tight integration,
chip scale, or wafer level packaging is used and any circuit not required is
turned off. A good example of this device is the low standby power 16-bit
MAXQ2010 microcontroller that only draws 1mA when operating at
1MHz and 2.7V, and just 370nA in stop mode. As with the MCUs from
other manufacturers, this device includes USARTs, timers, 64kbytes of
Flash-based program storage, 2kbytes of RAM, some general-purpose I/O
pins, a 312.5 ksample/s 12-bit successive-approximation ADC with refer-
ence, and a 160-segment LCD controller (Mossman, 2010; Ganesan, 2013).

4.2.2 Pumping and Sensing
To achieve flow rates measured in fractions of a μL, the motor driving the
pump is geared down and coupled to a threaded screwdriver that advances
the reservoir piston very slowly with many revolutions of the motor. This
concept is illustrated in Fig. 17 above. As a consequence motor angle need to
only be coarsely measured, typically with optical or Hall-effect sensors cou-
nting individual revolutions. Conventional DC micro-motors are generally
used, though some designers have used stepper motors or MEMS-based
pumps (Mossman, 2010; Ganesan, 2013).
Some innovative new pump technologies include an shape memory
alloy (SMA) motor used in the Omnipod pump and the V-Go, a disposable
spring-driven pump from Valeritas shown in Fig. 18.
Similar to a typical patch pump, the V-Go sticks to the body and delivers
a continuous stream of rapid-acting insulin at a rate of 20, 30, or 40 units of
insulin per 24h, depending on the model. At mealtimes, the user can press a
button to deliver an additional 2 units per push, for a maximum bolus vol-
ume of 36 units.
The Tandem Diabetes Care t:slim is another clever new pump design. It
consists of a small cylinder containing a reciprocating shuttle that transfers
the one-third of a unit from the reservoir to the infusion set with each cycle.
Because the whole reservoir is not pressurized to drive the infusion process,
the motor size can be reduced, power requirements decreased, and overall
safety improved.