Medical Electronics
Delivering drugs electronically
Chris Ferris looks at the design issues that have to be taken into account when developing electronically enabled drug delivery systems
F
or drug delivery devices produced in high volumes, often in excess of 10 million units per year, a low cost of goods is critical in maintaining a competitive market position. Hence, whilst there are obvious opportunities to add electronic intelligence to medical devices, moving away from traditional passive containment and dispensing materials such as foil pouches and plastic mouldings invariably raises the issue of value. Can the increased cost of a device which might actively help the patient with compliance, or improve the uniformity of delivery, be justified? On a straight per-use calculation, the answer would appear to be negative, but new technologies mean that manufacturers are able to revisit the economics of electronically enabled drug delivery devices. Firstly, the growing demand for home-
based treatment has increased the need for multiple-dose devices, giving manufacturers the challenge of providing the patient with information such as remaining doses, regularity of use (compliance) and consistency of delivery, such as the inhalation profile of the user. Traditional mechanical dose counters are being replaced by more intelligent electronic solutions which provide not only dose indication, but alarms, logging and analysis features as well.
As well as market pull from users,
increasingly complex drug formulations are pushing the need for actively controlled flow, and even aerosolisation phase change, to deliver their intended therapy. For these reasons, demand for batteries, sensors, and user interface components such as LEDs, buzzers, and LCDs is growing rapidly in the medical sector. And whilst these requirements are, on the face of it, no different from components that are readily available in consumer markets, the best design solution for a medical device is often arrived at only after detailed consideration of the particular needs of the market. In particular, safe operation of software is seen as a major requirement for market acceptance of intelligent devices.
Device disposability One of the biggest differentiators from consumer products is the relatively short life in service, lifespan of a drug delivery device. This doesn’t imply a low emphasis on reliability, however, because the device may have to meet extended storage requirements of up to two years, before
www.cieonline.co.uk
being activated and used for a few days, or a few weeks at most. For built-in sensors intended to make consistent accurate measurement of a medical condition, these storage requirements may stretch far beyond the normal stability period achieved by standard calibration methods, and accurate performance may be unexpectedly compromised by excessive drift due to ageing. Designing a method of secondary calibration during use is often the best, and sometimes the only way to achieve accurate sensing. And whilst instantaneous battery capacity might not be a challenge during the use of the device, the nominal capacity of batteries is hugely influenced by their own self- discharge characteristic, requiring a secondary barrier such as a tear-off activation strip or priming action to be completed by the user before first use.
Clinical considerations The operating environment of drug delivery devices is also of concern in making design choices. By their very nature, they are used in proximity to disease, and the prevention of cross-contamination is a priority. Single- use devices in clinical environments could simply be disposed of through the hospital waste control process, but multiple-use devices in the home environment must be designed with sustainability in mind. Often this means creating a distinct partition between the parts of the device that will come into contact with the user, and are at risk of infection, and the parts of the device that will be retained for use during the next cycle of treatment. In a cassette-based inhaler, for instance, whilst it may make sense from a usability viewpoint to have an active LED close to the site of dispensing, it makes more sense from a design viewpoint to house the LED in the main device body. Where adjacent indication is a necessity, it may be a better option to use a light pipe (a relatively low-cost component which is easy to clean/dispose of) to carry the indication to the correct location, without increasing cost or the risk of contamination. In many cases, the disposable part of the device will need to be assembled in a clean room, and stored in sterile packaging.
Designing the active parts of the device to be separable from this process is therefore often a necessity, and perhaps the most challenging design area in this regard is in the use of sensors. Electrochemical sensors, for instance to
measure blood glucose, and micro- electromechanical systems (MEMS) such as pressure sensors, are invariably part of the drug delivery path and must be assumed to be disposable, with the associated complexity of sterilisation adding to the cost-per-use. Alternative non-contact technologies such as ultrasonic, thermal and optical sensors often offer a more practical solution.
Active Retained systems As well as achieving the desired cost-per- use of the disposable parts of the drug delivery device, proper attention must be paid to the design of the reusable parts of the medical system. Should a display be large and complex, or small and simple? Should the system be powered from the mains, or packaged as a portable device. And if portable, how can users ensure that the batteries are always charged and ready to run? Recent advances in component technology, such as low resistance MOSFETs to control motors, miniature piezo-based actuators, high brightness LEDs operating at low voltages, high efficiency voltage boost converters available in small packages, and active matrix displays have all helped to improve the functionality of active medical devices, but increased functionality without adequate control can often unintentionally increase use error and ultimately affect the effectiveness of the device.
User-centric engineering For this reason, Team places human factors and usability design at the heart of its design process. From a regulatory standpoint, both the FDA and Notified Bodies in Europe recognise IEC 62366 as a harmonised standard which allows manufacturers and designers to analyse, specify, design, verify and validate usability, as it relates to safety of a medical device, and this pivotal standard forms an integral part of our device development plan. Whilst a usability assessment is mandated for all medical devices, actually designing the best system partition for any particular device requires close co-operation between product designers and engineering teams. Attracted by the technology already available in homes, where smartphones, wireless communication and tablet computers are the norm, it is often tempting to mimic the same functionality in medical systems. One advantage of this approach is that users
inherently understand how to correctly use medical devices that operate like consumer devices. During the design process, we can rapidly develop flexible user interfaces on Windows or Linux platforms, but consideration must always be given to the risk of operating system errors (which are largely beyond our control) unexpectedly compromising the performance of the device.
IEC 62304 defines a process for managing the medical software development lifecycle, and indicates the extent of testing and verification that must be completed during the design process. Often, where continuous safe operation is of critical importance the best solution is to migrate away from large operating systems to a customised platform running a robust and validated operating system such as SafeRTOS or QNX.
The OrganOx metra system, which is designed to control liver perfusion at normal body temperature for up to 24 hours, is an example of how a safety- critical software architecture can be implemented. By separating out the graphical aspects of the user interface, the majority of the testing effort can be concentrated towards ensuring reliability of the critical operation of the core control software. The most critical aspects of the software are also bounded by hardware limits to prevent catastrophic mis-operation in the event of a failure
Medical systems of the future Removing the overhead of large operating systems allows us to implement complex devices using the same mid-range processors that are used for portable consumer devices. Low power wireless technologies such as Bluetooth LE might also simplify the development of sterile active devices, allowing fully sealed, immersable or implantable devices to be applicable to a wider range of treatments. So whilst medical device development is distinctly different to consumer device development, convergence is increasing. As home-based treatment becomes the norm for an increasing number of illnesses, the demand for cost-effective, interoperable medical devices will continue to grow.
Team Consulting | www.team-
consulting.com
Chris Ferris is Head of Electronics and Software at Team Consulting
Components in Electronics May 2013 13
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64