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Published: September 1, 2009
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Implantable Devices: Challenges and Opportunities

Breakthroughs in technology are progressing at a phenomenal pace, but bringing them to the patient in the form of a device remains a challenge. The technical issues involved in designing an implantable device are outlined together with some potential solutions.
By: R. Srinivasan

DESIGN

A billion dollar market

The worldwide medical device market is projected to be valued at US$200 billion by 2015, approximately half of this being generated by the market in the United States. Conservative analysis suggests that the implantable medical device market segment will reach a sizeable US$30 billion, growing at a healthy rate of 10% annually. Improved understanding of human physiologies and breakthroughs in sensing are progressing at a phenomenal pace in laboratories. However, challenges remain in bringing these insights to the practising physician in the form of a device. Although all medical device designs present challenges, implantable devices can increase project and product complexity by an order of magnitude. This article reviews the significant technical issues that must be overcome and provides some direction towards solutions.


Shape and size


Size and shape factor significantly in the design of implants. Given that implants are essentially foreign objects inside the human body, their shape and overall size can be severely constrained depending on the anatomical region in which they are to reside. The following are basic points to consider when designing an implantable device.


  • The installation of the implant will require an incision the size of which is influenced by the shape and girth of the implant itself. The larger the incision, the longer it will take to heal, increasing the possibility of infection.
  • The path that the implant takes from the site of incision to the target location is paved by relocation of existing tissue. Excessive movement and manipulation of this tissue can result in trauma.
  • Implants destined for the vasculature must be small enough not to be obstructive and be of a shape that is consistent with the required fluid flow rates.
  • Ease of extraction should also be a significant consideration. A technology upgrade, component replacement or other corrective action may require access to the implant.
The optimal shape and size of the implant, although somewhat determined by its contents and purpose, is likely to come from the collaboration of surgeons and engineers with industrial design and human factors experts. Involvement of creative talent is already the norm for many medical device design processes and implantable devices could derive great benefits from the inclusion of design and human factors sensibilities.


Power


The diverse array of implantable devices available presents a variety of power demand profiles.


  • Defibrillators. These devices are basically dormant until the fibrillation is detected and then a momentary surge in power is required.
  • Insulin pump. In this case, the power demand is higher, but it is more uniform over a long timeframe. Glucose level sensing, insulin delivery and communication with an external device all contribute to the total power requirement.
  • Cardio assist devices. Here, the power demand is uniform, but much higher than for an insulin pump. These power requirements are unlikely to be met by a fully implanted battery without employing some type of external assistance.
Batteries are characterised by their chemistry, energy density, rechargeability and their discharge curves. Lithium carbon monofluoride (Li/CFx) chemistry is found in many neurostimulation devices and drug pump applications; lithium silver vanadium oxide (Li/SVO) chemistry may be more suitable for defibrillators. Regardless of the technology, monitoring battery performance is a difficult requirement for any implant design. Innovations in battery technology based on current chemistries, but utilising thin film technology can result in significant gains in energy density. Because the volume is smaller and the shape more flexible, much smaller implants become feasible; for example, currently, battery volume is a significant percentage of the overall volume of a pacemaker.


Battery life and thus implant life is a significant obstacle to the growth of implantable interventions. Depending on the battery technology, a replacement and/or upgrade strategy need to accompany any implantable system design. Some implants used for drug delivery require battery replacement every three years. Energy scavenging technologies are in their infancy now, but it is anticipated that in the coming decade implants will be developed that generate power from the movement of internal parts or the movement of the body to recharge implanted batteries, thus extending device life.


Biocompatibility, degradation and biofouling


Medical implant designers should be concerned with the biocompatibility of the entire implant, including the internal components that never come into contact with the patient such as circuit substrates, batteries and antennae. To handle any eventualities, components should be implant grade even if hermetically sealed inside the casing. Assessment of this is a significant issue for implant designers, who rely on suppliers to provide insights into the safety of a product for implantation. If a suitable part is found by a designer, then the supplier may go through an internal assessment of manufacturing, design controls, test procedures, traceability and documentation; based on this review, the supplier may agree to provide the part for an implantable application. Over a period of time most vendors build up a catalogue of parts that have been used in implantable applications, thus making it easier for the next application.


Degradation, the breakdown of implanted materials, is a significant concern for implants relying on biomedical microelectrical mechanical systems (bioMEMS), the tiny devices that manipulate cells and microorganisms. This is particularly true for drug delivery devices. Ideally, all byproducts will be removed by the body, but the potential for systemic toxicity and genotoxicity must be evaluated during device design. Designers of leading edge technology have to rely on published research for this evaluation, although there is not a lot of history about devices of this type despite the fact that the field is growing.


Biofouling refers to the deterioration of sensor functionality and is particularly problematic for materials used for bioMEMS. Macrophages and foreign body giant cells can attach themselves to the implant and result in slow, but imperceptible deterioration and cause the intervention to be ineffective. The surface microarchitecture of the implants, for example, the composition, polish, coating and other treatments, have been shown to have an influential factor in avoiding this attachment. The surface should be rendered incompatible to attachment by biomolecules.


Table1.jpg
Implant designers should be cognisant of these factors when selecting materials. Potential mitigation strategies include short duration studies with subcutaneous and muscle tissue of small and medium sized animals. For the long term, the guidelines recommended by standards bodies such as the International Organisation for Standards (www.iso.org) should be strictly adhered to; some of the standards are listed in Table I. Sterilisability and manufacturability are other related considerations in materials selection.


Communications


Most implants can benefit from being able to communicate with the external world. Pacemakers and other similar devices require a moderate level of testing and tuning on installation. Handheld devices are designed to be used by surgeons during implantation to set various therapy parameters for later diagnostics in a hospital setting during consultations. Increasingly, communication from the implant to the external world outside a clinical setting is expected for monitoring and alerting applications. In the case of some drug delivery implants such as insulin releasing devices, frequent human interaction may be required. Neurostimulation devices used for pain management allow the patient some control and provide the option to select different sites of stimulation, as well as the strength of stimulation.


The US Federal Communications Commission (www.fcc.gov) has designated the Medical Implant Communications Services (MICS) band (402–405 MHz) for use by implantable devices. Several suppliers of radio frequency (RF) chips have specialised solutions available for the MICS band. However, the implementations vary considerably in their sensitivity, modulation strategies and other session level protocols contributing to the cacophony of the wireless environment, particularly in hospitals. Table II provides an insight into the regulatory environment for implant communication in the worldwide market.


Table2.jpg
Table II. (click to enlarge) Regulated spectrums for implantables.

One component that is often underestimated in importance and complexity is the antenna. Its design is strongly influenced by the frequency chosen. Recognised and addressed early in the design cycle, antennae could help keep the power demands low, provide better coverage and be conformal to the shape of the implant itself, for example implantable neurostimulation devices. Whereas the control, sensing and communications electronics lend themselves to standardisation, antennae are custom designed for each implant.


Interoperability and security


One of the significant technological challenges in healthcare, and in hospital settings in particular, is the interoperability of medical devices. The proliferation of communication technologies (for example, universal serial bus, Bluetooth, ZigBee, Wibree, IEEE 802.11) and medical devices has resulted in many “islands of communication” around a patient. An EKG system for example may be a self-contained device with no electronic interfaces; a pulse oximeter may utilise Bluetooth to communicate with a PC; an implanted defibrillator may use the MICS band. From the doctor’s (and hence the patient’s) perspective time synchronised data from all these devices is extremely valuable, but difficult to achieve under these conditions. IEEE 11073, Personal Health Data, the Bluetooth Health Device Profile and the USB Personal Health Devices address this issue to some extent.1 The implant designer has many options in the overall design of a system, including communication technologies.


Once the implants have the ability to communicate security becomes an obvious concern. Even with the designated frequency bands and the relative obscurity of communication protocols to and from the implant, the risk of security breaches, malicious or inadvertent, must be managed and mitigated.


The trend in the security industry is towards silicon-based solutions. Implant designers can anticipate a confluence of RF transceiver and security technologies in the coming years. The transceivers are likely to include encryption capabilities integrated into the application.


Verification


Any medical device is challenging to verify, but implantable devices inherit all these challenges while presenting many of their own unique difficulties. The usual approach of animal and cadaver studies is helpful in most cases, but inconsistent with the repeatability and automation required for a successful verification strategy.


For communicating implants, the electromagnetic properties of the human living tissue add a special dimension to the verification. Special custom phantoms to mimic human tissue, anechoic chambers equipped with sophisticated network/spectrum analysers and robotics are required to characterise the communications performance of an implant. Coexistence with other implants and noisy ambience will also require deliberate verification strategies.


Over the next few years specialised testing facilities that target communicating medical implants will emerge. A much richer set of physiological parameters will be gathered during the trials to expand the knowledgebase.


Sensing and actuation


Advances in MEMS for pressure, temperature, pH, strain, flow and acceleration sensing have already led to many innovative implants.2,3 Nanomicroelectromechanical systems, microoptoelectromechanical systems and bioMEMS are all active areas of research ripe for the world of implantables.


Software tools


A significant gap that exists in many software based engineering tools is an understanding of the human body. Whether used for mechanical or electromagnetic design, these tools need to consider the concepts and restrictions of biophysics. For example, an RF engineer designing an implantable antenna faces a complex environment consisting of bone, muscle, fat and other tissues all of which have widely varying physical properties. Implant designers would benefit greatly from simulation tools that take advantage of the anatomical knowledgebase.


Multidisciplinary endeavour


In navigating the landscape of human implantable device development for diagnostic or interventional applications, it is clear that the path from laboratory to patient is complex and fraught with many challenges. Expertise in wide ranging disciplines is required to navigate the technical, commercial and regulatory pathways, each of which is developing at a tremendous pace. The optimum path towards a successful implant lies in collaboration between specialist companies in each area under the orchestration of the primary developer.


References





Rajagopalan Srinivasan is Principal Software Engineer at Cambridge Consultants Ltd, Science Park, Milton Road, Cambridge, CB4 0DW, UK, tel. +44 1223 420 024, e-mail: info@cambridgeconsultants.com www.cambridgeconsultants.com

Copyright ©2009 Medical Device Technology

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