The heart is a truly remarkable muscle. About the size of an adult’s fist, it beats on average 72 times per minutes when a human is at rest, pumping blood around the body. Over an average human lifetime, this indefatigable organ will beat more than 2 billion times – many more in some cases. It is the engine that powers every human body.
Not every heart, however, works as effectively as it should do. Around 600,000 pacemakers are implanted each year worldwide to maintain an adequate heart rate, either because the heart’s native pacemaker is not fast enough, or there is a block in the heart’s electrical conduction system. Pacemakers, which have been in use since the 1950s, are implanted into the patient’s upper chest; an insulated wire is passed through a vein into the heart. The wire is plugged into the pacemaker, which then delivers electricity to the heart.
Pacemakers have helped millions of people to live fuller lives, but they are by no means perfect. The average pacemaker battery lasts between six and eight years before it has to be replaced.
It is a problem that Dr Paul Roberts, a consultant electrophysiologist at Southampton University Hospital, believes he has found an answer to.
Working alongside a consortium of companies – Perpetuum,a company with a particular expertise in harvesting energy, Zarlink,a semi-conductor company based in South Wales, and InVivo Technology, a group of physicians (including Dr Roberts) who use their clinical knowledge to create new ideas – over the past two years, he has developed an in-body model microgenerator. This pacemaker lead, called the Self-Energizing Implantable Medical Microsystem, or SIMM) converts energy from the heartbeat into power for implanted medical devices. The project was backed by £560,000 in matched funding from the UK government’s Technology Strategy Board.
“What we have designed is a lead,” he explains. “A normal pacemaker system consists of a generator that contains the computer, circuitry and the battery, which sits underneath the skin and then there is a lead, or two leads, that go from that generator into the heart and it is the lead system that we have developed.
“What you have essentially got is a normal pacemaker lead that sits in the right ventricle of the heart. It utilises the pressure changes within the heart to capture some of the redundant energy from the squeezing motion of the heart.
“How the device works is that it has two compressible bladders, or balloons; one that sits in the right ventricle and another that sits further up the pacemaker lead in the right atrium. What this means is that when the right ventricle squeezes, it compresses that balloon and that pressure sends a small magnet up the lead towards the right atrium.
“When the right ventricle then relaxes and the right atrium squeezes, it sends the magnet back down towards the other end. It is this to-and-fro movement of the magnet in the lead that creates the electrical energy as it passes through some coils and it’s just a straightforward physical property of passing a magnet through coils and generating electricity.”
In a paper presented to the American Heart Association’s Scientific Sessions 2008 in New Orleans in November 2008, Dr Roberts and his fellow researchers found that, at a heart rate of 80 beats per minute (bpm), their device yielded an average harvested energy of 4.3 microjoules per cardiac cycle.
In addition, increasing changes in the heart rate produced corresponding increases in energy – at 104 to 128bpm, the harvested energy level increased 140%. Decreases occurred when the researchers slowed the heartbeat or lowered blood pressure, while implantation and surplus energy harvesting caused no significant injury to the lining of the heart’s chambers.
This first model produces 17% of the energy needed to power the circuitry of a pacemaker. The next generation, Roberts believes, should be far more effective at using energy produced by the heart. “This is very much the first study to prove the concept that it was possible for us to be able to do it,” he says.
“What we are looking at with the next generation is changing some of the materials, and what we are using is slightly different material which is more compressible. We think that that will make the system far more effective and we’re anticipating that we should be able to get up to 100% of the power of a standard pacemaker. That’s the next developmental phase – we’re working on the prototype for that and we’ll need to do some experimental testing of that to see if it lives up to what our expectations are.”
It will be some time before a product is available to the general public, however. Dr Roberts said: “In terms of the timing – any medical device takes many years to go through the basic testing and then to go through the very strict regulatory processes that will allow for normal, human medical use and so the usual timeframe for that is probably five to six years. In reality I think that’s what you’re looking at before something like this might be incorporated into a standard
In addition to prolonging the amount of time a pacemaker can last, this new development may allow other medically valuable features. It is possible that the energy generated could allow the pacemaker to do more. While current pacemakers work reasonably well, the possibilities for the new device – such as wireless technology –are tantalising.
Wireless technology enables home-based health monitoring, with patient health and device performance data transmitted to the physician’s office over a broadband network. Wireless technology also allows a range of new diagnostics and therapies, including implanted devices used to monitor and treat diabetes and neurostimulators that can alleviate chronic pain or lessen the debilitating effects of Parkinson’s disease.
Speaking about the traditional pacemaker, Roberts said: “When the point comes that the [pacemaker’s] batteries have run out, it is a matter of disconnecting the pacemaker from the lead, throwing the old one in the bin and putting a new pacemaker in. It’s a way of doing things that has stood the test of time, it works reasonably well.
“What we would hope from our device is that we may be able to significantly prolong that period of time before you need to have a new pacemaker put it. Also, we think this may have a more important use by providing an additional energy source to the pacemaker and allowing the pacemaker to do additional things.”
At the moment there is a lot of focus on wireless telemetry for pacemakers and implantable defibrillators. This means that communication with the device allows more diagnostic information to be taken from the patient on a regular basis.
All of these features are fairly power hungry, so by providing an additional energy source it may allow for pacemaker platforms to develop to allow other features.
One of the big strides by industry at the moment is to try and develop sensors so that the pacemaker is not only treating the patient but is also able to diagnose whether the problems are ongoing with the patient. There is a balance between what you would accept in terms of reducing the longevity of the device versus the benefit of what information you could provide.
If there is a method of generating additional energy with the pacemaker lead, it would allow for further functions to be developed that would allow greater diagnostic capabilities. The microgenerator will also improve quality of life for patients by enabling smaller devices with a longer operating life.
The idea for the device came from observing the impressive power of the heart – and the excess energy produced therein. It is something that no physician can fail to observe, Roberts says. “If you are ever doing cardiac surgery or procedures on the heart, most clinicians are always struck by the strength of contraction of the human heart – it generates very significant blood pressure and it can do this every minute, every hour,” he explains.
“What you can see is that there is a huge amount of reserve in what the heart does. If you suddenly have to run, your blood pressure goes up, your heart rate goes up and there is a clear, significant reserve there and I think that people over the years have wondered whether it is possible to tap into that reserve. You don’t get anything for nothing, and our calculations are that we are taking a very small fraction of the energy of each heartbeat – significantly less than 1% of the energy of each heartbeat. It really is very minimal from that point of view.
“People have looked at this before but nobody has ever come up with an approach that is minimally invasive. It uses the same technology and techniques to put in a pacemaker lead.
“I’m not aware of anybody else having come up with this sort of solution before – we’ve obviously looked around in the medical literature and not found anything at this point in time.
“There have been various schemes looking at different energy sources – solar power, for example – but nothing actually using the heart motion to generate energy. I think people have looked at other body motion – wrist watches that charge on movement. I’m not aware of anything specifically related to the heart’s movement.”
There are more challenges ahead before the device will be ready for medical use. Roberts says that the materials used for the device will have to be rigorously tested. The next stage of the project is using different materials.
“One thing we haven’t looked at is the chronic elements: leaving one of these leads in for a long period of time and seeing what the effects are,” he says. “What we are essentially saying is that these leads would allow pacemakers to stay in significantly longer. You’ve got to have leads that will allow for that. Any medical device has to be phenomenally robust. That would be one of the important steps of the development, making a lead that is strong enough to be able to stand the test of time.”