Theranostic implants

Twelve Fraunhofer Institutes led by the Fraunhofer Institute for Biomedical Engineering IBMT have joined forces to work on the Fraunhofer lead project on Theranostic Implants.

Until now most implants have been of the purely passive type – a typical example is orthopedic devices for bone repair.

But there is a growing interest in active ''theranostic'' implants that combine therapeutic and diagnostic functions in a single medical device. These devices create a closed feedback loop in which vital parameters are recorded and provide the input for therapeutic intervention.

Pacemakers, for example, are capable of responding to the need for increased blood flow to the muscles, for instance during physical exercise, by adjusting the stimulation pulse rate.

Theranostic implants record numerous different biosignals, which they process, analyse and transmit to an external receiver. These signals then provide the basis for therapeutic intervention, which can take the form of electrical, biochemical or mechanical stimulation.

Theranostic implants must be able to function reliably in vivo for many years, preferably throughout the patient's life, despite being exposed to constantly fluctuating cell growth in the damp and warm environment of the human body.

This is one of the greatest challenges facing the engineers developing these highly complex sensor-actuator systems, which also need to be as small and light as possible.

A high degree of biocompatibility with the surrounding tissue is therefore a fundamental requirement, because the implants might otherwise cause a rejection response.

The partners in this project will develop three demonstration models. Their choice of applications was based on diseases that account for a high proportion of the costs borne by German health insurers. The top items on this list are cardiovascular diseases, skeletal system diseases and neuromuscular diseases.

In each of these cases, medical implants are being used more and more frequently to heal the disease or attenuate its symptoms, and to improve the patient's quality of life.

Smart hip-joint prostheses – skeletal system demonstrator
Artificial hip joints are mostly implanted in patients suffering from arthrosis, a widespread disease in modern society. It is generally caused by wear to the joint cartilage, and subsequent damage to the surrounding bone structure, muscles, joint capsules and ligaments. The main symptoms are joint pain and restricted mobility.

Older people are at greater risk of developing arthrosis, which means that, given the current demographic trends, there are an increased number of people requiring artificial hip replacements.

The hip prosthesis being developed by Fraunhofer researchers as part of this lead project is equipped with electronic sensors and actuators that enable the physician to monitor the fit of the artificial hip joint and the bone in-growth without further surgical intervention, and to readjust the position of the implant if necessary.

Conventional prosthetic hip joints have a tendency to work loose because they are unable to adapt to changes in the bone structure. This usually means that they have to be replaced after 10 to 15 years.

But hip revision surgery is a complicated medical procedure and carries with it a high risk to the patient's health.

It is therefore important that any loosening of hip joint prostheses should be detected in good time.

Until now, computed tomography (CT) scans have been the only way of detecting this at an early stage.

However, this method isn't suitable for routine checkups, partly for cost reasons and partly because it exposes patients to a high level of radiation.

This is why the researchers are now developing a non-invasive technique that can also be used in an outpatient care setting.

Their solution works by monitoring changes in the resonant frequency of the biomechanical system comprising hip prosthesis, ball and socket joint and femur.

To do so, a mechanically operated actuator is integrated in the hip prosthesis. The necessary magnetic or electrical energy is transmitted wirelessly to the hip prosthesis from an external micro-controller.

Sensors integrated in the ball of the hip joint, or femoral head, measure the extremely small vibrations in the prosthesis and transfer the data back to the microcontroller using RFID technology.

If the hip prosthesis is too loose, the integrated actuators are activated in order to adjust its fit so that it is seated precisely on the femoral bone again. This is made possible by thin strips of shape-memory alloy attached to the hip prosthesis.

This material has the property of expanding when heated. An integrated heating element serves as a local source of heat, which causes the alloy strips to expand and tighten the fit of the prosthesis on the femoral bone.

Thanks to the smart hip-joint prosthesis, implant patients no longer need to undergo hip revision surgery.

This in turn helps to reduce the costs of medical care. In 2009, German health insurers spent 3.5 billion euros on a total of 385,000 prosthetic hip and knee implants and 53,000 surgical revision procedures.

Sensor implant for monitoring blood circulation – cardiovascular demonstrator
For persons suffering from circulatory diseases such as hypertension or stroke, long-term monitoring could be very helpful. Ideally, the therapy for these patients should include continuous monitoring of the pressure in the blood vessels of the cardiovascular system.

Current methods only permit short-term monitoring, which has to be carried out in an intensive care unit using catheters and intravenous filters. Such interventions are unsuitable for long-term monitoring because they carry the risk of infection and can lead to complications.

The challenge that the researchers have set themselves is to develop smart sensors based on microsystems technology and to encapsulate them in such a way that they can be durably implanted in the patient's body.

Apart from measuring blood pressure, they can also be used to measure other parameters such as acceleration and temperature, and transfer the data to an external receiver. The data obtained in this way will facilitate early diagnosis and improve the disease prognosis by enabling optimised drug treatment. Other advantages include less time in hospital and reduced treatment costs.

The statistics clearly illustrate the applicability of this type of implantable sensor: The proportion of patients with high blood pressure (hypertension) amounted to 37.3 per cent in the year 2000 and is expected to rise to 42 percent by 2025. Hypertension is responsible for 62 percent of stroke cases and 49 percent of all cases of coronary heart disease.

Myoelectric prosthetic hand controller – neuromuscular demonstrator
The loss of an arm, a hand or a leg is always a dramatic event that changes the patient's life forever and inevitably results in severe restrictions in their daily lives.

Roughly a million people around the world have to deal with the loss of a hand due to injury or amputation. Currently available prosthetic hands are less than ideal because they are not sufficiently flexible.

Their movements are limited and they are too complicated to operate efficiently. And above all they are unable to transmit the sensory information required to feel an object held in the prosthesis-wearer's hand.

To solve this problem, Fraunhofer researchers are developing a myoelectric prosthetic hand controller capable of providing sensory feedback. The movement of each finger of the artificial hand is controlled on the basis of muscular contractions and the associated changes in bioelectric potential.

This allows for complex movements. An array of electrodes, which records the myoelectric signals generated by muscular contractions, provides a direct interface between the technical and biological systems.

The control of complex prosthetic systems requires a coordinated response involving the greatest possible number of independently generated myoelectric signals. In amputation patients, these signals originate either from the still-intact arm muscles or from the chest muscles after targeted muscle re-innervation (TMR).

The aim of sensory feedback is to make it easier for the patient to control the prosthetic device. Pressure sensors integrated in the fingers of the prosthetic hand measure the applied gripping force.

They emit signals that drive an implanted stimulator. At the same time, implanted microelectrodes stimulate nerve fibres that activate the central nervous system. The resulting sensory response mimics the act of grasping an object.

Hence, for the first time it has been possible to implement a fully implantable system capable of recording myoelectric signals and stimulating nerves as the precursor to the development and manufacture of a truly bionic hand.

The newly developed sensors and implantation methods must be tolerant to a wide range of changing factors in the surrounding tissue, preferably without any loss of accuracy throughout the life of the implant, so as to avoid the necessity of revision surgery and thereby minimize costs.

For example, electrodes and circuits must be sufficiently flexible to accommodate more than a million muscle contraction and relaxation cycles.

An application-specific integrated circuit (ASIC) with eight analog amplifier inputs has been developed to register and preprocess the acquired signals. It also contains four stimulation outputs.

Other requirements include a wireless power supply and bidirectional data transfer, also wireless. After preprocessing, the recorded bioelectric signals are transferred together with other functional data pertaining to the implant to an external base station. Using the same wireless link, signals are sent to the implant to stimulate the peripheral nerves and to control the integrated devices. Bidirectional wireless and optical interfaces are developed for this purpose.

Project outlook
Funding for the ''Theranostic Implants'' lead project has been granted for a period of four years. The objective is to create a set of technological tools for creating a platform that will provide a basis for developing and manufacturing implantable medical devices and systems quickly and according to a modular approach. In order to carry out this project, the participating Fraunhofer Institutes have pooled their expertise in many different specialist areas in order to create an all-round solution to the need for theranostic implants. This approach guarantees results at the cutting edge of technological and scientific progress.