Introduction• Monitoring of vital organs is important for treating.• Methods until now have drawbacks, technological limitations, health hazards

involved or complex procedures.• Electrocardiogram (ECG) lacks specificity or gives insufficient results.• Imaging methods such as those involving ionizing radiation or ultrasound are

unsuitable for continuous monitoring.• Radiation based methods are unsafe for long term exposure.• Ultrasonography needs constant attention.• Other implantable systems are difficult to be safely implanted and even more difficult

to remove.• Hence a system is needed that is specific, accurate, does not affect the

body/concerned organ, can be implanted and removed easily and can be used for continuous monitoring.

Introduction• A 3-axis accelerometer based system provides the necessary requirements• Acceleration data from the vibrations of the heart is collected by a sensor.• The small size allows it to be implanted directly into the heart muscle.• It should be able to work not just during the operation but post operation too, for

continuous monitoring.• Suggested by practicing cardiac surgeons from Oslo University Hospital’s Intervention

Center, that the sensors should be implanted inside the heart muscle

HardwareThere are three major components in the system:

1. Sensor:• 3-axis sensor with 3mm outer diameter limit after encapsulation.• The CMA3000A was selected with analog interface to ensure software compatibility.• The CMA3000A is a MEMS-on-chip device where the ASIC is attached to the sensing die by

flip chip bonding.• The sensor has 8 I/O terminals and requires external capacitors for low-pass filtering.

2. Integrated flexible cable-substrate:

• Few critical requirements:the stretch of cable has to be fairly robust and substrate must be closely tailored to the sensor’s dimensions.

• The accelerometer sensor requires a set of decoupling capacitors located next to sensor to minimize interference from inductive crosstalk.

• The bandwidth should harmonize with the frequency spectrum of the heart motion (0 – 100 Hz). This was achieved by combination of the output resistance and low-pass filter capacitors(68nF for O/Ps and 100nF for I/P(VDD) giving bandwidth of 74 Hz).

• The dimensions of the cable-substrate: 504.2mm in length (4.2mm substrate part), the width of the design was 2.2mm. The tail end with larger contact pads was 6mm wide. The thickness of the cable-substrate was 0.2mm

3. Encapsulation Design:

• Initially silicone was used to encapsulate the sensor.

• Stable fixation of the implant inside the myocardium was required. In temporary pacing leads this is accomplished by either a helical or a zigzag attachment. The thickness of the “anchor” was chosen to be 1mm in diameter.

• The actual sensor needed to be smaller than the recommended maximum of 3mm, thick enough to be a barrier against moisture and to retain mechanical integrity, the shape must avoid sharp angles and have a shape that would prevent rotation – this is particularly important as the sensor merely works in translation axis and not in rotation.

Fabrication• Mould for Silicon is made of Aluminium using a CNC.

• The mould has cavities for the sensor, without any screws.

• The Silicon should be cured at 165 degrees, but it is 80 degrees than the maximum operating temperature of the sensor. Hence, a longer time duration and a lesser temperature is used.

• Empirical formula for curing:10 degrees below the actual curing temperature = double the time required

• The formula guarantees a clear, strong, transparent, and no trapped air bubbles.

Highlights of the Design

• One of the major advantages of this system is that it could provide monitoring of the heart function during the surgery and for the period after it.• The compact size of the sensor enables it to be easily implanted and

removed when necessary.• The 3-axis sensor (CMA3000A) comes with an analog interface thus

ensuring compatibility with softwares developed for older generations of this system.• The sensor is provided with a power supply and also a connection to

send the data to an external data acquisition system.

Limitations• During molding parting, the high forces and stresses and can damage the sensors.• A thin film called flash usually forms on the boundary between two halves of the mold

and its manually removal is a tedious process.• Adhesion primer restricts de-encapsulation of the sensor which forbids the repair of

sensor.• The “anchor” used for ensuring implant fixation (with a thickness of 1mm) was unable

to properly hold the implant in place.• The stiffness of the cable-substrate in the lateral direction has the potential to damage

the delicate tissue of the myocardium. Also this stiffness can affect the sensor’s fixation and unseat it.• The implantation of the sensor does not involve integration with any surgical tools, so

it is upon the surgeon to determine the problems and abnormalities.

Future ScopeElectrical interconnection• In subsequent designs it should be replaced with a round cable Encapsulation• The problems with flash (a thin film that forms on the boundary between two halves

of the mold) needs to be addressed in future work Carefully weighing the silicone to avoid excess, or polishing the mold surfaces to achieve better contact between the two halves with no room for silicone to penetrate could be two ways to solve the flashing problem.

Handling and implantation• A recommended way of implanting the device would be to use a needle with thread

attached to the capsule housing the sensor, the needle would then be used to make a channel in the myocardium and the capsule could be pulled in to the channel. This technique is known as a blunt dissection. Silicone may not be the best option for this type of encapsulation, as the flexibility of the material would transfer the mechanical stresses to the sensor.

Design considerations:

• The possibility of moving the capacitors out of the implantable part, and the performance of the sensor depending on capacitor placement needs to be investigated, keeping in mind that this may leave the sensor more susceptible to high frequency noise.• Further functionality could be attained in the areas of wireless

communication of the sensor. This could allow the sensor to become a remote patient monitoring tool whilst being non-invasive and non-intrusive which better ensures the quality of life for the subject

Future Scope

Result• Finally, it can be seen that a 3-axis accelerometer based system

proved to be quite a successful alternative to the pre-existing methods.

• It was more accurate and specific, compact, easily implantable, and can be used for continuous monitoring.

• Despite these, it was limited by the certain hardware-based reasons and because it is not integrated with any surgical tools which makes it entirely dependent on the surgeon.

References

IEEE Research paper titled “3-axis MEMS Accelerometer-based Implantable Heart Monitoring System with Novel Fixation Method” published in 2013, Fjodors Tjulkins*, Anh Tuan Thai Nguyen, Nils Hoivik, Knut E. Aasmundtveit, Erik Andreassen, Lars Hoff , Kristin Imenes IMST-Department of Micro and Nano Systems Technology, HiVe-Vestfold University College in collaboration with Intervention Centre – Oslo University Hospital