MEMS ANTENNA FOR WIRELESS BIOMEDICAL

MICROSYSTEMS

Extremely Small Antenna for RF Receivers in Implantable Devices

P. M. Mendes

DEI, University of Minho, Campus de Azurém, 4800-058 Guimarães, Portugal

L. A. Rocha

DECE, FEUP, University of Porto, Rua Dr. Roberto Frias 4200-465 Porto, Portugal

Keywords: MEMS, Wireless Communications, Antenna, Bio-Telemetry, Implantable.

Abstract: This paper presents an extremely small antenna, together with its model, for wireless biomedical devices.

Most of the implantable devices require very small dimensions. On way to achieve it is to use

microtechnologies to obtain the required size reduction. One of the most challenging devices to integrate is

the antenna, required if we want to communicate with the device trough a wireless link. The proposed

antenna uses a MEMS structure to convert the incoming electromagnetic field into a voltage. This antenna

allows the reception of signals using a carrier in the kHz range and uses only a chip area of 2x2 mm

1 INTRODUCTION

Invasive and implantable biomedical devices used

for diagnostic and therapy, ranging from neural

prosthesis to video-capsule endoscopy (VCE)

systems, are emerging innovative technologies and

they are expected to originate significant business

activity in the near future. The success of such

systems is in part due to the advent of

microtechnologies, which made possible the

miniaturization of several sensors and actuators, as

well their integration with readout and

communication electronics.

Several people from all ages suffer from

incontinence or other urinary pathologies. The

bladder and the intestines perform their function in

an autonomous way, independently from the

individual will. However, any disorder in the healthy

behavior leads to the problem of urinary

incontinence, bladder infections, low bladder

capability and fecal incontinency.

The healthy working of the urinary tract is

essential for health and well being in general, and

even more critical for patients with lesions in the

spinal cord. In this situation, catheters are commonly

used to control the daily volume of urine inside the

bladder. However, the complications related to the

use of catheters, together with the fact that, most of

the times, the spinal segments which controls the

bladder are intact, are driving the development of

several devices to improve the control the inferior

urinary system (Gaunt and Prochazka, 2005).

The new biomedical devices offer the possibility

of improved quality of life, as well cost savings

associated with health care services. However, one

open challenge is to communicate to and from a

biomedical device placed inside the human body

with devices outside the human body. The lack of

antennas, small enough to be integrated with the

sensing microsystem, is a difficult task to overcome

because such communications must be made at

relatively low frequencies, due to live tissue signal

attenuation (Kitchen, 1993). The straightforward

solution is to increase the devices size to dimensions

where it becomes possible to integrate an antenna.

Up to now solutions, use conventional antennas

together with miniaturization techniques to achieve

the smallest antennas possible. However, the size of

such devices is usually limited by the antenna and, is

some cases, also by the batteries size.

In this paper, it is first discussed the need for

small wireless biomedical devices, paying special

attention to patients suffering from urinary

M. Mendes P. and A. Rocha L. (2008).

MEMS ANTENNA FOR WIRELESS BIOMEDICAL MICROSYSTEMS - Extremely Small Antenna for RF Receivers in Implantable Devices.

In Proceedings of the First International Conference on Biomedical Electronics and Devices, pages 70-75

DOI: 10.5220/0001054400700075

 SciTePress

pathologies. Afterwards, MEMS structures,

previously used for non-conventional front-ends,

will be introduced and investigated, having in mind

the new application. The MEMS structure will be

modeled when operating as an antenna.

2 IMPLANTABLE DEVICE

In patients with spinal cord injury at a level that

leaves the sacral segments intact, detrusor

hyperreflexia and detrusor sphincter dyssynergia

(DSD) develops after an initial phase of spinal

shock. This type of bladder is responsible for

important morbidity. The hyperreflexia impairs the

reservoir function of the bladder and the DSD causes

a high resistance against micturition. This results in

reflex incontinence, recurrent urinary tract infection,

and autonomic dysreflexia in high lesions and

threatens these patients with renal failure.

All of these severe disturbances may be well

managed by sacral deafferentation (SDAF) and

implantation of an anterior root stimulator (SARS).

2.1 Electrical Stimulation

Fig. 1 shows the commonly adopted system

architecture to control the inferior urinary system.

nal

enerator

Internal inductor

External inductor

Cuff electrode

Transmission cables

Figure 1: Schematic view of the overall system used for

bladder control.

The system has a signal generator that generates the

appropriate stimulus to activate, e.g., the bladder.

That stimulus is transmitted to the external coil,

which induces the signal in the internal coil.

Reaching the biologic environment, a receiver

module delivers the stimulus trough the transmission

cables that carry the signal to the cuff electrode.

Since the internal coil is placed in the frontal region and

the electrodes are in the back, the transmission cables must

go through the body and are one main cause of system

failure. Moreover, the existence of these cables requires a

small opening in the duramater, not good for the spinal

cord integrity. One main benefit of the microsystem

approach is the possibility to avoid cables trespassing the

duramater.

2.2 Anatomy of spinal cord

Fig. 2 shows the anatomy of the spinal cord. This is

the place where the microsystem must be designed

to operate.

Figure 2: Spinal cord cut, showing the region where the

microdevice must be placed (1– intervertebral disk, 2-

vertebral body, 3- duramater, 4– epidural space, 5- spinal

medulla, 6- subarachnoid space).

From the figure we see that the microdevice must fit

in a very small region, inside the duramater. It can

be placed in two places, or in region 4 or in region 6.

The best place is 6, the subarachnoid space, since the

duramater can be totally closed after surgical

intervention. The available space in region 6 varies

between 3 mm and 9 mm [4]. This is room enough

to accommodate a small microdevice.

The conventional surgical procedure requires the

duramater opening to place the electrodes in contact

with the sacral roots (Fig. 3).

The electrodes are connected to the leads coming

from the stimulator, leaving a small opening in the

duramater. As we can see from Fig. 3, there is plenty

of room to place the microsystem in the implant

region.

MEMS ANTENNA FOR WIRELESS BIOMEDICAL MICROSYSTEMS - Extremely Small Antenna for RF Receivers in

Implantable Devices

Figure 3: Placement of electrodes (testing the nerves to be

used).

3 MICROSYSTEM

The need to reduce the failure associated with long

wires that are used, to reduce the risk of infection or

shifts in the wires is driving researchers to find a

solution using microthecnologies. Also, and very

important, is the internment period associated with

the surgical intervention. Due to the highly invasive

intervention that is required using the traditional

technique, the patients, even when there are no

complications, are required to stay a few days in the

hospital. The availability of a device to allow a less

invasive method would be more comfortable for the

patient, reducing also the hospital costs associated to

the surgery.

To make it possible to use, the device must be

small enough to fit inside the spinal cord, it must be

able to deliver the required stimulus (power and

timing) and it must be possible to communicate with

the device using a radio-frequency signal. This

requires the use of a microsystem completely

integrated, from sensors to communications, thus

requiring the use of integrated antennas. Moreover,

the antenna integration requires the availability of an

electrically small antenna fabricated on materials

compatible with the fabrication of integrated

circuits. This integration requires the use MEMS

techniques, like micromachining and wafer level

packaging.

The microsystem must be designed taking into

account the place where it will be required to operate

because we cannot change the human body electrical

system. As we saw, the human body anatomy will

place constrains on available room for the

microdevice, on power required for system powering

and/or telemetry, and on required power for

stimulation.

The final device to implant must not fail during its

lifetime, must be stable and must be biocompatible

(citotoxity, sensibilization, genotoxity, chronic

toxity, carciginosity, and intracutaneous irritation).

3.1 CMOS Microsystem

The most suitable technology to implement the

microsystem is the CMOS technology. It is the

cheapest technology, with low power consumption,

and adequate for this device since there are no high

power delivery requirements.

In face of constrains, we have three solutions to

design the microsystem. One is to design a chip,

which acts only as a stimulus converter. The second

option is to implement a full microsystem, with the

ability to be remotely powered and controlled

(Piella, 2001). The third is to use a microsystem with

local power and a wireless receiver (Carmo et al.,

2006). The first solution is the simplest, however the

second and third are more flexible and, despite its

higher complexity, fit in the available room for

implant. The main drawback of third solution is the

need of a local source of energy, battery or energy

harvesting.

3.2 Wireless link

Antenna integration is a hard task to accomplish

since it requires joining the knowledge from

antennas, microwaves, circuit design, and materials.

Moreover, the on-chip antenna integration requires

an electrically small antenna, due to wafer cost and

devices size constrains, and operating on a substrate

that was not initially intended for that purpose

(Mendes and Correia, 2007).

Despite it is used to transmit data or to power the

device, it is necessary to have a wireless link

operating at one frequency. As is well known, the

human body shows higher attenuation to higher

frequencies. This means that, the lower the

frequency, the higher the power we receive in the

implant. Moreover, the attenuation is highly

dependent on water content in tissues. The water

content depends on the type of tissue and part of the

body.

Micro-Electro-Mechanical Systems (MEMS) are

becoming an available option for RF communication

systems since they can offer, simultaneously,

devices with improved performance and they use IC-

compatible materials, allowing their integration in a

silicon chip, side by side with semiconductor

Electrodes

erves under test

BIODEVICES 2008 - International Conference on Biomedical Electronics and Devices

circuits. Up to now, MEMS have been used for

antenna applications to obtain non-conventional

front-ends with improved, or new characteristics.

However, some preliminary tests have shown that

some MEMS structures could have the ability to

operate as an antenna itself and this solution would

have the potential to be smaller than the

conventional antennas.

The basic principle of micromachined

cantilevers offers an interesting possibility to

measure a variety of physical parameters (Lange et

al., 2002). When used as a sensor, a MEMS

structure requires the use of a sensing mechanism

and the most widely used is the capacitive method.

The moving structure, and a fixed plate, forms a

parallel plate capacitor, where the structure

movement is translated into a capacity change.

4 MEMS ANTENNA

4.1 Cantilever Antenna

The U-shaped cantilever, proposed to detect a time-

varying magnetic field, is presented in Fig. 4.

Figure 4: Cantilever used to detect a time-varying

magnetic field.

To measure magnetic fields with cantilever

structures, the Lorentz-force is used on a current

carrying lead (Keplinger, 2004). A cantilever of this

type measures only the magnetic flux density in the

direction parallel to the arms of the cantilever, i. e.,

x-axis of Fig. 4. The Lorentz-force acting on a lead

is used to bend a micromachined cantilever.

Deflections, which are small compared to the length

of the cantilever, are a directly proportional measure

of the applied force. To reach the highest possible

sensitivity it is advisable to use a resonant

mechanism, where the cantilever is excited by an

AC current with a frequency equal to an eigen-

frequency of the elastic structure. Due to the high

quality factors of Si structures, which are at least

several hundred, this is an efficient way to enhance

the sensitivity.

Electromagnetic field can be sensed using an

optical, capacitive, or piezoelectric sensing solution.

The most attractive options are capacitive and

piezoelectric. These solutions can be easily

integrated with the MEMS structure and have the

potential for low power consumption (except the

optical solution). Since the desirable displacement

depends on structure dimensions and material

properties, electrostatic actuation can be used as the

actuation mechanism for MEMS micro-antennas.

However, if large displacements are required or if

the MEMS structure area becomes too small for

capacitive detection, the use of a piezoelectric

material can be the solution since it can act both as

sensor and actuator. Moreover, the operation is only

voltage based, leading to low power driving

operation. Furthermore, it produces a voltage in

response to a deflection leading to simple readout

electronics.

4.2 Antenna Packaging

Fig. 5 shows a solution to integrate the proposed

antenna structure. It consists of three stacked wafers,

where the bottom wafer is used to place the reading

and controlling electronics, the middle wafer is used

to implement the U-shaped cantilever, and the

bottom wafer encapsulates the device, enabling a

very small microsystem with integrated antenna.

Figure 5: Use of WLCSP to integrate the proposed MEMS

antenna.

4.3 Antenna Modelling

The proposed MEMS structures were engineered to

have the desired electrical and geometrical

properties, as well the requirements to be used in a

post-process module compatible with integrated

circuit (IC) fabrication.

MEMS ANTENNA FOR WIRELESS BIOMEDICAL MICROSYSTEMS - Extremely Small Antenna for RF Receivers in

Implantable Devices

Fig. 6 shows the 3D FEM (finite element

modelling) model being used to analyse the

receiving properties for a cantilever operating as an

antenna. FEM modelling is a very powerful

technique to predict the interaction between different

domains (electrical, mechanical, and

electromagnetic).

Figure 6: Model of structure used to sense the

electromagnetic field.

However, it is also very time consuming and, in this case,

very complex to set up a simulation and get data we can

rely on it. In this way, a simple model was derived to

understand the requirements and potentials of a structure

like this one, if operated as an antenna.

4.4 Modulation Modelling

Considering the structure proposed in Fig. 4, we can

represent it using the simplified model of Fig. 7. The

electrical behaviour is modelled by the beam of

length L and the current I, whereas the mechanical

behaviour can be described using the spring k.

Considering that the magnetic field, B, is applied

perpendicularly to the current I, trough the length L,

the resulting Lorentz force, F

, will be given by:

LIBF

)(

(1)

The MEMS structure will move when the

Lorentz force becomes higher then the elastic force

(Rocha et al., 2004).

Figure 7: Simplified model of a cantilever operating as an

antenna.

Due to the applied force F

, the structure will move,

and the following equation can be written:

FkuLIBF =

)(

(2)

where F

stands for elastic force, u for displacement

and k is the spring constant of the structure. From

the previous equation, we can find the displacement:

LIB

)(

(3)

Independent of the detection method, optical or

capacitive, the displacement will give origin to a

received voltage, V

, which will be proportional to

the applied magnetic field, B.

BuV

∝

(4)

If the magnetic field was originated by a

modulated signal:

))()(2cos()()( ttftAtB

(5)

the received voltage will be:

))()(2cos()()( ttftA

GtV

φπ

(6)

where G represent the system gain.

4.5 Antenna Analysis

From equation 6 it is possible to conclude that the

proposed device will actuate as a signal receiver if

the spectral properties of the modulated signal are

chosen to fall inside the device operating frequency

limit.

The first parameter requiring analysis is the force

such a structure can produce when in presence of a

magnetic field. From equation 2, if we consider

I = 100 mA, L = 1,1 mm and B = 10 mT, then the

resulting force will be 1.1 μN. This is enough force

to produce movement in this kind of structure.

A key advantage of this solution is that it can

deliver gain trough the increase of the current I. If

the resistivity of the beam material is kept low, then

we can have a low power device, since the voltage

drop will be small.

The other requirement is that the modulation

type must be selected carefully. Even if the

modulating signal bandwidth is inside the MEMS

device bandwidth, if we chose, e.g., FSK, the

BIODEVICES 2008 - International Conference on Biomedical Electronics and Devices

resulting bandwidth may be outside the structure

bandwidth due to the instantaneous required

frequency shifts. In this way, a continuous

modulating method must be used instead.

To check the ability to operate as a radiating

element, some preliminary tests were also

conducted, where it was used a scaled model of the

proposed structure. A commercially available

magnetic sensor was connected to a signal

acquisition board that was connected to a personal

computer. A current was injected into a scaled

structure of Fig. 4, and the signal was recorded with

the magnetic sensor. When the transmitting structure

was oscillating at 100 KHz, it was possible to easily

detect that signal with the magnetic sensor.

5 CONCLUSIONS

This paper described the design, and modelling of

chip-size MEMS antennas for short-range wireless

microsystems. These antennas allow the fabrication

of an implantable microsystem with integrated

wireless communications. The antenna integration is

based on wafer-level packaging techniques, which

enables the integration of new materials with the

standard silicon processing steps, as well the

fabrication of complex three-dimensional structures,

in an economically acceptable way.

MEMS were explored as a new solution to

obtain structures that can sense electromagnetic

fields. Thus, instead of having the need to design

very advanced antenna structures to achieve antenna

size reduction, the standard MEMS devices, e.g.

cantilevers, will be used to save system space and

improve system integration. A novel electrically

very small antenna using MEMS structures, and a

model to describe the operation of that structure; is

presented.

The present solution envisions power saving,

smaller volume, lower cost, and increased system

lifetime, which are very important features in

biomedical microsystems for diagnosis and therapy.

ACKNOWLEDGEMENTS

We would like to acknowledge Medical Doctor

Paulo Vale for his support in this research work.

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MEMS ANTENNA FOR WIRELESS BIOMEDICAL MICROSYSTEMS - Extremely Small Antenna for RF Receivers in

Implantable Devices