MEMSSensing in Textiles

Ashish Kapoor2013TTE2756

Micro-Electro-Mechanical Systems is a technology that in its most general form can be defined as miniaturized mechanical and electro-mechanical elements that are made using the techniques of micro fabrication.MEMS are made up of components between 1 to 100 micrometres in size (i.e. 0.001 to 0.1 mm), and MEMS devices generally range in size from 20 micrometres (20 millionths of a metre) to a millimetre (i.e. 0.02 to 1.0 mm).

Because of the large surface area to volume ratio of MEMS, surface effects such as electrostatics and wetting dominate over volume effects such as inertia or thermal mass.

Micro machines are divided into two functional groups: the sensors and the actuators.A sensor is defined as a device that provides a usable electrical output signal in response to a signal. When a sensor is integrated with signal processing circuits in a single package (usually a polysilicon chip), it is referred to as an integrated sensor or smart sensor.An actuator is a device that converts an electrical signal, which may be taken from a sensor to an action.A transducer is considered as a device that transforms one form of signal or energy into another form. Therefore, the term transducer can be used to include both sensors and actuators.Smart SensorsSmart sensors have all the electronic integrated in a MEMS structure. A photo of a silicon wafer with one hundred microstructures.

Principles Used in SensorsPhysical principles or effects grouped according to the six forms of physical energy.

Advantages of MEMS devices•The function is replicated numerous times giving a higher accuracy to the measurement.•Due to the replications, failure of some sensors would not affect the system performance. Such system is usually referred to as an array of sensors.• A small device interferes less with the environment that it is trying to measure when it is of a smaller size.• They can be placed in small places where traditional macro devices could not fit.• A higher precision is achieved with actuators. Motions of micrometer range are precisely achievable.

MEMS FABRICATIONThe micromechanical components are fabricated using compatible "micromachining" processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices. Typical MEMS are miniature sensors and actuators.

Approaches to integrate the MEMS on textiles .

The first approach is trying to develop yarn-like electronics and transducers using existing and new flexible materials in order to stitch up the sensors on the textile directly, which may result in limited sensing capabilities and computation capabilities.

The other approach is trying to design and fabricate the silicon-based flexible sensors with MEMS technology and then sew the flexible sensors array on the textile.

A MEMS device in general is rigid so that it cannot be bent. However, if MEMS devices of the rigidity of these small size are fabricated on a flexible substrate in the form of isolated islands, one flexible silicon sensor skin is then obtained The flexible substrate is patterned with metal wires that are to be used as interconnects between the MEMS and processing circuits. Then the intelligent textiles are obtained by sewing up this flexible silicon sensor skin on the fabric.

In order to achieve flexible skin, there are two approaches to fabricate MEMS devices on the flexible substrate: (1) First we fabricate MEMS devices using "surface additive processes" after depositing a layer of polymer on a four-inch wafer, and make MEMS devices isolate each other and obtain flexible silicon sensor skin promptly to strip off the coating polymer on the wafer.

(2) Second, here first we deposit a layer of polymer on front side of the wafer after fabricating MEMS devices using "bulk subtractive processes", then corrode the reverse side of the wafer to make MEMS devices form the detached islands in isolation each other, later deposit a layer of polymer and obtain the base skin of flexible silicon promptly on the wafer reverse side again.

The bulk subtractive processes is more practical and cost effective.

The principle of the thermo resistive transducer is that the resistance changes according to material heat change and the resistance (R) of the material can be calculated according to the following formula:

where ᵨ is resistance coefficient of the material, L is the length of the material, and A is the

area of the material. For being compatible with MEMS, we have chosen the p-Doped silicon as the resistance material, and the resistance coefficient of the p-Doped silicon can be calculated according to the following formula:

where p is the carrier concentration, q is charge on electron, and µp is the hole mobility.

MEMS Fabrication on FabricsFabrics present a very different substrate compared with a silicon wafer– Rough, uneven surface with pilosity (hairiness).– Flexible and elastic– Suitable for low temperature processing– Limited compatibility with solvents and chemicals

To use standard printing techniques to deposit a range of custom inks in order to realise freestanding mechanical structures coupled with active films for sensing and actuating.

SCREEN PRINTINGAlso known as thick-film printing, this is normally used in the fabrication of hybridised circuits and in the manufacture of semiconductor packages.

Inkjet PrintingNon contact direct printing onto substrate, used for fabrics and electronicsapplications.

Printed MEMS Process

Sacrificial layer requirements: Printable Solid Compatible Easily removable without damaging fabric or other layers.

Structural layer requirements Suitable mechanical/functional properties.

Piezoresistive layer

Interface layer

Sacrificial layer

Structural layer

Fabric

Electrode

Case Study: Strain GaugeExploits the piezoresistive effect: the resistance of a printed film changes as it is strained (stretched) due to a change in the resistivity of the material. Useful for sensing movement, forces and strains.Printed SensorSilver electrodes printed using a low temperature polymer silver paste.Piezoresistive paste is based on graphite. Cured at 120-1250C

Ink types required

Printed Heater•Simple heater is a current carrying conductive element.•Existing heaters integrated in textiles by weaving or knitting.•Woven approach limited by direction of warp and weft.•Knitted solution requires specialist equipment .

Heated car seat element(BMW)

Interface layerOvercomes surface roughness and pilosity of fabric substrate providing a continuous planar surface for subsequent printed layers.

Cross-section SEM micrograph of 4screen printed interface layers on

polyester cotton fabric

Screen DesignHeater has three layers: Interface, Conductor and Encapsulation.

• Interface layer improves heater performance but increases fabric coverage to ~40% - still below limit of 50%.

Finished Print

Piezoelectric FilmsPiezoelectric materials expand when subject to an electrical field, similarly they produce an electrical charge when strained.

Ideal material for sensing and actuating applications.

Piezoelectric Structure Piezoelectric material sandwiched between electrodes.

Polarising voltage required after printing to make the piezoelectric active.

Cured at temperatures below 150 oC.

Textile-based (MEMS) Accelerometer for Pelvic Tilt Mesurement

An accelerometer is a device that measures proper acceleration (in relativity theory, proper acceleration is the physical acceleration experienced by an object. It is thus acceleration relative to a free fall, or inertial, observer who is momentarily at rest relative to the object being measured. Gravitation therefore does not cause proper acceleration, since gravity acts upon the inertial observer that any proper acceleration must depart from (accelerate from). A corollary is that all inertial observers always have a proper acceleration of zero. The proper acceleration measured by an accelerometer is not necessarily the coordinate acceleration (rate of change of velocity). Instead, the accelerometer sees the acceleration associated with the phenomenon of weight experienced by any test mass at rest in the frame of the accelerometer device.

Micro Electro Mechanical System (MEMS) accelerometer is an electro-mechanical device that measure acceleration force exerted on it. The development of textiles-based MEMS for pelvic tilt measurement is an effort to reduce the cost in medical sensor devices.

The piezoresistive effect describes change in the electrical resistivity of a semiconductor or metal when mechanical strain is applied. In contrast to the piezoelectric effect, the piezoresistive effect only causes a change in electrical resistance, not in electric potential.

Sensor DesignThe accelerometer sensor is designed as a cantilever beam structure with suspended mass at one end.

(a)Schematic drawing of accelerometer design. (b) Close-up drawing on conductive section of accelerometer. (c) Actual photo of textile cantilever accelerometer.

Advantages

1. Textile-based accelerometer provides an alternative to the costly and hazardous radiographic measurement of pelvic tilt.

2. The flexibility of textile structure makes it more advantageous to conform to body contour than rigid digital inclinometer and more accurate than indirect trigonometric

3. measurement4. Textile material is relatively low-cost, flexible, lightweight, readily available,

environmental friendly and easy to use.

Silicon flexible skins

Other research areas and future scope

Monitoring warp end tension and breaks during fabric formation. A custom designed micro machine sensor has been designed is being fabricated. It will replace the off shelves sensors currently used to measure warp tension.

Manipulating and aligning micro fibres in up to 6 axes is the first step towards a micro weaving machine. Future work could absolutely be the fabrication of this micro weaving machine.

References1. Rakesh B. Katragadda, Yong Xu, A novel intelligent textile technology basedon silicon flexible skins, ECE Department, Wayne State University, Detroit, MI 48202, USA.2. S Beeby, M J Tudor, R Torah, K Yang, Y Wei, MICROFLEX Project: MEMS onNew Emerging Smart Textiles/Flexibles, Electronics and Computer Science, University of Southampton.3. Maozhou Meng, Yong Xu, Honghai Zhang, and Sheng Liu, Intelligent Textiles Based on MEMS Technology, Division ofMOEMS, Wuhan National Laboratory for Optoelectronics and Institute of Microsystems, Huazhong University of Science and Technology1037 Luo Yu Road, Wuhan, Hubei 430074, China and Electrical and Computer Engineering, Wayne State University, Detroit, Michigan, USA .4. Nik Nur Zuliyana Mohd. Rajdia, Azam Ahmad Bakira, Syaidah Md. Saleha, and Dedy H.B.Wicaksonoa, Textile-based Micro Electro Mechanical System (MEMS) Accelerometer for Pelvic Tilt Mesurement, International Symposium on Robotics and Intelligent Sensors 2012 (IRIS 2012).5. S Beeby, R Torah, K Yang, Y Wei, J Tudor, MICROFLEX Project - Microtechnology in Smart Fabrics, Electronics and Computer Science, University of Southampton.

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