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31 Cards in this Set
- Front
- Back
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Define MEMS? |
Micro electro-mechanical system is a functional unit that contains electrical and mechanical components for sensing or actuation, characteristically on the micron scale. |
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What are the 2 categories of MEMS devices? |
- Sensors - Actuators |
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Give an overview of what a sensor does? |
Measures a physical quantity or event from its surrounding and transducers into a signal that is interpretable by humans. |
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Give an overview of what an actuator does? |
Takes energy in an electrical domain and converts it into a mechanical domain to produce a motion It acts through highly controlled movements to execute a given command. |
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What does the transducer in the sensor do? |
Made up of a transducer that transforms signals in various domains eg. radioactive, magnetic, thermal, chemical or mechanical, into a signal that can be discerned by humans such as the electrical domain to interface with computers and recorders. |
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How are MEMS made? |
Basic microfabrication techniques 1) Patterning - photolithography, e-beam lithography 2) Deposition - thin layers of material are deposited onto a base 3) Etching - and then selectively etched away leaving a microscopic 3-dimensional structure. |
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What does scalable manufacturing mean? |
If you produce it on a research scale it is going to be expensive But if you scale up the fabrication and produce more of those devices it is cheaper, and with small devices lots can be produced in parallel (as apposed to a car) |
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What is stiction, what is its relevance in BioMEMS? |
Friction which tends to prevent stationary surfaces from being set in motion Protein molecules aggregate if 2 of the moving parts within the MEMS are too close togather and they touch. It is not possible to then move them again. |
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What is piezoelectric material? |
Ceramic and crystalline materials that display a linear electromechanical response to stress or an electric field. |
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What are the main principles by which piezoelectric material works? |
Linear electromechanical response If the material is deformed, the way the charge is distributed within that material will change, and it will give a voltage output. Or if there is an input voltage to that material, it will be deformed. |
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How does piezoelectric material function in a MEMS? |
If the piezoelectric material is attached to a lever, and input voltage will deform the material and push the ratchet, so a motion can be evoked from an electrical input. |
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What are the 2 main uses of piezoelectric material in MEMS? |
1) activate a mechanical movement 2) transduce a mechanical response to a discernible electrical response |
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What is a microcantilever? |
Long projecting beam fixed or anchored at one end and free at the other end. |
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What is the difference between piezoelectric and piezoresistive? |
Piezoresistive is a semiconductor, rather than a ceramic material Changes in the piezoelectric material produce a voltage, changes in the piezoresistive material produce a change in resistance, they do not convert any form of stimulus to a voltage or current. Piezoelectric materials can work both ways; converting mechanical energy to electrical energy. They also can operate in reverse, converting electrical energy into mechanical energy such as sound or vibration. Piezoresistive material convert mechanical energy into proportionate levels of resistance, they are not reverse active, they cannot convert resistance levels to some other form of energy. Piezoresistive transducers tend to be larger due to their use of semiconductor materials |
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What is the piezoelectric effect? |
the ability of certain materials such as ceramic or crystal to generate an electric charge (voltage) in response to applied mechanical stress eg. pressure or vibration |
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What material is used for a piezoelectric component? |
Lead zirconate titanate (PZT) |
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What material is used for a piezoresistive component? |
Silicon |
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Describe the electrostatic effect? |
Two plates in a MEMS with a potential different formed across these "electrodes" forms a positive and negative charge, so the plates sense each other. This makes an electrostatic actuator |
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How is does an accelerometer function by design? |
- It is two electrodes very close together creating an electrostatic effect on a proof mass beam (flexible beam). - The acceleration in the direction the accelerometer is turned causes the two electrodes to become closer together. When they come closer together the capacitance changes When an object on which the accelerometer is mounted moves or vibrates, the proof mass detects and copies the movement |
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Define capaticance? |
the ability of a system to store an electric charge |
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Put simply what does the piezoelectric component do? |
convert mechanical energy to electrical energy and vice versa |
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Put simply what does the piezoresistive component do? |
convert mechanical energy to resistance values and that’s it |
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What applications use MEMS accelerometers? |
- Iphones - Gaming devices - Airbag deployment in cars - Computer stabilisation - Camera stabilisation - GPS devices - Pacemakers |
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What type of MEMS senses pressure? |
Piezoresistive material |
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What applications use MEMS pressure sensors? |
- Gauges eg. to tell is oil is too low - BP sensors in head of a hypodermic needle - Intracranial pressure - Cerebrospinal fluid pressure - Intraocular pressure - Endoscopes for organ pressure |
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What applications use MEMS as actuators? |
- Inkjet printer Piezoelectric element has a voltage applied to it, it deforms and lets a fine jet of ink out. - DNA chips - Protein chips - Cells chips |
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Describe how the atomic force microscope works? |
- A cantilever is deflected at a set point away from a surface - an optical lever is shined at the cantilever tip with a laser beam - this is sent to a photodiode - as the photons hit the photodiode it is converted to an electrical signal to tell us the exact position of the cantilever sharp tip. - as the tip is moved across the surface the PZT scanner moves up and down allowing us to give the atomic resolution of the surface |
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What are the modes of operation of an atomic force microscope? |
- Contact mode - Non-contact mode - Tapping mode |
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What are microneedles ? |
- They have the ability to deliver drugs and sample fluids painlessly because they are short enough to avoid nerve endings located in the dermis. - so can be used for the totally painless transdermal drug or vaccine delivery |
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Why use microneedles for transdermal delivery? |
- Skin creams have chemicals that reduce the skin barrier to get into the body, but they are therefore limited to relatively small water-soluble molecules
- Drugs with larger molecular sizes of hydrophobic drugs would not get through with a cream - A lot of people will not comply with a hypodermic needle - pain - Shallow penetration reduces chance of infection - Can deliver to where drugs are most effective allowing for a lower dose (not metabolised) |
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How does the scanning probe microscope work? |
A microcantilever with a sharp tip is fabricated in silicon and can image surfaces at a resolution fractions of a nanometer which is 1000x better than that of a light microscope Essentially the cantilever is rubbed over the surface of an object to give its topography. |
