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BME 432
Lab on a Chip
Spring 2020
HW #3
Due Thursday, March 5
Name:
Answer the following questions on engineering paper. [40 pts]
1. SPROMs
[20 pts]
You are designing a microfluidic system using SPROMs. The general pattern for your design is shown
below.
100 μm
500 μm
A
R1
Vent
x
500 μm
B
R2
Given:
Surface tension of water-based solution, γ = 0.072 N/m
Contact angle, θ = 120°
Channel height is uniform throughout the design, h = 200 μm
a.) Calculate the pressure drop across R1 in units of Pa.
b.) Pick the width, x, of R2 to ensure the fluid fills Chamber A before it fills chamber B. Show your
work. (I recommend a factor of 1.5 difference in the pressure to ensure this will happen).
2. Pumps
[20 pts]
A thermopneumatic check-valve pump delivers a maximum flow rate of 34 μL/min and has a maximum
back pressure of 5 kPa. The heater that powers the pump has a resistance of 15 Ω, which is driven by a
symmetric square signal with a maximum voltage of 6V and a 50% duty cycle (i.e., it is on for half of the
time and off the other half). What is the pump efficiency?
Microfluidics
Components
(Pumps, mixers)
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Pumps
 Functions
 Transport of sample/reagents
 Delivery of pulsatile flow
 Generating pressure differences
 Moving cooling fluids
 Two major categories
 Mechanical pumps
 Non-mechanical pumps
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Example
 Centrifuge + microfluidic disk
Slide courtesy of J. Papu
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The centrifuge has failed!
Design a pump for a microfluidic system
that will function if the centrifuge fails
Must be suitable for low-resource
environment
Be creative!!
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Pumps
 Mechanical Pumps
 Integrated
• Pump integrated within microfluidic system
 External
• Pump located outside of microfluidic system
Integrated
Check-valve
Peristaltic
Valve-less rectification
Rotary
Ultrasonic
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External
Centrifugal
Syringe
Capillary
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Pumps
 Non-mechanical pumps
 Add momentum to the fluid by converting another nonmechanical energy form into kinetic energy
– Electro-osmosis
• Magnetic Potential
– Megnetohydrodynamic flow
• Surface tension driven
– Electrochemical
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Pumps
 Maximum flow rate
 Qmax
 Volume of liquid per unit time delivered by the pump at zero
back pressure
 Maximum back pressure
 Pmax
 Max pressure the pump can work against
 Power of pump
Ppump 
Pmax Qmax
2
 Efficiency

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Ppump
Pactuator
x100%
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Mechanical Pumps
 Check-valve pump
 Membrane movement drives pump
Membrane
Actuator
Inlet valve
Stroke volume
Outlet valve
 Requires check-valves to control flow direction
 Membrane deflection determines stroke volume
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Mechanical Pumps
 Peristaltic Pump
 Synchronized movement of multiple membranes
Fig. 7.15 from N.-T. Nguyen, S. T. Wereley, Fundamentals and
Applications of Microfluidics, Artechouse, 2002, p. 304.
 Requires several controllable actuators
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Mechanical Pumps
 Valve-less rectification
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Fig. 4.6 from O. Geschke, H. Klank, and P.Telleman, Microsystem
Engineering of Lab-on-a-Chip Devices, Wiley-VCH, 2008, p. 45
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Mechanical Pumps
 Rotary Pump
From G.T.A. Kovacs, Micromachined Transducers Sourcebook, p. 844.
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Mechanical Pumps
 Ultrasonic pump
 Uses mechanical traveling wave to create acoustic
streaming
 No moving parts, heat, or electric field required
 Wave induced by interdigitated piezoelectric transducers
• Requires integrated electrodes
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Mechanical Pumps
 Centrifugal pump
 Uses external actuator to drive centrifugal force
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Mechanical Pumps
 Syringe Pump
 e.g. – New Era NE-1000
http://syringepump.com/NE-1000.htm
 Features
• Infusion and/or withdrawal
• Max pressure ~200 kPa
• Flow rate: 1.7 nL/min – 35 µL/min
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Design an alternative to syringe
pump
Interfaces with syringe
Eliminates need for electricity
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Mechanical Pumps
 Capillary pump
 e.g. – Springfusor (Go Medical Industries)
http://www.gomedical.com.au/products/springfusor.php
 Features
• Spring-driven
• Re-usable, low-cost, compact, no power
• Flow rate: 2 µL/min – 2 mL/min
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Non-Mechanical Pumps
 Electro-osmosis
 Uses electric field to provide pumping
1
100 mm
3
4
2
 Simple integration (just requires electrical interconnects)
 Uniform flow velocity (plug-like)
 Requires conductive fluid, dielectric materials (E-field)
 Flow dependent on chemistry
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Non-Mechanical Pumps
 Magnetohydrodynamic pump
 Uses applied magnetic field
_
+
B
F I
 Requires conductive solution to generate current
 Requires electrodes on channel walls and interconnects
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Non-Mechanical Pumps
 Electrochemical pump
 Uses pressure of gas bubbles generated by electrolysis of
water
C. Lui, Lab Chip, 2010, 10, 74-79
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Pump Design Considerations
 Integrated vs External
 Does our pump need to be on-chip?
 Is portability an important design consideration?
 Will our pump selection affect materials we can use for
fabrication?
 Will it affect the complexity of the system?
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Pump Design Considerations
 Mechanical vs. Non-mechanical
 Mechanical pumps have traditionally been used in macroscale fluidic systems
 Non-mechanical pumps show advantages in microscale
• High viscous forces and small dimensions lead to high fluidic
resistance
– Thus very high pressures needed, which consumes power
1 µL/min
10 µL/min
100 µL/min 1 mL/min
New pumps
Electrokinetic
Magnetohydrodynamic
Ultrasonic
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10 mL/min
Mechanical pumps
Check-valve
Peristaltic
Valve-less rectification
Microfluidics
Components
(Channels, valves)
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Microfluidic Components
 Major components of microfluidic systems
 Microchannels
 Valves
 Pumps
 Mixers
J.S. Shim, et al., Proc. SPIE, vol. 6465. From G.T.A. Kovacs, Micromachined Transducers Sourcebook, p. 824, 844.
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T.M. Squires, Rev. Mod. Phys., vol. 77, July
2005., 977-1026.
Microchannels
 Uses
 Connection between components
 Reactant delivery
 Reaction zone
 Separation
 Types
 Bulk
 Near-surface
 Surface
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Microchannels
 Bulk Microchannels
 Channel is formed directly in substrate material
 Bonding
• Achieved with flat coverslip
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Microchannels
 Bulk Microchannels

Simplicity
Possibility of coating channel walls
Robust channels
Large channel cross-section
• Microvoids possible
• Bonding required
• Poor access
 By far the most common type of microchannel
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Microchannels
 Near-Surface Microchannels
 Similar to bulk channels, but typically shallower
 Sealing achieved by deposition of thin film
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Microchannels
 Near-Surface Microchannels

Close to substrate surface
Clear walls
Some control over channel depth
No bonding
• Poor control over channel dimensions and shape
• Fragile
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Microchannels
 Surface Microchannels
 Channels built on top of substrate using sacrificial layer
 Sacrificial layer removed to form channel
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Microchannels
 Surface Microchannels
• Precise control of channel dimensions
• Simple access
• Fragile walls
• Small cross-sectional areas
• Long clearance times for removal of sacrificial layer
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Microfluidic Valves
 Valves are critical components of microfluidic
systems
 Function
 Used to shut off or otherwise modify the flow of a fluid that
passes through it
 Types
 Passive valves
• No energy needed for operation
• Can use energy from flow
 Active valves
• Require external energy to function
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What are the desired
characteristics of valves?
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Microfluidic Valves
 Desired characteristics
 No leakage
 Minimal power consumption
 Large closing force (pressure range)
 Maximum capacity
 Zero response time
 Linear operation
 Insensitivity to particulate contamination
 Compatible with wide range of fluids
 Other important characteristics to consider
 Normally open or closed
 Proportional or digital
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Passive Valves
 Check Valve
 Allows flow in forward direction
From G.T.A. Kovacs, Micromachined Transducers Sourcebook, p. 823.
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fast response time, simple response
leaky
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Passive Valves
 Biological example
http://www.sjm.com/assets/popups/valvereplace.gif
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Passive Valves
 Biomimetic hydrogel
 Hydrogels
• materials that can change some physical parameter (typically
volume) based on some physical or chemical stimulus
• pH sensitive hydrogel swells or stays in shrunken state
– Swells = blocks channel
– Shrunken = flow continuous
Fig. 4.2 from O. Geschke, H. Klank, and P.Telleman, Microsystem
Engineering of Lab-on-a-Chip Devices, Wiley-VCH, 2008, p. 41.
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responds to sample pH
difficult to integrate
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Passive Valves
 Geometric restriction
 Use geometry of channels to control flow
 Requirements
• Hydrophobic surfaces
• Abrupt changes in channel geometries
w2
w1
 Operation

Fluid arrives at interface between large and small channels
Significant pressure is required to overcome surface tension
Fluid stops until higher pressure is achieved
Flow continues
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Passive Valves
 SPROMs
 Structurally Programmable Microfluidic Systems
 Series of microfluidic channels with passive valves located at
strategic locations
• Fill channels in specific sequence
C.H. Ahn, et al., Proc. IEEE, vol. 92, no. 1, 2004, pp. 154-173.
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Passive Valves
 SPROMs
 Structurally Programmable Microfluidic Systems
 Series of microfluidic channels with passive valves located at
strategic locations
• Fill channels in specific sequence
(a)
C.H. Ahn, et al., Proc. IEEE, vol. 92, no. 1, 2004, pp. 154-173.
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Passive Valves
 SPROMs
 Structurally Programmable Microfluidic Systems
 Series of microfluidic channels with passive valves located at
strategic locations
• Fill channels in specific sequence
(b)
C.H. Ahn, et al., Proc. IEEE, vol. 92, no. 1, 2004, pp. 154-173.
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Passive Valves
 SPROMs
 Structurally Programmable Microfluidic Systems
 Series of microfluidic channels with passive valves located at
strategic locations
• Fill channels in specific sequence
(c)
C.H. Ahn, et al., Proc. IEEE, vol. 92, no. 1, 2004, pp. 154-173.
Western New England University
BME 432
Passive Valves
 SPROMs
 Structurally Programmable Microfluidic Systems
 Series of microfluidic channels with passive valves located at
strategic locations
• Fill channels in specific sequence
(d)
C.H. Ahn, et al., Proc. IEEE, vol. 92, no. 1, 2004, pp. 154-173.
Western New England University
BME 432
Passive Valves
 SPROMs
 Structurally Programmable Microfluidic Systems
 Series of microfluidic channels with passive valves located at
strategic locations
• Fill channels in specific sequence
(e)
C.H. Ahn, et al., Proc. IEEE, vol. 92, no. 1, 2004, pp. 154-173.
Western New England University
BME 432
Passive Valves
 SPROMs
 Structurally Programmable Microfluidic Systems
 Series of microfluidic channels with passive valves located at
strategic locations
• Fill channels in specific sequence
(f)
C.H. Ahn, et al., Proc. IEEE, vol. 92, no. 1, 2004, pp. 154-173.
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Active Valves
 Operation
 Working state is determined by closure of element (valve seat)
 Actuator provides mechanical action to control position of valve
seat
 Types
 Digital operation
• Normally open
– Requires energy to close and remain closed
• Normally closed
– Requires energy to open and remain open
• Bistable
– Can be open or closed
– Requires energy for the transition between states
 Analog operation
• Proportional operation
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Active Valves
 Actuation principles
 Pneumatic
 Thermopneumatic
 Thermomechanic
 Piezoelectric
 Electrostatic
 Electromagnetic
 Electrochemical
 Evaluation criteria
 Operating force (pressure range)
 Response time
 Power requirement
 Integration
 Stroke displacement
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Active Valves
 Pneumatic
 Uses compressed gas
T.M. Squires, Rev. Mod. Phys., vol. 77, July 2005., 977-1026.
 Simplest actuation concept
 Requires external pressure source and interconnects
• Not suitable for compact designs
 Large pressure range
 Slow response time
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Active Valves
 Pneumatic
T.M. Squires, Rev. Mod. Phys., vol. 77, July 2005., 977-1026.
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Active Valves
 Pneumatic
Silicone
membrane
Fluid inlet
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Fluid outlet
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Active Valves
 Pneumatic
Applied Pressure
Silicone
membrane
Fluid inlet
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Fluid outlet
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Active Valves
 Thermopneumatic
 Using heated fluids
Fig. 4.3 from O. Geschke, H. Klank, and P.Telleman, Microsystem Engineering of Lab-on-a-Chip Devices,
Wiley-VCH, 2008, p. 42.
 Large pressure range
 Relatively slow response time
 High power consumption
 May heat sample
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Active Valves
 Thermomechanic
 Convert thermal energy directly into mechanical stress
• e.g. bimetallic valves
heat
heat
 High pressure range
 Relatively slow response
 High power consumption
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Active Valves
 Thermomechanic
 Convert thermal energy directly into mechanical stress
• e.g. shape-memory alloys
Fig. 6.17c from N.-T. Nguyen, S. T. Wereley, Fundamentals and
Applications of Microfluidics, Artechouse, 2002, p. 270.
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Active Valves
 Piezoelectric
 Material expands due to applied voltage
Fig. 4.4 from O. Geschke, H. Klank, and P.Telleman, Microsystem Engineering of Lab-on-a-Chip Devices,
Wiley-VCH, 2008, p. 42.
 High pressure range
 Low displacements
 Large power consumption
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Active Valves
 Electrostatic
 Uses electrical attraction/repulsion
Fig. 6.21a from N.-T. Nguyen, S. T. Wereley, Fundamentals and
Applications of Microfluidics, Artechouse, 2002, p. 280.
 Non-linear forces
 Large movements
 Very fast response time
 Low power (but high voltages)
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Active Valves
 Electromagnetic
 Uses applied magnetic field
Fig. 6.22c from N.-T. Nguyen, S. T. Wereley, Fundamentals and
Applications of Microfluidics, Artechouse, 2002, p. 282.
 Large movements
 Low power
 Fast response time
 Have to integrate magnetic materials
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Active Valves
 Electrochemical
 Uses gas bubbles generated by the electrolysis of water
Fig. 6.23a from N.-T. Nguyen, S. T. Wereley, Fundamentals and
Applications of Microfluidics, Artechouse, 2002, p. 285.
 Low pressure
 Slow response time
 High power consumption
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Passive vs. Active Valves
 Passive
• No power
• Simple design
• Relatively simple integration
• Cannot modify operation
• Leaky
 Active
• Can modify operation
• Power required
• Complex design/integration
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Valve Design Considerations
 There is no optimum valve for all applications
 Important questions
 What kind of fluids will come into contact with the valves?
 How often do we need the valves to operate?
• Many times per minute
• A few times total
 Can we afford to exhaust energy on actuating the valves?
 Will an off-chip method be better?
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