What is microfluidics
the science and technology of systems that deals with the behaviour, precise control and manipulation of microliter and nanoliter volumes of fluids using channels with dimension of tens to hundreds of micrometers
Advantages of microfluidics?
SINF
1. sample savings - nL instead of mL
2. Integration - combine lots of steps into a single device
3. faster analysis - shorter reaction time in small volumes
4. novel physics - diffusion, surface tension and surface effects dominate, leading to faster reactions
Why miniaturization?
- Small components: reduced weight and size, reduced energy consumption
- Small sample amts: reduction consumption of material, reduced waste production, accurate dosing
- Batch fabrication: reduced price (disposable)
- Potential for higher yield and fewer defects: integration of sensors, parallel process
- Device performance: scaling law for new effects, increased heat exchange, fast mass transport
Bio-MEMS
Biomedical Micro Electro Mechanical Systems
Micro: small size, microfabricated structures
Electro: electrical signal/control
Mechanical: mechanical functionality
Systems: Structures, Devices, Systems, Control
What is a lab-on-a-chip/lab-on-a-disc?
Miniaturized device that integrates into a single chip one or several analyses, which are usually done in a laboratory;
analyses such as:
DNA sequence
Biochemical detection.
relies on microfluidics and molecular biology
What do we learn from scaling theory?
- length decreases by X, surface by X^2 and volume by X^3.
- the behaviour of fluids at the microscale is different from macroscale in factors such as surface tension
surface tension force decreases
capillary effect is caused by surface tension.
look at equations in slide 11
What is scaling theory?
a valuable guide to what may work and what will not work. by understanding how phenomena behave and change as their scale size changes, we can gain some insight and better understand the profitable approaches
What are scaling laws?
- relates to geometry and its implications on physical forces
- applies to phenomenological behaviour
- mathematically speaking phenomena that have large power dependencies on length will have a reduced effect at smaller length scales
- phenomena with lower power dependencies play an important role in the design of micro/nano systems
what is reynolds number
a dimensionless number that is the ratio of inertial forces to viscous forces and consequently it quantifies the relative importance of these two types of forces for given flow conditions.
re = puL/v
p = density, u = viscosity, v = velocity, L = characteristic length
What reynolds number with give what flow regimes?
Re > 2,300 is turbulent (inertial forces dominate)
Re < 2,000 is laminar (viscous forces dominate)
3,000 > Re > 2,000 is transitional
what is hydraulic diameter?
Dhydro = 4A/P, where A is the cross sectional area of the channel and P is the wetted perimeter
it is used instead of characteristic length, so:
Re = puD/v
Expression for diffusion
tao = L^2/aD proportional to l^2
- mixing occurs through diffusion at the microscale
- chemical reactions happen faster at the microscale
What is poiseuille flow
pressure driven flow in which a pressure difference exists between the ends of a microchannel
FLuidic resistance
- ratio of applied pressure (for driving the flow) and the volume flow rate (in analogy with electrical resistance: R = V/I)
- for a microchannel segment of length L: R = deltaP/Q
check slide 27 for actual formulas for resistance depending on cross section shape
Hydraulic resistance (hagen=poiseuille law)
R hydr = driving force/ flow rate = delta P / Q
Q = h^3w delta P/12uL so R hyd = 12uL/h^3w in the case of flow in a section of an infinite parallel plate system
this will also change depending on cross section
CAD and simulation of microfluidics
computer aided design (CAD) softwares are employed to create the microfluidic configuration, including channels and chambers etc
computational fluid dynamics (CFD) software can solve complex transport phenomena equations to test, iterate, validate and optimate the designs
difference between flow in macrochannels vs microchannels
look at slide 36 - 38
what happens at small scales?
- at small scales (10-1000mm) some interesting and unintuitive properties appear
- inertia means nothing
- viscosity is important
- flow is laminar
- surface is important
- fluidic resistance dominates
What are some unique characteristics of microfluidics?
- DEterministic feature due to laminar flows at microscale:
—> flow patterns and concentrations are mathematically predictable –> experimental and theoretical results agree well - micrometer size of channels: fits well with biological cell size –> suitable for probing cell behaviour, handling single cells, imitating physiological parameters
- easy integration for automatic fluid control (quake valve) –> avoid human errors and operator costs
- low cost: batch fabrication, high density integration, high throughput, multiplexed assays
- low consumption of reagents, smaller footprint –> potentially portable
What are the two types of synthesis/fabrication methods
- bottom-up fabrication methods (wet-chemistry, physical/chemical deposition, electrodeposition)
- top down (lithography)
top-down lithography?
- materials (glass, fused silica, silicon)
- resist (e-beam, photo)
- lithography (e-beam/photo)
- etch (wet/dry)
what is needed for photolithography?
- a photomask for selective exposure of resist
- a photoresist that is an optically sensitive polymer
–>with a positive PR, light will remove the exposed areas
–> with a negative PR, light will do the opposite
What is the difference between wet and dry etching?
wet:
- isotropic
3 spatial directions
- spherical cavities
- HF for glass and fused silica
- large depth
dry:
- anisotropic
- depends on crystallographic orientation
- flat surfaces, such as wells or channels
- reactive ion etching, CHF3 for silica
- small depth
What are the properties of fused silica?
thermal: diffusion bonding at temp over 1000C is possible
chemical resistance to acids (not HF0 and solvents
optical: high optical clarity which makes it suitable for single molecule resolution microscopy
