S series AFA system
The Covaris
Process is based on fundamentally different physical processes to that
has typically been applied into the life science industry. This has enabled
a broad range of beneficial applications to be developed, and more importantly,
in some instances a higher level of performance than currently available
processes. The Covaris Process, termed Adaptive Focused Acoustics (AFA), was derived
from incorporating advances of the highly developed industries of medical
diagnostic ultrasound imaging and therapeutic lithotripsy applications,
such as the acoustic disruption of kidney stones.
The process works by sending acoustic waves from a dish-shaped
transducer that converges and focuses to a small-localised area At
this focal point there is a large amount of intense energy controllably
focused into the sample of interest.
For example, when driven at low intensity,
this can create a controlled mixing environment. Whereas, when driven
at high intensity, this can create a shock wave environment ideal for
tissue disruption. A sample introduced into the focal point
of the instrument, is treated with non-contact, controlled mechanical
energy.
Treatment variables
Sonication disclaimer
Low frequency 20khz sonication is
also an acoustic-based process and been used for a number of years
in the life sciences industry, however, it is intrinsically distinct from
adaptive focused acoustics (AFA) for a number of reasons.
One key to the difference lies in the operating wavelength
of each system. Sonication has a wavelength of 10's of centimetres. This
is a fundamental problem as a typical tube used in a laboratory environment
will be anywhere from 10 - 100mm long with the sample occupying typically
less than 20mm in height. It is therefore impossible to control the waveform
into this sample with a 100mm wavelength. This results in scattering and
uncontrolled energy transfer. In other woards, the energy is not correctly
scaled to the sample. In contrast, AFA wavelengths are scaled to the process.
For example, for a 96 well plate the wavelength is approximately 1mm.
This allows AFA to be focused into a localized area of the sample.
The
wavelength difference is in excess of 100 fold.Another benefit of AFA is that the processes may be run
isothermally. This is not the case with sonication which is known to pump
large degrees of heat into samples. Again, this is related to the long
wavelength.
Another benefit of AFA is the convergence of the energy.
A short welength enables the energy to traverse materials, such as sample
tubes, prior to achieving a peak density inside of a sample tube.
How
does AFA Work?
AFA energy is a form of mechanical energy. As acoustic
/ mechanical energy transfers through the sample, the material undergoes
compression and rarefaction (expansion). At high intensity with fluid
samples, this is typically embodied as cavitation events.Cavitation is
the formation and subsequent collapse of bubbles.
In other words, the
acoustic energy applied to a sample causes bubbles to form from the naturally
occurring dissolved gases and vapors of biological and chemical fluids.
When the energy is then removed the bubble collapses. As the bubbles collapse,
a jet of solute (typically water) is created. This jet travels over a
very short distance but at a velocity of >100m/sec. As the number of
bubbles is high and the time interval is short (micro seconds), the mixing
and disruption power capability of the process is significant. A key point
is the precise, reproducible control that is obtainable with the Covaris
instrument systems.
Treatment acoustic setting variables
Acoutic treatment are made up of 3 variables
1-duty cycle which relates to the % on time
2-mV or intensity which is the ampluitude of the wave form
3-cycles per burts which is how many waves at 500khz arer applied durning the on time
By varying all of these it is possible to greatly alter the cavitation bubble size and therfore cavitation bubble collapse power.
Acoustic cavitation is the generation and action of cavities, or bubbles, in a liquid. Acoustic waves move through a liquid and produce variations in the liquid's pressure. When the liquid pressure momentarily drops below the vapor pressure during the low-pressure portion of the acoustic wave, small evacuated areas, or cavities, are formed. These cavities quickly become filled with gas (a foreign contaminant such as dissolved oxygen or air) and/or vapor (a gaseous form of the surrounding liquid). These tiny bubbles are set in motion by the acoustic wave.
The tiny bubbles can expand and contract in the liquid. Bubble expansion is caused by reducing the ambient pressure in the liquid. The bubbles can become large enough to be seen by the unaided eye. The bubbles may contain gas or vapor or a mixture of both. If the bubbles contain gas, their expansion can be caused by rectified diffusion.
Rectified diffusion is the diffusion of dissolved gas from the liquid into the bubble, and vice versa, with the pressure oscillations resulting in a net diffusion into the bubble. This net inward diffusion occurs because the bubble surface area increases during inward diffusion and decreases during outward diffusion; a higher surface area leads to more diffusion.
If the ambient liquid is not saturated with gas, then rectified diffusion must compete with ordinary diffusion from the bubble to the liquid. In that case, the sound pressure amplitude must exceed a certain value for the bubbles to significantly increase in size.
The pressure oscillations that created the bubbles can also cause them to expand and contract. If the pressure variation is great enough to reduce the local liquid pressure down to or below the vapor pressure in the negative parts of the acoustic cycle moving through the liquid, any minute cavities or bubbles that are present will grow larger. If the range of the pressure variation is increased to produce zero and then negative pressures locally in the liquid, then bubble growth is increased. Gas from the liquid diffuses into a bubble during expansion, and leaves the bubble during contraction.
When the bubble reaches a size that can no longer be sustained by its surface tension, the bubble will expand and then collapse, or implode, which is an important action of the cavitation phenomenon. The bubble action of cavitation has sufficient energy to overcome particle adhesion forces and to dislodge particles attached to substrates in the stream of bubbles. Imploding cavitation bubbles generate shock waves that dislodge particles from substrate surfaces. Cavitation breaks down the molecular force by which a particle is held to surface either by direct impact from bubble implosion or by the fatiguing action caused by repeated bombardment.
Cavitation implosion force varies with the size and contents of the bubble. Larger bubbles are unstable and implode with larger force; smaller bubbles are stable and collapse with less force. Vapor collapses more quickly, resulting in larger implosion force, whereas gas cushions and slows the collapse, resulting in a smaller implosion force.
acoustic transducer and focal zone