Comparison of Liquid Handling Technologies
Considering the large number of liquid handler models in the lab automation marketplace and the variety of liquid handling technologies available, it can be difficult for the researcher or manager to make sense of the pros and cons of each device and technology. A particular dispenser may be highly suited to certain applications and a poor choice for others because of its underlying technology. This Technical Briefing explains the different liquid handling technologies, including Innovadyne's, and compares them on a number of criteria, such as precision, reliability, ability to aspirate, speed, cross-contamination issues, and so on.
Liquid handlers fall into three main categories as follows:
Non-contact instruments
Manifold or flow-through non-contact/contact devices
Traditional liquid handlers
A section of this document is devoted to each of these three major categories, with subsections on each of the technologies in that category. At the end of the document a matrix is provided in which each technology is rated on a variety of criteria.
Non-Contact Dispensers
Non-contact dispensing means liquid dispensing that does not rely on a touch-off action to break the surface tension of the liquid as it emerges from the tip. The liquid is projected as droplets onto the destination across a distance, rather than dragged against a surface to detach it via capillary action.
The following technology classifications can be applied to non-contact liquid handlers:
Solenoid based
Flow-through solenoid based
Aspirate-dispense solenoid based
Isolated solenoid (Innovadyne)
Acoustic wave/ultrasonic
Piezo-electric
These are described in subsections below.
Solenoid-Based Dispensers
Microsolenoid valves or solenoids (sometimes called ink-jets) are incorporated in liquid handlers to improve dispensing precision. They are rapidly switched back and forth between their open and closed states to permit the flow of pressurized liquid in very small, rapid pulses. The ability to actuate the solenoids quickly makes this type of dispenser ideal for non-stop reagent dispensing.There are three kinds of solenoid-based dispensers:
Flow-through solenoid based
Aspirate-dispense solenoid based
Isolated solenoid (Innovadyne)
Flow-Through Solenoid-Based
Flow-through, solenoid-based dispensers are dispense-only devices. Reagent is pressurized in a pressure reservoir by means of an air pump, creating a pressurized reagent behind the solenoid valve. The solenoid valve pulses to release reagent, as shown:
| Flow-Through Solenoid Based |
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Advantages
Speed. Because the reagent does not have to be aspirated up into the tips, these are faster than devices that must aspirate prior to dispense. This can be an advantage in a manufacturing environment, as there is often a desire to place fixed tips above a conveyor that moves plates or membranes.
Dispensing precision. The solenoid valve enables very tight control over the volume dispensed.
Disadvantages
Reliability. Because these systems have a reagent flowing through the solenoid valve, and solenoid valves have a small internal diameter that is compromised over time by viscosity, particulates, and adhesion, they are unreliable except when only used with the most inert, non-viscous, non-particulate reagents. Most of these systems are also pressurized with air which, through in-gassing, can cause micro-bubbles to form in the reagent fluid. The increased compressibility results in deteriorating performance, particularly in the sub-microliter range.
Dead volume. A large amount of reagent is required in the reservoir and flow path relative to the amount dispensed, which makes these systems unsuitable for expensive or rare reagents.
Changing reagent. Many researchers need the ability to switch reagents quite often. Flow-through systems must be thoroughly cleaned prior to the introduction of a subsequent reagent. The inline valves are particularly difficult to clean due to their internal design.
Difficult to diagnose dispensing problems. Most systems deploy multiple channels with multiple solenoids. Since many reagents have characteristics that can cause them to adhere to valve parts, such as the plunger or seal, it is almost impossible to diagnose the common situation of performance degradation caused by valve failure. To help detect this problem the researcher must put process control in place in the form of additional standards. These additional standards allow the user to monitor the pipetting performance but require additional diligence (cost).
Aspirate and Dispense Solenoid-Based
Some liquid handlers provide aspirate-and-dispense capability in a microsolenoid-based device. Reagent is aspirated by means of the syringe or pump, with the solenoid set in its open position. Reagent is dispensed by pressurizing the reagent in the tubing using the syringe or pump, and pulsing the solenoid valve.
| Aspirate and Dispense Solenoid-Based |
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Advantages
Dispensing precision.
Ability to function in either aspirate-dispense or dispense-only modes.
Disadvantages
Reliability. Valve failure is common except with the most inert, non-particulate, non-adhesive reagents, due to the same reasons that this occurs in flow-through solenoid systems.
Uneven aspiration of viscous materials. Often these systems use a manifold aspirating pump or vacuum causing the preferential aspiration of liquids of less viscosity resulting in uneven aspiration.
Difficult to diagnose dispensing problems. As above.
Isolated Solenoid Aspirate-Dispense
Innovadyne's proprietary technology, which can be classified as "isolated solenoid aspirate-dispense", uses separate, isolated actuators for aspiration and dispensing -- aspiration is by means of syringes, and dispensing is by means of microsolenoids. Since the sample or reagent line is valve free, reagent only contacts the tips and a short length of tubing, The remainder of the system, including the syringes and solenoids, contains deionized water. Aspirated reagent is separated from the deionized water by means of an air gap, and is dispensed as a result of the displacement of deionized water through the solenoids. The pressure reservoir is pressurized by means of helium from a helium tank, regulated with a digital pressure regulator (DPR). The aspiration and dispensing paths are shown below:
| Isolated Solenoid -- Reagent Aspiration |
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| Isolated Solenoid -- Reagent Dispense |
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For an in-depth description of Innovadyne's technology, refer to the Innovadyne Technology Briefing document entitled "High-Precision Non-Contact Dispensing".
Advantages
Dispense precision. The combination of the microsolenoid valve and a degassed system fluid creates highly precise dispensing, with excellent performance down to 100 nL and below.
Reliability. Because the solenoids are only in contact with de-ionized water, they are extremely reliable.
Low dead volume / use of an air gap. No reagent is expended except for the reagent that is aspirated because air gaps can be used to isolate reagent from the system fluid in most cases. The use of air gaps is unique to Innovadyne because it is not possible to use air gaps in systems that incorporate an inline valve.
Performance with difficult reagents. Viscous reagents, beads, cells and slurries are readily aspirated and dispensed without compromising performance, reliability or cell viability. In the event of a tip failure due to clogging, Innovadyne tips can be replaced/recovered at a fraction of the cost of valve inline systems.
Changing reagents. Changing reagents requires only a standard wash cycle.
Ease of washing. The tips and the portion of tubing in contact with reagent can be rinsed by propelling deionized water out through the tips, and externally washed in a wash station on the instrument. Syringes, solenoids and switching valves do not require washing because they don't contact reagent. In the event that additional wash solvents are required they may be readily aspirated into the valve-free flow path followed by flushing with water.
Negligible cross-contamination. Due to flexible wash routines as above.
Disadvantages
Speed. Not as fast as flow-through dispense-only devices.
Volume range. Some volume limitations because the system is limited to the amount of fluid that can be aspirated into the valve-free flow path. The result is multiple aspirate/dispense cycles for the dispensing of large volumes (greater than 40 microliters).
Acoustic Wave (Ultrasonic)
An acoustic wave or ultrasonic dispenser generates an acoustic wave pulse via an ultrasonic head that causes a fluid drop to be ejected from a reservoir and dispensed into a destination plate placed upside down. It has been shown to be very accurate at sub-nanoliter volumes and is now used for compound transfers and “hit picking,” predominantly in 1536-well plates. The technology doesn’t require any wash steps.
Advantages
Excellent dispensers for compound transfer.
Disadvantages
Typically not suitable for reagent dispensing. The volume that can be dispensed with an acoustic wave device is generally in the picoliter range making the dispense of microliter volumes too slow to be practical.
Piezo-Electric Dispensers
These devices use ink-jet dispensing technology equivalent to that found in an ink-jet printer to propel reagent out of an extremely small orifice (approx. 50 microns).
Advantages
Extremely low volume. Well suited for printing genomic arrays.
Disadvantages
Reliability. Highly unreliable for high-throughput applications, which caused this design to be abandoned by most manufacturers in the late 1990's.
Manifold and Flow-Through Peristaltic Devices
This class of systems consists of devices that propel reagent from an unpressurized reservoir through tubing to the tip(s). These devices are generally dispense only and are non-contact. Most devices in this class have a dispense range above 5 µl. This class includes manifold and peristaltic devices.
Manifold Devices
Often the manifold devices use a single syringe, pump or actuator to push fluid through a manifold that is attached to 8 tips.
| Manifold Devices |
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Advantages
Low capital equipment cost
Fast
Well-suited to high-volume (>10 µL) dispensing (where there are no differences in viscosity and high precision and accuracy are not requirements).
Disadvantages
High reagent costs due to system dead volume. Relatively large dead volumes are required to fill each channel (milliliters).
High maintenance costs. Many users replace cartridges with every run resulting in a large maintenance budget.
Precision. This type of system cannot reliably aspirate or dispense liquids of varying viscosity or reliably dispense small volumes (sub-microliter).
Changing reagents. These systems require thorough cleaning prior to the introduction of a new reagent.
Peristaltic Devices
In a peristaltic-pump-based dispensing device, a peristaltic pump squeezes the tubing from the outside to propel reagent, eliminating contact between pump parts and the reagent (reagent only touches the inside of the tubing). The pump is in-line between a reagent reservoir and the tips. These systems cannot aspirate.
| Peristaltic Devices |
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Advantages
Lower capital equipment cost
Fast.
Well-suited to higher volumes (where viscosity differences are not an issue)
Disadvantages
High reagent costs due to dead volume. The design requires a large back-filled dead volume.
High maintenance costs. Many users replace cartridges with every run resulting in a large maintenance budget.
Reliability at low volumes. For low-volume dispensing, the peristaltic tubing used is reduced in size and wound tightly over the pump rollers, resulting in reduced MTBF (mean time between failure) rates and high cost per test.
Difficulty of cleaning / changing reagents. These systems require thorough cleaning prior to the introduction of a new reagent.
Traditional Syringe-Based or Positive Displacement Devices
This class includes systems that incorporate one syringe per channel, those that use a syringe array (most often 96 or 384 syringe plungers moved by a single plate), and disposable tip systems. Their principle of operation is similar, but disposable tip and syringe array systems employ a plunger assembly, not a syringe. Generally, these devices are not suited to non-contact dispensing.
Syringe Per Channel
Syringe-per-channel systems utilize a syringe in-line between the pressure reservoir and tip for each tip in the system. They can be used as aspirate-dispense or dispense-only devices. For dispense-only mode, the syringe valve alternates between routing flow from the reservoir for the aspiration and to the tip for the dispense. For aspirate-dispense mode, the flow path is only to and from the tip.
| Syringe-Per-Channel |
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Syringe-per-channel systems are occasionally used in non-contact mode but the more common usage is to dispense using the contact of the tip to liquid already in the well, causing the last of the liquid on the tip to be wicked off onto the point of contact.
Advantages
Independent control of each channel.
Low dead volume.
Easy to clean.
Disadvantages
Excessive washing of the tips is required because the tips must touch-off often in a well that contains another reagent or sample
Reliability. The duty cycle is placed on the syringes and replacement is required fairly often.
Syringe Array
Syringe array systems do not incorporate syringe valves and have no through flow wash capability. They utilize a plunger in the tip above the area where the reagent is aspirated, and are aspirate-dispense devices. They are generally used as compound transfer devices rather than reagent dispensers, and 96-tip devices are fairly common. These systems are often used in protein crystallography applications.
| Syringe Array |
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Advantages
Low dead volume.
Low capital costs for the number of tips
Disadvantages
Precision/contamination from plate. Generally unsuitable for non-contact dispensing, with the result that precision and contamination of reagent from the plate are issues, and reliable low-volume dispense onto dry surfaces is not possible.
Speed/washing. Syringe array devices are relatively slow due to the wash step, which requires the repetitive aspiration and dispense of a cleaning solution between cycles.
Disposable Tip
Disposable tip systems, like syringe array systems, are aspirate-dispense devices. Instead of a plunger down near the bottom of the tip, directly contacting the reagent, a disposable tip system has the plungers well above the reagent in the tip, and aspirate and dispense utilizing air displacement. This makes it possible for the portion of the tips in contact with reagent to consist of removable, disposable tips.
| Syringe In Tip |
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Advantages
Cross-contamination between reagents is eliminated by discarding tips before changing reagents
Low dead volume.
Disadvantages
Slow for reagent dispensing. Must discard tip between dispenses and then re-aspirate reagent.
Precision/contamination from plate. Generally unsuitable for non-contact dispensing, with the result that precision and contamination of reagent from the plate are issues, and reliable low-volume dispense onto dry surfaces is not possible.
Expensive per use. Requires a disposable tip per use resulting in a high cost per test.
Differentiation Matrix
| Innovadyne Nanodrop | Flow-Through Solenoid | Aspirate-Dispense Solenoid | Manifold | Flow-Through Peristaltic | Syringe Per Channel | Syringe Array | Disposable Tip | |
| Contact dispensing | yes | no | yes | no | no | yes | yes | yes |
| Non-contact dispensing | yes | yes | yes | yes | yes | poor | no | no |
| Aspirate and dispense | yes | no | yes | yes | no | yes | yes | yes |
| Solenoids or pump in flow path | no | yes | yes | yes- | yes | no | yes | no |
| Speed - Reagent Dispense | medium/fast | fast | medium/fast | fast | fast | slow | slow | slow |
| Speed - Sample Transfer | slow | X | slow | X | X | slow | medium | fast |
| Reliability | excellent | poor | poor | good | poor | poor | good | good |
| Precision, 1 µL- 10 µL | excellent | good | good | poor | poor | good | good | poor |
| Precision, sub-1 µL | excellent | good | good | poor | poor | fair | fair | poor |
| Wash required between dispenses | no | no | no | no | no | yes | yes | X |
| Difficult reagents (high-viscosity, cells, beads) | yes | no | no | some | some | some | poor | some |
| Dead volume | low | high | low | high | high | low | low | low |
| Ease of cleaning | good | poor | poor | poor | poor | good | poor | X |
| Independent dispense channel articulation | yes | yes | yes | no | no | yes | no | no |
| Dispenses to dry surfaces | yes | yes | yes | some | some | no | no | no |
Technologies