If a pipette gives variable amounts after being calibrated correctly and pipetting properly, it is usually the tip of the pipette that is the problem. The reliability of sealing, minimal dead volume, and consistent liquid delivery over repeated cycles of operation are critical and depend on the design of the silicone tip for the pipette. Well-designed silicone tips provide improved conformability and cushioning and are useful in manual pipettes and electronic and automated pipetting systems, where flexibility needs to be combined with precision.
We are able to provide a wide range of silicone pipette tips for numerous OEM’s, which both preserve the accuracy of the volume, and reduce operator fatigue and the compatibility with non-standard cones. This guide covers the key design parameters—material properties, geometry, tolerances, ergonomics and validation—such that tips can be specified to reliably perform in practical laboratory applications, ranging from PCR set-up to high throughput screening.
How Pipette Tips Influence Accuracy and Repeatability
The stability of the air cushion and the air tightness between the cone and the tip are essential for pipette systems. Any leakage, excessive deformation or dead volume within the device will have a direct impact on the delivered volume, particularly at microliter scales. Many of the resources are about the operator technique: immersion depth or pre-wetting, but tip design and material properties have variables that can quietly affect the results.
Silicone tips are excellent for situations where the cone or the applications themselves have slight dimensional changes that don’t allow standard polypropylene tips to seal or for applications that place greater emphasis on ergonomic requirements by limiting the force required to insert and eject the cone. There is, however, a balancing act needed for this advantage: if there is too much flexibility, it can collapse under aspiration force or when handling viscous samples; and if it is not flexible enough, it can leak or it will require high operating force.
Comparing Silicone vs Rigid Plastic Tip Behavior
The rigid plastic tips offer good dimensional stability and are the standard in many standard systems. They can be manufactured with an accuracy of micromillions from one production lot to another, but require the perfect fit of cones and may add to the mechanical stress applied to the pipette and user.
The silicone tips, on the other hand, take advantage of the material’s inherent elasticity, making it more flexible and resilient to slight changes in the geometry of the cones, thus providing superior seals on custom and worn pipettes. They also absorb small shocks when the robot is moving, and can help to prevent thumb strain when working lots of manual operations. The compromise comes when softness results in ballooning or varying rebound, so the design of the silicone tip for pipettes needs to be optimized for each application.
Many OEMs now collaborate with specialized manufacturers to develop precision custom silicone tips that match their pipette cone geometry and deliver consistent results across production lots.
Material Properties – Flexibility, Durometer, and Surface Behavior
Tip load behavior is directly dependent upon the durometer (hardness) of the silicone. Softer (30-50 Shore A) formulations are more comfortable and have a better sealing ability, but will not hold its shape under high-aspiration or in handling thick liquids. Higher Shore A (60-70) grades provide geometry that maintains high accuracy, but may need more insertion force and are less forgiving for misaligned cones.
Elastic recovery is also of note. The tip should be able to fit snugly onto the cone, and return to its original shape after thousands of times of attaching and detaching. Surface characteristics also play a role, as silicone is naturally hydrophobic, which reduces droplet retention; in cases of very low volume or sticky samples, optimized surface modifications can be used to reduce the amount of residual sample even more.
Choosing Material Parameters for Different Liquid Types
A medium durometer silicone (approx. 50 Shore A) with optimized wall thickness is generally the best choice for standard aqueous buffers and reagents for both flexibility of the silicone tips on the pipettes and performance stability.
For viscous liquids, like glycerol, serum or detergent solutions, slightly softer compounds with more generous internal bores are useful to minimize flow resistance and tip collapse. Tips that are silicone should be used with viscous volatile liquids and must be carefully engineered for a quick rebound speed, preventing carry over or dripping after each dispense.
If volatile solvents (such as ethanol or acetone) are used, they need more precise geometry and surface stability to avoid evaporation and wicking. In such instances, use of a harder silicone along with a minimum dead volume helps to maintain pipette tip design for high precision.
Geometry and Tolerances – The Heart of Pipette Tip Design
The geometry of the tip dictates whether or not it can make an airtight fit without over-draining. Internal taper should be as close to the pipette cone as possible and wall thickness should change gradually from a thick, sturdy hub to a thin, controlled orifice. Having inconsistent dimensions results in an inconsistent fit, leaks or tips that are too tight to wear comfortably.
Other factors, such as length, orifice diameter, and concentricity have also a part to play in aspiration depth, droplet formation and the amount of residue. In low volume applications, relatively small changes in orifice size can affect meniscus formation and accuracy.
Critical Dimensions to Lock into Your Drawings
When you are ordering silicone tips for lab equipment, consider the following:
- Primary sealing diameter (inner and outer diameters).
- The angle is designed to be compatible with your pipette cone.
- The inner diameter of the orifice and the concentricity with the tip axis.
- The distribution of thickness along the length of the wall
- The overall length of the tips and the location of the filter (if used)
Ask for capability information like Cpk values from the manufacturer, don’t just take tolerances. This means that each lot will be treated the same in manual and automated systems.
Ergonomics, Fit, and Compatibility with Manual and Robotic Pipetting
Good ergonomic pipette tips directly influence the productivity of everyday lab work. Too much attachment force can lead to repetitive strain injuries and too loose tips can fall off during mixing or plate handling. Lower forces are possible with Silicone, but the design must be tested for each family of pippettes.
Whereas force variations, even small ones, multiply across the head in multi-channel setups (8-, 12- or 96-channel), it is important to ensure they are consistent in manufacturing.
Design Considerations for Robotic Liquid Handling
There are extra challenges with robotic systems: consistent pick-up height, reliable seating detection and resisting bending during quick motion or occasional collisions with the deck. The custom silicone tips for automated liquid handling systems should be dimensionally stable over temperature changes and repeated use and should be able to withstand shaking of plates or rapid transportation without drop.
Validation, using worst-case scenarios, in the real robot is critical. Flexibility, if not properly addressed, is crucial for a good tip to perform robustly under high-speed robotic operation, but could otherwise fail if the tip is operated manually.
Validation and Line Testing – Proving That the Design Works
Design validation should be carried out at the system level at the end of the design phase, covering the whole volume range and the appropriate liquid types, by gravimetric or photometric measurement. Add forward and reverse pipetting, multi-dispense protocols and stress testing using challenging and aqueous samples.
Monitor accuracy and precision (CV%) in actual use, including any sterilization procedures that the tips will experience. These data are used for final adjustments of durometer, geometry, or surface finish.
What to Ask from Your Manufacturing Partner
A good partner offers:
- Multiple cavity and lot dimension/durometer reports.
- Samples that are collected for use in the laboratory.
- Inclination to repeat the validation process if needed and modify the tooling.
- Material certifications for your application (food grade, medical grade, etc.)
Using the manufacturer more as an engineering resource than just a supplier shortens the development process and enhances the final product quality.
Practical Design Checklist for OEM Engineers
Use this checklist when specifying your next set of silicone pipette tips:
- Verify the reasons why silicone is used over stiff plastic in this application
- Explain the concepts of accuracy, precision, range of liquids and targets.
- Choose the type of sealing required, the ergonomics and the type of sample to select the durometer range.
- Protect critical geometry and tolerances with capability data.
- Conduct system level testing similar to real-life scenarios, including robot runs as applicable
- Put in place continuous quality monitoring procedures for production lots
The design of silicone tips for pipettes needs to be balanced between flexibility and accuracy, and this will involve considerations of material science, mechanical engineering and laboratory requirements. If these tips are done properly, it will make manual and automated platforms more effective and user-friendly.
Our engineering team at HT Silicone can help you select the right silicone material for your pipette system, design your mold, and manufacture in production quantities, if you are creating or optimizing an existing pipette system and need assistance in liquid handling accuracy with silicone pipette tips.
