Melt Processing COP for Low Auto Fluorescence Micro Fluidics
Mark Kinder, Engineer, Plastic Design Company
As science and chemistry advance, a demand for more sensitivity in test platforms is being created. This is especially true when fluorescence scanning is used to measure test results. Litmus paper has been replaced by reagents and modified strands of RNA that will fluoresce when excited by a light beam of given wave length and the correct reaction has occurred. The fluorescence is measured by a photovoltaic cell and recorded as a voltage. This technology is used in a wide range of applications from High Throughput Screening (HTS) for drug discovery to single molecule detection systems. Combined with DNA replication techniques such as Polymerase Chain Reaction (PCR), an explosion of test platforms have evolved.
The actual test and measurement occur in either a microtitre plate or on a custom micro fluidic chip. Historically these have been moulded from a variety of materials such as polystyrene or polypropylene due to their low cost and in the case of polypropylene, chemical compatibility. Many of today’s test techniques require more sensitivity from the test platform. Today’s microtitre plates and micro fluidic chips need to be moulded from materials with specific properties such as; optical clarity for through scanning, low auto fluorescence for discrimination of reading, chemical compatibility and purity for reliable test results, and high heat deflection temperature to allow thermal cycling. One thermoplastic that meets all these requirements is Cyclo Olefin Polymer or COP. It is being specified for use in many platforms that require a high level of sensitivity.
Designing for COP
The same part design practices apply for COP as for any other injection moulded part. The critical nature of a micro fluidic plate requires that these rules be followed completely. The thickest nominal wall section must be accessible for the gate or gates. Sharp edges and rapid transitions of wall sections must be eliminated to reduce shear. Intersecting walls can only be 70% of the nominal wall to avoid sink. Vertical walls require draft to reduce pulls and reduce induced stress.
Auto fluorescence of the micro fluidic plate will increase background noise and reduce the sensitivity of the test. While COP is very low in auto fluorescence, it is can still fluoresce. Poor processing technique can increase the auto fluorescence to an unacceptable level. In addition to a well-designed part, there are two considerations to successfully moulding COP. The first is to adhere to good moulding practices. The second is understanding what occurs when COP is exposed to oxygen at elevated temperatures (melt temperature for moulding).
Good moulding practices include minimizing residence time, keeping shear low, keeping residual stress low, and keeping all surfaces that come in contact with the material clean.
Residence time is the total time the material has been converted from a solid state as the resin pellet until it returns to a solid state as the finished part. Most of this time is spent in the barrel of the moulding machine where the resin is plasticized for moulding. If the barrel size is not matched to the part, or parts in the case of a multi cavity mould, then molten plastic may remain in the barrel for an extended period. This can lead to degradation, forming particles of degraded material in the finished part. These particles will light up like a neon sign when hit with an excitation source. To a lesser extent hot manifold systems can add to residence time and must be considered when calculating total time.
Shear is the term used to describe the mechanical breakdown of the polymer chain when it is forced under high pressure into the mould. Shear is inevitable and to a certain extent necessary as the energy released when the molecular bond is broken adds to the heat of the resin. This additional heat keeps the viscosity in a range that allows the plastic to fill the mould. What we do not want is excessive shear. In addition to reducing the molecular weight, which reduces tensile strength; enough shear heat can be generated to degrade the resin. This leads to those particles that fluoresce.
Residual stress is primarily the result of poor product design where excessive pack pressure is needed to reduce sink in in thick wall sections. Even in well-designed parts, careful process development must be used to correctly transition from fill to pack without any plastic pressure spikes.
It is assumed that this type of moulding will be done in a cleanroom with the appropriate protocols in place to manage particulates and bio burden. Additional attention is needed in cleaning the plasticizing unit; the barrel, screw, screw tip and nozzle. Any degraded material attached to these components will certainly become glowing dots on the moulded part. All material handling equipment; hoppers, hoses, and dryers must be meticulously cleaned before the introduction of the COP resin pellets.
Oxygen is the nemesis to COP moulding. How hard you work to get rid of it depends on the type of part you are moulding. If the part is pigmented with carbon black, has low mechanical requirements, and the test is not sensitive to auto fluorescence, you may get away with simply drying the resin pellets prior to moulding. Note, you are not drying in the classic sense to eliminate absorbed moisture. COP has a very low moisture absorption rate. In fact it is an excellent moisture barrier. What you are actually doing is driving any absorbed oxygen out of the pellet. This is very important. Most commercial drying systems use heated air passing a hopper with the resin pellets, recirculating through a desiccant bed and back through the hopper. At PDC we have the ability to purge our dryers with nitrogen to further reduce the introduction of oxygen.
Next, we use compressed nitrogen to convey the resin pellets to the feed throat of the moulding machine. All of our machine that mould COP have been modified with nitrogen injectors in the feed throats. This creates a nitrogen blanket in the area of the plasticising unit where the pellets start to melt. Again, this is all done to reduce the chance of oxygen being present as the pellets are melted.
It still Glows in the Dark
So you have done everything right and you still have auto fluorescence issues with your plate. Now what? First, do not assume anything. Determine the appearance of the auto fluorescence. This will help determine the source. Is it small dots, large blobs, streaks through the part or cloudy areas? This requires equipment capable of excitation at the same wavelength as used on the test platform and the ability to observe the fluorescence.
Small dots, up to 2 µm are typically un-melted contamination that was introduced somewhere prior to the plasticizsng of the resin pellet. Blobs, 3- 00 µm are often degraded material that have broken loose from the plasticizing system. Large blobs should be visible with the naked eye as imbedded black specks. Again these will most likely be due to degraded material in the plasticizing system.
Streaks can be caused by a couple of different things. Look at where the streak seems to start. The streak itself will usually follow the fill pattern of the part. If the streak starts at an ejector pin, you can look at gas residue in the ejector pin hole or possible lubricant used on the pin. Note, these moulds need to run dry and be maintained at set intervals. If the streak starts at the gate or at a sharp edge, you may be experiencing shear degradation.
Cloudiness can be as simple as a dirty mould surface. Residue from solvents used for mould cleaning can transfer to the plastic part. The surface finish of the mould can create fluorescence that will appear cloudy. The finish of the vertical walls, perpendicular to the viewing surface, can create refractive conditions that in turn create a fluorescing pattern.
Is it worth all the effort to design and mould in COP? It depends. If a material like polystyrene will meet all of your product requirements, it would be not be cost effective to use COP. Where COP comes into play is where the sensitivity of the test demands it. Also the cost of poor test results should be considered. This can apply equally in clinical situations where inaccurate test results lead to incorrect treatments; and in research labs where poor results lead to wasted time. In these instances, the COP parts become very cost effective and worth the effort to produce them.