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|Title: ||Polymer bonding by induction heating for microfluidic applications|
|Authors: ||Knauf, Benedikt J.|
|Keywords: ||Microfluidic systems|
|Issue Date: ||2010|
|Publisher: ||© Benedikt J. Knauf|
|Abstract: ||Microfluidic systems are being used in more and more areas and the demand for such systems is growing every day. To meet such high volume market needs, a cheap and rapid method for sealing these microfluidic platforms which is viable for mass manufacture is highly desirable.
In this work low frequency induction heating (LFIH) is introduced as the potential basis of a cost-effective, rapid production method for polymer microfluidic device sealing. Thin metal layers or structured metal features are introduced between the device s substrates and heated inductively. The surrounding material melts and forms a bond when cooling.
During the bonding process it is important to effectively manage the heat dissipation to prevent distortion of the microfluidic platform. The size of the heat affected zone (HAZ), and the area melted, must be controlled to avoid blockage of the microfluidic channels or altering the channels wall characteristics. The effects of susceptor shape and area, bonding pressure, heating time, etc, on the heating rate have been investigated to provide a basis for process optimisation and design rules. It was found that the maximum temperature is proportional to the square of the susceptor area and that round shaped susceptors heat most efficiently. As a result of the investigations higher bonding pressure was identified as increasing bond strength and allowing the reduction of heating time and thus the reduction of melt zone width. The use of heating pulses instead of continuous heating also reduced the dimensions of melt zones while maintaining good bond strength. The size of the HAZ was found to be negligible.
An analytical model, which can be used to predict the heating rate, was derived. In validating the model by numeric models and experiments it was found that it cannot be used to calculate exact temperatures but it does correctly describe the effect of different heating parameters. Over the temperature range needed to bond polymer substrates, cooling effects were found not to have a significant impact on the heating rate.
The two susceptor concepts using thin metal layers (metal-plastic bonds) or structured metal features (plastic-plastic bonds) were tested and compared. While the metal-plastic bonds turned out to be too weak to be useful, the bonds formed using structured susceptors showed good strength and high leakage pressure.
Based on the knowledge gained during the investigations a microfluidic device was designed. Different samples were manufactured and tested. During the tests minor leaks were observed but it was found that this was mainly due to debris which occurred during laser machining of the channels. It was concluded that induction bonding can be used to seal plastic microfluidic devices.
The following guidelines can be drawn up for the design of susceptors and process optimisation:
Materials with low resistivity perform better;
For very thin susceptors the effect of permeability on the heating rate is negligible;
The cross-sectional area of the susceptor should be as large as possible to reduce resistance;
The thickness of the susceptor should be of similar dimensions to the penetration depth or smaller to increase homogeneity of heat dissipation;
The shape of the susceptor should follow the shape of the inductor coil, or vice-versa, to increase homogeneity of heat dissipation;
The susceptor should form a closed circuit;
Higher bonding pressure leads to stronger bonds and allows reduced heating times;
Pulsed heating performs better than continuous heating in terms of limited melt area and good bond strength.
The drawbacks of the technique are explain|