Henniker shows plasma treatment of silicone improves adhesion

Henniker Plasma has completed testing of its plasma treatment technique to improve bonding in silicone devices.

Its research found that polydimethylsiloxane (PDMS), a member of a group of polymer organo-silicones, improves its adhesive properties after plasma treatment.

PDMS is in applications ranging from medicines and cosmetics to silly putty. It has a number of properties which make it favourable for use in creating microfluidic devices, as it is biocompatible, cheap, has low fluorescence and it is transparent.

One key limitation of PDMS when used in this application is poor adhesion to glass, which leads to premature device failure.

Henniker used its HPT-200 system to produce consistent plasma treatment performance for repeatable bonding of PDMS. Both the glass and PDMS substrates are treated with air plasmas, at low pressure, with all settings under microprocessor control.

On both substrates, the treatment is effective at removing hydrocarbon groups (CxHy) leaving behind silanol groups on the PDMS and OH groups on the glass substrate respectively. This allows strong Si – O – Si covalent bonds to form between the two materials.

Contact angle measurements indicate whether a surface is hydrophobic (over 90°) or hydrophilic (under 90°) by the angle a water drop makes with the surface. Increasing the time or power of an air plasma treatment on PDMS leads to the surface becoming more hydrophilic.

Bonding of PDMS to glass is a key issue in the use of the material in fabricating microfluidic devices - using a Henniker model HPT-200 benchtop plasma system to treat PDMS substrates alongside glass substrates, this problem has been addressed. Plasma treatment has been shown to improve the wettability of both surfaces, leading to increased bonding between the two substrates  - the system can also be used to bond PDMS to thermoplastics.