Expert insight: Biocompatible bonding

The use of lab-on-a-chips in life sciences has grown strongly over the past few years. In-vitro diagnostics (IVD) and point of care (POC) tests have become crucial in almost every healthcare segment. Test devices are often made of polymer materials. Due to their typical properties, such as transparency, ease of manufacturing and high volume compatibility, polymers are extremely popular in biological, pharmaceutical and medical applications. The production of polymer microfluidic chips is carried out using various microfabrication methods such as micromachining, hot embossing, extrusion coating and injection molding. With these techniques, the micro-channels are fabricated in a polymer substrate or foil, after which, in the majority of cases, the channel structure has to be sealed by a next layer of material. Joining the top and bottom layers and hermetically sealing the microchannel network can be done through several bonding methods. Often the distinction is made between direct and indirect bonding. Indirect bonding involves the use of an adhesive layer (like glue or tape) or other assisting sticky material to seal two substrates and encapsulate the microstructure. This opposed to direct bonding, which is carried out without any additional materials. Direct bonding often includes a heating step to soften the material and break the polymer chains at the connecting surfaces.

Developers in the biomedical field will wonder: how do abovementioned methods behave when biological material comes into play? This indeed changes things radically, because now not only the properties of the polymer, but also the extreme sensitivity of the biological contents must be taken into account.

The Need for Biocompatibility

There are different ways in which biological materials are used in IVD. Mostly, they are applied during the assay in a liquid form. In this case, it is of extreme importance that the channel surfaces are clean. If the channels would contain residues of harmful substances used during bonding, it can seriously affect the biomaterial in the liquids and subsequently, negatively affect the results of the assay or diagnostic test.

A strongly upcoming process is the use of chips that are pre-loaded with biomaterials. The chip surface is loaded with reagents, often in a dried form, we call this surface functionalization. During the assay, the dried substance is liquified and becomes part of the assay. In this case, the biomaterial is applied to the substrate in an early stage of the production process of the device. This means that any following production step, in particular bonding, could harm the biocontent. This is why we are working hard to further develop methods of biofriendly or biocompatible bonding.

What are the challenges of Biocompatible Bonding?

Successful biocompatible bonding methods must meet a number of conditions.

• The use of chemicals (e.g. solvents, adhesives) should be avoided, or at least limited to applications where they would not have a negative impact. Chemicals can damage any preloaded biocontent, and residues in the channels can contaminate fluids that are used in the testing phase.
• High temperatures can only be applied during the bonding process as long as there are no biomaterials on the chip. Elevated temperatures would ‘kill’ preloaded biomaterials.
• The bond must be strong enough. To function properly, a microfluidic chip must be a hermetically sealed system.

There are several bonding methods that meet these requirements. Each of these methods has its own pros and cons.

We have a white paper with a detailed overview of Micronit’s biocompatible bonding techniques ready for you. Download it now!