# A Microfluidic Approach for Biosensing DNA within Forensics
## 4 Biocompatibility of Materials for Forensic Microfluidic Devices  
The material choice for (forensic) microfluidic chips is very critical. Each material has its own characteristics in the areas of, for instance, transparency, chemical resistance, thermal conductivity and certainly also biochemical compatibility, as shown in Table 1. Although more chip materials are available, such as poly(methyl methacrylate) (PMMA), polystyrene (PS), polyethylene terephthalate (PET) and polytetrafluoroethylene (PTFE). In this article only glass, silicon and the plastics PDMS and COC are considered, since these materials are mostly reported as chip materials for forensic microfluidic devices. \begin{table}
\begin{tabular}{c c c c c} \hline  & **Class** & **Silicon** & **PDMS** & **COC** \\  & **Borofiat** & **With Layer of Native Oxide** & **Sylgard 184** & **Topos 6013** \\ \hline \multirow{2}{*}{Wetability} & Hydrophilic & Hydrophilic & Hydrophobic & Hydrophobic \\  & \(\sim 35\)[34] & \(\approx 57\)[35] & \(\approx 105\)[36] & \(\approx 88\)[37, 38, 39] \\ \hline \multirow{2}{*}{Inhibitory (to amplification)} & Yes & Yes & Yes & Yes \\ Applied coating (to avoid & PEG 8000, & Surfacil, C\({}_{\mathrm{H}}\)S\({}_{\mathrm{H}}\)S\({}_{\mathrm{C}}\)I & Parylene, BSA [40, 41, 42] & BSA, PEG [45, 46] \\ inhibition) & PVP, EPDNA [47, 48] & C\({}_{\mathrm{H}}\)C\({}_{\mathrm{J}}\)S\({}_{\mathrm{H}}\)S\({}_{\mathrm{C}}\)I [49, 50] & \\ \hline \multirow{2}{*}{Transparency} & Transparent & Opaque [17] & Transparent & Transparent \\ Light transmission (530-750 nm) & \(\approx 91\%\)[51] & 0\% & \(\approx 90\%\)[52] & \(\approx 91\%\)[53] \\ \hline \multirow{2}{*}{Thermal conductivity} & Medium & High & Low & Low \\  & \(1.29\) (\(90\)\({}^{\circ}\)) (\(51\),\(54\)) & 156 (600 K) [58] & 0.27 (unknown K) [54, 56] & 0.12-015 (20 \({}^{\circ}\)) [53] \\ \hline \multirow{2}{*}{Chemical resistance} & High & High & Low & High \\  & HF, NaOH [51] & KOH [57] & Chloroform, benzene, & Non-polar \\ \hline \end{tabular}
\end{table} 
Table 1: Characteristics of different chip materials. By using microfluidic devices for the amplification reaction, instead of polypropylene tubes or well plates in combination with a conventional thermocycler, an enormous gain in time is achieved. Forensic chips are for single-use only, i.e., disposable, and therefore these chips can be fabricated of a cheap base-material, provided that it meets all requirements. However, material selection is not always simple and easy, because the material itself can disadvantageously influence (some of) the steps in the process of DNA analysis, such as PCR and detection. Biochemical compatibility of materials for forensic chips is of great importance, since an interaction of the used chemicals with the walls of a microfluidic channel can result in slowing down of the reaction or even total inhibition. The high surface-to-volume ratio of microfluidic devices is a disadvantage in this case. However, such unwanted interaction between the chip material and used chemicals can be avoided by either passivation of the walls of the reaction chamber prior to use, called passive coating, or by so-called dynamic coating by adding components to the used chemicals, e.g., in the amplification mixture [60]. Besides biochemical compatibility, also the wettability of the chip material is of great importance. Microfluidic devices for droplet PCR are usually made of PDMS [61; 62; 63; 64; 65; 66], since the interior of a PDMS channels is hydrophobic and aqueous droplets-in-oil are easier generated in channels of which the surface has this wetting state (compared to a hydrophilic state of the surface of a microfluidic channel). In case of water-in-oil droplets, the template DNA that is present in the aqueous phase cannot adsorb to the interior of the fluidic channel because of the hydrophobic property of the oil [67]. On the other hand, the PCR mixture, as well as other reaction mixtures used for DNA analysis (e.g., lysing agents), are aqueous solutions.