Heat and pressure-resistant room temperature irreversible sealing of hybrid PDMS–thermoplastic microfluidic devices via carbon–nitrogen covalent bonding and its application in a continuous-flow polymerase chain reaction

Bonding Strategies for Thermoplastics Applicable for Bioanalysis and Diagnostics

Microfluidics is a multidisciplinary science that includes physics, chemistry, engineering, and biotechnology. Such microscale systems are receiving growing interest in applications such as analysis, diagnostics, and biomedical research. Thermoplastic polymers have emerged as one of the most attractive materials for microfluidic device fabrication owing to advantages such as being optically transparent, biocompatible, cost-effective, and mass producible. However, thermoplastic bonding is a key challenge for sealing microfluidic devices. Given the wide range of bonding methods, the appropriate bonding approach should be carefully selected depending on the thermoplastic material and functional requirements. In this review, we aim to provide a comprehensive overview of thermoplastic fabricating and bonding approaches, presenting their advantages and disadvantages, to assist in finding suitable microfluidic device bonding methods. In addition, we highlight current applications of thermoplastic microfluidics to analyses and diagnostics and introduce future perspectives on thermoplastic bonding strategies.

Microfluidics for High Pressure: Integration on GaAs Acoustic Biosensors with a Leakage-Free PDMS Based on Bonding Technology

Microfluidics integration of acoustic biosensors is an actively developing field. Despite significant progress in “passive” microfluidic technology, integration with microacoustic devices is still in its research state. The major challenge is bonding polymers with monocrystalline piezoelectrics to seal microfluidic biosensors. In this contribution, we specifically address the challenge of microfluidics integration on gallium arsenide (GaAs) acoustic biosensors. We have developed a robust plasma-assisted bonding technology, allowing strong connections between PDMS microfluidic chip and GaAs/SiO₂ at low temperatures (70 °C). Mechanical and fluidic performances of fabricated device were studied. The bonding surfaces were characterized by water contact angle measurement and ATR-FTIR, AFM, and SEM analysis. The bonding strength was characterized using a tensile machine and pressure/leakage tests. The study showed that the sealed chips were able to achieve a limit of high bonding strength of 2.01 MPa. The adhesion of PDMS to GaAs was significantly improved by use of SiO₂ intermediate layer, permitting the bonded chip to withstand at least 8.5 bar of burst pressure. The developed bonding approach can be a valuable solution for microfluidics integration in several types of MEMS devices.

Recent Advances in Thermoplastic Microfluidic Bonding

Microfluidics is a multidisciplinary technology with applications in various fields, such as biomedical, energy, chemicals and environment. Thermoplastic is one of the most prominent materials for polymer microfluidics. Properties such as good mechanical rigidity, organic solvent resistivity, acid/base resistivity, and low water absorbance make thermoplastics suitable for various microfluidic applications. However, bonding of thermoplastics has always been challenging because of a wide range of bonding methods and requirements. This review paper summarizes the current bonding processes being practiced for the fabrication of thermoplastic microfluidic devices, and provides a comparison between the different bonding strategies to assist researchers in finding appropriate bonding methods for microfluidic device assembly.

Facile Patterning of Thermoplastic Elastomers and Robust Bonding to Glass and Thermoplastics for Microfluidic Cell Culture and Organ-on-Chip

The emergence and spread of microfluidics over the last decades relied almost exclusively on the elastomer polydimethylsiloxane (PDMS). The main reason for the success of PDMS in the field of microfluidic research is its suitability for rapid prototyping and simple bonding methods. PDMS allows for precise microstructuring by replica molding and bonding to different substrates through various established strategies. However, large-scale production and commercialization efforts are hindered by the low scalability of PDMS-based chip fabrication and high material costs. Furthermore, fundamental limitations of PDMS, such as small molecule absorption and high water evaporation, have resulted in a shift toward PDMS-free systems. Thermoplastic elastomers (TPE) are a promising alternative, combining properties from both thermoplastic materials and elastomers. Here, we present a rapid and scalable fabrication method for microfluidic systems based on a polycarbonate (PC) and TPE hybrid material. Microstructured PC/TPE-hybrid modules are generated by hot embossing precise features into the TPE while simultaneously fusing the flexible TPE to a rigid thermoplastic layer through thermal fusion bonding. Compared to TPE alone, the resulting, more rigid composite material improves device handling while maintaining the key advantages of TPE. In a fast and simple process, the PC/TPE-hybrid can be bonded to several types of thermoplastics as well as glass substrates. The resulting bond strength withstands at least 7.5 bar of applied pressure, even after seven days of exposure to a high-temperature and humid environment, which makes the PC/TPE-hybrid suitable for most microfluidic applications. Furthermore, we demonstrate that the PC/TPE-hybrid features low absorption of small molecules while being biocompatible, making it a suitable material for microfluidic biotechnological applications.

Chemically robust succinimide-group-assisted irreversible bonding of poly(dimethylsiloxane)-thermoplastic microfluidic devices at room temperature

This study investigates surface chemical modification using anhydride silane and amino silane reagents at room temperature (RT) to realize bonding between silicon-based PDMS and non-silicon thermoplastics. The anhydride silane shows vigorous activity against water, forming a terminal dicarboxylic acid in the plasma-activated elastomeric poly(dimethylsiloxane) (PDMS) surface, and it can readily react with amino-silane-modified thermoplastic surfaces, resulting in a permanent bond via the formation of a stable succinimide group without the requirement for high temperature or additional pressure to initiate the bonding. The modified surfaces of PDMS and thermoplastics were successfully characterized by water contact angle measurement, fluorescence measurement, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The bond strength values of PDMS-thermoplastic assemblies, measured by the tensile test for PDMS-polystyrene (PS), PDMS-poly(methyl methacrylate) (PMMA), PDMS-polycarbonate (PC), and PDMS-poly(ethyl terephthalate) (PET) assemblies, were found to be approximately 519.5 ± 6, 259 ± 15, 476.6 ± 8, and 458.2 ± 27 kPa, respectively. Moreover, the bond strength was further examined by performing a burst test for PDMS-PMMA, PDMS-PS, PDMS-PC, and PDMS-PET microfluidic devices, which were found to have the maximum pressure values at approximately 344.73, 448.15, 413.68, and 379.21 kPa, respectively. Based on these results, the hybrid microfluidic devices can be used for high-pressure experiments such as blood plasma separation and continuous-flow polymerase chain reaction (CF-PCR). We have also performed the large area bonding of the PDMS-PC assembly (10 × 10 cm2), ensuring the high robustness and reliability of the proposed surface chemical bonding method.