Lingqian Zhang

Restraining non-specific adsorption of protein using Parylene C-caulked polydimethylsiloxane.

Non-specific adsorption (NSA) of proteins on surface is a critical issue in polydimethylsiloxane (PDMS)-based microfluidics, which may either considerably decrease the efficiency of a continuous flow reaction or cause a large background noise in a heterogeneous sensing. This work introduced a new method to restrain NSA of protein by caulking PDMS with Parylene C, i.e., forming a Parylene C-caulked PDMS (pcPDMS) surface. The caulking depth of Parylene C inside PDMS matrix was characterized by laser scanning confocal microscopy based on a detectable autofluorescence intensity difference between Parylene C and PDMS after being annealed at 270 °C for 2 h in nitrogen. NSA of bovine serum albumin (BSA) on the inner surfaces of PDMS and pcPDMS microchannels was experimentally compared. The results indicated that the adsorbed BSA on the pcPDMS surface were 35.2% of that on the pristine PDMS surface after the BSA solution flowing through the microchannels at a flow rate of 2000 nL/min, a typical scenario of the continuous flow reaction. In a case mimicking the heterogeneous sensing, after a 60 min washing of phosphate buffered saline flow on a pre-saturated BSA adsorbed surface, the residual BSA on the pcPDMS surface was only 4.5% of that on the pristine PDMS surface. Adsorption/desorption coefficients of BSA on the PDMS and the pcPDMS surfaces were extracted from the experimental results based on the first-order Langmuir model, which indicated that the pcPDMS has a lower adsorption coefficient (Ka ) and a higher desorption coefficient (Kd ), compared to those of the pristine PDMS. A preliminary experiment also indicated that Taq polymerase kept 93.0% activity after flowing through a pcPDMS microchannel, while only 28.9% activity was left after passing a pristine PDMS microchannel under the same operation condition.

Fabrication of reusable whole PDMS biochip for mesenchymal stem cell separation and enrichment

A novel biochip, which was made from Polydimethylsiloxane (PDMS) and integrated separation and enrichment structures, has been presented. The biochip mainly composes of the spiral microchannel with pillared filtration structures being used to simultaneously and continuously separate cells, and the U-shaped filter being applied to enrich the cells. The protocol of the soft lithographic methods for the whole PDMS biochip is described. The reversible PDMS-PDMS bonding technique is important for the reusable biochip, therefore, the biochip will reuse after separation and enrichment of cells, which can reduce the cost. The simple fabrication processes are attractive for mesenchymal stem cell (MSC) or other cell cultivation, which has potential for clinical application.

Caulking polydimethylsiloxane molecular networks by thermal chemical vapor deposition of Parylene-C

Surface functionalization of polydimethylsiloxane (PDMS) is important in developing high-performance microfluidic devices. This work applied the thermal chemical vapor deposition (t-CVD) of Parylene-C onto PDMS to caulk the molecular network while retaining the original surface properties for the oxygen plasma bonding. The very low deposition rates (for example, a nominal rate of 0.12 Å min-1 at 135 °C) of Parylene-C at elevated substrate temperatures enabled the reactive Parylene-C monomers to penetrate into the PDMS matrix up to 4.6 ± 0.1 μm (135 °C), which was verified for the first time by a scanning electron microscope with an energy dispersive X-ray analysis (SEM-EDAX). The Parylene-C caulked in the molecular network of PDMS matrix guaranteed an excellent resistance to small molecule permeations. Meanwhile, only discrete nucleation islands were formed on the top surface rather than a continuous Parylene-C layer as observed under the AFM scan, which made the processed PDMS surface ready for device assembly. This surface functionalization method has better long-term stability than the other wet-type rivals. The barrier for oxygen plasma bonding in previously reported dry surface treatments was also avoided, thereby, facilitating the device assembly. The present work successfully developed a novel pcPDMS (Parylene-C caulked PDMS) technique, which overcame the bonding difficulty in the previous works but retained the low small molecule permeability as before. Caulking a molecular network through the t-CVD of Parylene-C also demonstrated a new strategy of functionalizing polymer surfaces and preparing new hybrid materials for wide lab-on-a-chip applications.

High-efficiency blood separation utilizing spiral filtration microchannel with gradually varied width

A hybrid microfluidic separation device that uses the crossflow and the centrifugation effect to separate human plasma from blood cells has been designed and fabricated. The chip mainly consists of the spiral microchannel, which is divided into inner and outer mcirochannels by micropillar arrays. The slits between the micropillars act as filter. Clogging and jamming in this filtration structure are efficiently alleviated. Remarkably, the width of the inner microchannel is gradually decreasing from inlet to outlet in order to increase separation efficiency. The performances of the separation device were investigated theoretically and experimentally. Due to high separation efficiency and compact structure, it is envisaged that this device can be integrated with other microfluidic device for point-of-care diagnostics in the near future.

SF6 Optimized O2 Plasma Etching of Parylene C

Parylene C is a widely used polymer material in microfabrication because of its excellent properties such as chemical inertness, biocompatibility and flexibility. It has been commonly adopted as a structural material for a variety of microfluidics and bio-MEMS (micro-electro-mechanical system) applications. However, it is still difficult to achieve a controllable Parylene C pattern, especially on film thicker than a couple of micrometers. Here, we proposed an SF6 optimized O2 plasma etching (SOOE) of Parylene C, with titanium as the etching mask. Without the SF6, noticeable nanoforest residuals were found on the O2 plasma etched Parylene C film, which was supposed to arise from the micro-masking effect of the sputtered titanium metal mask. By introducing a 5-sccm SF6 flow, the residuals were effectively removed during the O2 plasma etching. This optimized etching strategy achieved a 10 μm-thick Parylene C etching with the feature size down to 2 μm. The advanced SOOE recipes will further facilitate the controllable fabrication of Parylene C microstructures for broader applications.

Parylene C autofluorescence for on-chip highest processing temperature sensing

This paper reported an on-chip sensing method to record the highest processing temperature that the structure experienced by using the autofluorescence of Parylene C. Temperature properties of Parylene C autofluorescence was studied, that with the increment of processing temperature, the fluorescence intensity kept increasing. Therefore, the Parylene C was deposited on the chip as a temperature indicating label to sensing the highest temperature in a plasma etching process. Test results showed that the heating effect of 5min to 25min plasma etching was successfully presented by the Parylene C autofluorescence. As the deposition process of Parylene C was conformal and fabrication compatible, this method was promising to achieve temperature sensing on micron and submicron scaled structure.

Bonding-friendly pcPDMS: Depositing Parylene C into PDMS matrix at an elevated temperature

This paper reported a simple and effective process of bonding-friendly Parylene C-caulked PDMS (pcPDMS) for low-permeability required microfluidics. Parylene C was deposited into PDMS matrix at an elevated temperature (higher than 135°C) to caulk the permeable sites. The so-prepared pcPDMS can be directly bonded with oxygen plasma treatment just as pristine PDMS. SEM EDAX and Laser scanning confocal microscopy (LSCM) were introduced to characterize the Parylene C caulked status in the PDMS matrix based on the specific Cl element component and the firstly-found temperature-sensitive autofluorescence of Parylene C. The preliminary results indicated that the present bonding-friendly pcPDMS can successfully suppress the diffusion of small molecules into the PDMS matrix.

3D morphology reconstruction of high aspect ratio MEMS structure by using autofluorescence of Parylene C

This paper reported a MEMS fabrication compatible, damage free method for in-process 3D morphology reconstruction of high aspect ratio microstructure. As a novel morphology tracer, Parylene C thin film was conformally deposited onto the structure and annealed at high temperature under N2. The autofluorescence of Parylene C was considerably enhanced by the annealing, which made it possible to image the microstructure. By scanning with a confocal microscopy, 3D morphology of the microstructure was reconstructed. The preliminary result indicated that microstructure with width of 8 μm and depth of 34 μm (34.1 μm actual depth by SEM) was successfully and accurately measured by this method.

Advanced conformal parylene fabrication for micro/nano devices

Herein, we reported several advanced parylene fabrication techniques for various micro/nano devices by taking advantages of the conformal deposition capability and overcoming related restrictions when depositing in high aspect ratio structures, including deep-trench filling by Parylene C for thermal isolation in silicon microdevices, parylene molding technique for high porosity filter membrane preparation, Parylene C caulked PDMS (pcPDMS) for low permeability microfluidics applications, and ultra-thin parylene deposition for flexible electronics.