**4.1. Direct incorporation of membranes into microfluidic devices**

The commercial membrane can be incorporated into the microfluidic devices directly. The membrane can be fabricated as per the requirement by following the traditional membrane fabrication techniques described above and then bonded to the microfluidic chip. Russo et al. [46] directly incorporated polymeric membrane into silicon-based lab-on-chip device. Silicon substrate coated with a thin nitride film was used to serve as a support structure for the track-etched membrane. Patterning was conducted by UV exposure through chrome glass mask and CF4 reactive ion etching to transfer the pattern to the nitride layer. The process was repeated on the other side of the wafer by using the second mask with pores on it. The membrane was finally incorporated into the PDMS device.

The membrane can be placed between two microfluidic chips and make a sandwiched structure. This is also another way of using a membrane directly in the microfluidic devices. By using this technique, a three-dimensional microfluidic network was designed by Ismagilov et al. [47] to investigate the interactions of chemical and biochemical reagents. They used a polycarbonate membrane between two PDMS microfluidic devices to make the sandwiched structure.

Membrane integrated with microfluidic device plays an essential role in the medical application [48–51]. To study the complex phenomena inside the vascular system different types of membrane with the microfluidic devices are used. A microfluidic device was fabricated by sandwiching polyester membrane between microfluidic chips and used to study the interaction of cancer cells with a vascular endothelium and to prevent the metastatic disease [49]. A membrane with microfluidic device was also used to demonstrate the lungs injury [50] by toxic substances [51]. Huh et al. made a microfluidic airway system with an approximate diameter of respiratory bronchioles (narrowest airways of the lungs) to explore the cellular-level lung injury [50]. To make a sandwich structure of a membrane in a microfluidic device, bonding of the membrane and the device is a critical issue to deal with the leakage. PDMS mortar film, which is made by mixing PDMS and toluene, can be used to effectively make the bond [52, 53]. Young et al. [52] fabricated such kind of devices to measure the biomolecule permeability across the porous membrane. PDMS prepolymer was cured, and 3 mm diameter holes were punched through the cured PDMS. PDMS mortar layer was then generated on a glass support, and the PDMS substrate was placed on the support so that the holes were not in contact with the mortar layer. On the other side, the membrane was pressed down into the mortar layer. Finally, the membrane was placed between two substrates and bonded with PDMS mortar layer. Using an additional PDMS separator with the membrane can be another way to prevent the leakage [54].

#### **4.2. Membrane fabrication as a part of the microfluidic device fabrication**

**4. Membranes in microfluidic devices**

brane was finally incorporated into the PDMS device.

mask and CF4

300 Microfluidics and Nanofluidics

**4.1. Direct incorporation of membranes into microfluidic devices**

of the equal diameter of 50 μm. Figures are taken with permission from Ref. [6].

The commercial membrane can be incorporated into the microfluidic devices directly. The membrane can be fabricated as per the requirement by following the traditional membrane fabrication techniques described above and then bonded to the microfluidic chip. Russo et al. [46] directly incorporated polymeric membrane into silicon-based lab-on-chip device. Silicon substrate coated with a thin nitride film was used to serve as a support structure for the track-etched membrane. Patterning was conducted by UV exposure through chrome glass

**Figure 5.** (a) Schematic of an microfluidic chip, (b) top view of the micropillar array, and (c) SEM image of micropillars

repeated on the other side of the wafer by using the second mask with pores on it. The mem-

The membrane can be placed between two microfluidic chips and make a sandwiched structure. This is also another way of using a membrane directly in the microfluidic devices. By using this technique, a three-dimensional microfluidic network was designed by Ismagilov et al. [47] to investigate the interactions of chemical and biochemical reagents. They used a polycarbonate

Membrane integrated with microfluidic device plays an essential role in the medical application [48–51]. To study the complex phenomena inside the vascular system different types

membrane between two PDMS microfluidic devices to make the sandwiched structure.

reactive ion etching to transfer the pattern to the nitride layer. The process was

The membrane can be fabricated as a part of a microfabrication process instead of using the traditional membrane fabrication technique. Karnik et al. fabricated a composite membrane of copper, aluminum, spin-on-glass (SOG), and palladium for the water gas shift reaction experiment [55]. Silicon nitride was deposited on both sides of silicon wafers by chemical vapor deposition process. A thin layer of aluminum acted as an adhesive layer of the palladium. Photolithography and wet etching were used to pattern holes on the copper-aluminum layer and to obtain a microchannel. Ookawara et al. [56] fabricated a microchannel as a microseparator for oil and water separation. They made 10 mm curved radius and 112 μm width slits on 80 μm thick SUB308 plates by photolithography. A stack was made by putting the plates with and without slits in turn and diffusion bonded to make microchannel feature. Heyderman et al. [28] fabricated nanopore membrane chip by combining the techniques of hot embossing and photolithography. Silicon (Si) master mold with nanopore arrays was fabricated by using electron beam lithography, and the pores were replicated on PMMA by a hot embossing technique. Various etching processes were used to transfer the pores on Si<sup>3</sup> N4 to fabricate the final nanopore membrane. Though they used PMMA resist with chromium, Si, and silicon nitride (Si<sup>3</sup> N4 ), the final membrane they obtained was made of Si<sup>3</sup> N4 . The pores diameter varied from 100 nm to 450 nm of Si<sup>3</sup> N4 nanopore membrane. To analyze and separate the biological cell Dong et al. fabricated micromachined separator with soft magnetic micropillar arrays that could act as a membrane to observe the performance of the cell separation [57]. A membrane with embedded channel was used to study the hydrodynamic behavior and the fouling formation on the membrane during filtration of synthetic wastewater made of polystyrene particle [58]. They used square-shaped silica capillaries to template the membrane. Polyvinylpyrrolidone (PVP) and N-methylpyrrolidone (NMP) were used as polymer and solvent for membrane preparation, respectively. The silica capillaries were glued to a glass plate and the polymer solution was cast on the glass plate at room temperature. The structured membrane was then kept in a vapor bath and tap water bath for coagulation and phase separation. After making the final structured membrane the silica capillary is placed in the channel of the membrane. The membrane was then placed between two lamination sheets to seal the chip.

the experiment and computer simulation. The research on microfluidic membrane mimic has been mainly focused on fouling phenomena in porous media. For instance, Marty et al. [34] fabricated microfluidic devices with straight, interconnected and staggered channels to observe the biofouling nature in the microfluidic device due to biofilm. They studied the

> **technique of membrane**

Commercial membrane

Soft lithography

membrane and MEMS fabrication

photolithography

Soft lithography

membrane

**Incorporate membrane in microfluidic device**

Microfluidic Membrane Filtration Systems to Study Biofouling

http://dx.doi.org/10.5772/intechopen.75006

303

casting and sandwich the membrane in between the microfluidic devices

Membrane fabrication as a part of microfluidics device fabrication

**Different types of applications**

**1.**Biological analysis

**3.**Medical Application **4.**Fouling characterization

**1.**Oil-water separator **2.**Magnetic micro separator

**3.**Fouling analysis **4.**Biofouling study

**2.**Investigate chemical or biochemical interaction

**Membrane pore size Fabrication** 

Cellulose acetate [46] MWCO: 350 Da [46] Casting Direct

3–8 μm [58]

10 μm thick [47]

10 μm thick and 3 μm and 20 μm pore [53]

effective diameter [51]

200 μm or 170 μm and Depth: 50 μm [33, 34] Constriction: 20 μm smallest width: 50 μm

**Table 2.** Summary of different types of microfluidic membrane device fabrication.

8 μm [48] Track etching

60, 200 and 500 nm [55] Composite

Micro-slit, 112 μm [28] Hot embossing &

Polycarbonate [47, 53] 0.1–1 μm vertical pore

Polyamide [54] RO: MWCO: 200DA [54]

PDMS [51] 10 μm thick and 10 μm

SUS304 Plate[56] 500, 330, 140 nm [56] PDMS [33, 34, 60] width: 10 μm, Length:

[60]

Cellulose ester [61] 5 μm [61] Commercial

1 μm [52]

Polyester [49, 50, 53] 400 nm [49, 50]

**Different materials and polymers used in the fabrication process**

Polyetherimide (PEI), Polyvinylpyrrolidone

N-methylpyrrolidone

(PVP), and

(NMP) [58]

Polyethylene terephthalate (PET) [48]

[52]

Cyclopore polycarbonate regular and thin clear, nuclepore polycarbonate

Copper, aluminum and palladium [55]

PMMA, Si3N4, Si, Si3N4

and Cr [28]\*
