**3.2. Microfluidic device fabrication**

pores can be obtained. Dense membranes (symmetric and asymmetric) are mainly synthesized by solution casting and interfacial polymerization of two monomers on a substrate. A detailed explanation of membrane preparation techniques is available in the literature [38].

**Pore size Pressure** 

Porous 10−1–10 μm

Porous 10−2–10−1μm

Porous 10−3–10−2μm

Dense/ Porous 10−4–10−3μm **range (bar)**

**Flux range (l.m−2. h−1.bar−1)**

0.1–2.0 >50 Separation of

1.0–5.0 10–50 Separation of

5.0–20 1.4–12 Separation of

10–100 0.05–1.4 Separation

**Application**

macromolecular to cellular size particles (Bacteria/ fat and some proteins)

molecular to macromolecular size particles (all proteins)

Ionic molecular size particles (Lactose)

of ions (all minerals)

**Fabrication technique**

Phase inversion, stretching track etching

Phase inversion, solution wet-spinning

Interfacial polymerization, layer-by-layer deposition Phase inversion

Phase inversion Solution casting

**Table 1.** Summary of different types of pressure-driven membrane processes [38–41].

**Membrane process**

298 Microfluidics and Nanofluidics

Microfiltration (MF)

Ultrafiltration (UF)

Nanofiltration (NF)

Reverse osmosis (RO) **Polymer used in the fabrication process**

**Figure 3.** Preparation methods of polymeric membrane.

Polyacrylonitrile (PAN), polysulfone (PS), poly (phthazine ether sulfone ketone) (PPESK), poly (vinyl butyral) PVDF PES

Polyamides, polysulfones, polyols, polyphenols

Cellulose acetate/ triacetate aromatic polyamide, polypiperzine, polybenziimidazoline

Polyvinylidene fluoride (PVDF), poly (tetraflurethylene) (PTFE), polypropylene (PP), Polyethylene (PE), polyethersulfone (PES)

> There are many types of fabrication techniques available for making micro/nano devices such as photolithography, etching, soft lithography, hot embossing, injection molding, E-beam lithography, and micro-stereolithography. Photolithography and etching are two popular fabrication techniques. Soft lithography is a well-known method for microfabrication. McDonald et al. [42] fabricated microfluidic system with PDMS by a soft lithography technique to make 20–100 μm microfluidic structure. This technique has also worked well on hydrogel polymers (calcium alginate) to fabricate microfluidic network of 100 μm wide and 200 μm deep and 25 × 25 μm cross-section [43]. A complex structure with feature sizes larger than 20 μm can be achieved by using rapid prototyping [44]. The fabrication of 500–2000 μm diameters and 200–1000 μm height cylindrical columns [45] is possible by hot embossing technique. A schematic diagram of a microfluidic device is shown in **Figure 5**. This device is used to observe the biofilm behavior and the change of hydrodynamics of the fluid flow through the channel [6]. The chip has one inlet and one outlet and is made by traditional photolithography using polydimethylsiloxane (PDMS).

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].

Microfluidic Membrane Filtration Systems to Study Biofouling

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

301

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

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>

fabricate the final nanopore membrane. Though they used PMMA resist with chromium, Si,

N4

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

), the final membrane they obtained was made of Si<sup>3</sup>

and silicon nitride (Si<sup>3</sup>

N4

diameter varied from 100 nm to 450 nm of Si<sup>3</sup>

N4 to

. The pores

N4

nanopore membrane. To analyze and separate

**Figure 5.** (a) Schematic of an microfluidic chip, (b) top view of the micropillar array, and (c) SEM image of micropillars of the equal diameter of 50 μm. Figures are taken with permission from Ref. [6].
