Chapter 2

The Art of Making Polymeric Membranes

K.C. Khulbe*,1, T. Matsuura1 and C. Feng1,2

1IMRI, Chemical and Biological Engineering Department, University of Ottawa, Canada

2Chemical Engineering and Applied Chemistry Department, University of Toronto, Toronto, Canada

*Corresponding author: khulbe@eng.uottawa.ca

Abstract

Polymeric membranes are becoming increasingly important in the field of separation processes in the pharmaceutical industry, and artificial organs. Some polymers are obtained from natural sources (natural polymer) and then chemically modified for various applications, while others are chemically synthesized (synthetic polymer). Polymeric membranes can be fabricated in different configurations such as flat sheet, tubular, hollow fibers, nanofibers, etc., via using different techniques. Membranes can also be fabricated in different forms such as composite, porous, nonporous, supported, mixed matrix membrane (MMMs), etc. Since the performance of the membrane is largely controlled by its surface (active layer), the design of membrane surface and its characterization, either by chemistry or morphology, is extremely important. Hence, emphasis is placed on the membrane surface in this review article. Membrane surfaces can also be modified by different techniques. It is an art to make desirable polymeric membrane.

Keywords: Polymeric membrane, phase inversion, hollow fiber, electrospinning, modification of membranes, graft polymerization, mixed matrix membrane, UV irradiation, tensile strength

2.1 Introduction

The concept of a membrane has been known since the eighteenth century, but was used little outside of the laboratory until the end of World War II. Since then, the U.S. medical membrane device market was valued at $2.1 billion in 2010 and is forecast to grow to $2.9 billion by 2016, with a compound annual growth rate (CAGR) of 5% [1].

A membrane is a selective barrier that allows the passage of certain constituents and retains other constituents found in the liquid [2]. The influent of a membrane is known as the feed-stream, the liquid that passes through the membrane is known as the permeate, and the liquid containing the retained constituents is the retentate or concentrate.

A membrane can be complex in both structure and function. It may be solid or liquid, homogeneous or heterogeneous, isotropic or anisotropic in its structure. A membrane can be a fraction of a micrometer or several millimeters thick. Another characteristic property of membrane is its permselectivity, which is determined by differences in the transport rates of various components in the membrane matrix.

In general, a membrane is a thin, porous film (polymeric, metal, paper, etc.) through which a fluid (liquid, gas) is filtered in order to carry out separation. Types of filtration are subdivided according to the size of the particle or technology used.

i. Microfiltration membranes have pores ranging between 0.1 and 10 µm. The membranes thus eliminate all of the bacteria. Viral contamination is partially retained by this process, although the viruses are much smaller than the pores of this type of membrane. Indeed, the viruses can bind to bacterial biofilm. Microfiltration is thus used in many treatment processes where particles of a diameter larger than 0.1 µm must be eliminated. ii. Ultrafiltration membranes have pores from 0.001 to 0.1 µm and are used for complete elimination of viruses. The recently developed technique of nanofiltration is used primarily in the process of water purification, such as softening (particle and salt removal) dye removal, and elimination of micropollutants. iii. The last type of filtration to consider is reverse osmosis.

Membranes can be made from any materials such as glass, polymer, metals, etc. Polymeric membranes are becoming increasingly important in the field of separation processes in the pharmaceutical industry and artificial organs. In the field of artificial organs, many polymeric materials have already been used; e.g., artificial hearts are designed using polyurethane, artificial kidneys using cellulose and polysulfone, and membrane oxygenators using porous polypropylene. Polyethersulfone membranes are widely employed in biomedical fields such as artificial organs and medical devices used for blood purification; e.g., hemodialysis, hemodiafiltration, hemofiltration, plasmaphoresis and plasma collection [3,4]. Synthetic membranes are of major importance in the medical field, in particular in a number of life-saving treatment methods. Membranes are used in drug delivery, artificial organs, tissue regeneration, diagnostics devices, as coating medical devices, in bioseparation, etc. The biggest part of the medical market involves membranes for use in drug delivery, hemodialysis, other artificial organs (oxygenation, pancreas, etc.) and tissue engineering.

Collagen is a naturally occurring or biologic polymer, and is widely used in medical science or surgery. It comprises the most substantial group of structural proteins in connective tissue and represents one third of total body proteins. A search for a simplified synthetic alternative to collagen began in the early 1960s [5]. Prior to that time, collagen was the only commercially available biodegradable material. Schmitt and Palistina [6] reported that the alpha polyester analogs of the alpha polyimides are bioresorbable, and they are useful materials for surgical applications. Now, there are several synthetic polymeric materials that are available for applications in the medical/pharmaceutical field. Synthetic polymers have several advantages over naturally occurring materials, which are as follows:

i. They can be prepared reproducibly under carefully controlled conditions. ii. If needed, they are available in almost unlimited quantities. iii. They are available with a wide range of physical, chemical and mechanical properties. These properties can be altered by modification.

Synthetic membranes are fabricated in two main geometries:

i. Flat sheet - utilized in the construction of flat sheet, disc, spirally wound, plate and frame modules. ii. Cylindrical - utilized in tubular and capillary, or hollow fiber, modules.

Today membranes play a dominant role in medical applications such as artificial kidney and blood oxygenator capillary.

As the latest development of membranes for biomedical applications, nanofibers are attracting much attention. Compared with other materials, the surface area of nanofibers, allowing for the adhesion of cells, proteins and drugs, is much larger. Kanani and Bahrami [7] wrote a review in which they explored the application of electrospun nanofiber scaffolds in biomedical engineering (tissue engineering). Electrospun nanofiber scaffolds can be classified based on their functional applications into wound dressing, drug delivery, blood vessel (vascular tissue engineering), bone tissue engineering, heart tissue engineering, cartilage tissue engineering, etc.

In 1944, Kolff et al. [8] demonstrated the first successful artificial kidney. Since then, the use of membranes in artificial organs has become a major life-saving procedure. Membrane systems are competitive with conventional biological treatment in terms of price and cost.

The choice of a membrane material for medical applications is based on specific physical and chemical properties, since these materials should be tailored in an advanced way to obtain favorable products.

2.2 Types of Membranes

Three basic types of membranes can be distinguished based on structure and separation principles:

i. Porous membranes, ii. Nonporous membranes, iii. Liquid membranes, iv. Asymmetric membranes.

2.2.1 Porous Membranes

These are membranes that have many pores or other small spaces that can hold a gas or liquid or allow it to pass through. Membranes of this class induce separation by discriminating between different particle sizes. Porous membranes are intended for separation of larger molecules such as solid colloidal particles, large biomolecules (proteins, DNA, RNA) and cells from the filtering media. Porous membranes find use in microfiltration, ultrafiltration, and dialysis applications. High selectivities can be obtained when the solute size is large relative to the pore size in the membrane. The selectivity is mainly determined by the pore size in relation to the size of the particles to be separated.

2.2.2 Nonporous Membranes

Nonporous membranes are also known as dense-film membranes. Membranes from this class are capable of separating molecules of approximately the same size of each other. Separation takes place through differences in solubility and/or differences in diffusivity. This means that the intrinsic properties of the polymeric material determine the extent of selectivity and permeability. Small molecules of penetrants move among polymer chains according to the formation of local gaps by thermal motion of polymer segments. The free volume of the polymer, its distribution and local changes in distribution are of the utmost importance. The diffusivity of a penetrant depends mainly on its molecular size. Temperature has a very significant effect on the permeation of gases, etc., from these types of membranes.

2.2.3 Liquid Membranes (Carrier Mediated Transport)

With membranes of this class, transport...