Membrane Systems

For Bioartificial Organs and Regenerative Medicine

Loredana De Bartolo, Efrem Curcio, Enrico Drioli

• Publisher: De Gruyter
• Year: 2017
• Print ISBN: 9783110267983
• eBook ISBN: 9783110390889
• Edition: 1
• Topic: Medicine
• Subtopic: Biochemistry in Medicine

Membrane processes today play a signifi cant role in the replacement therapy for acute and chronic organ failure diseases. Current extracorporeal blood purifi cation and oxygenation devices employ membranes acting as selective barriers for the removal of endogeneous and exogeneous toxins and for gas exchange, respectively. Additionally, membrane technology offers new interesting opportunities for the design of bioartificial livers, pancreas, kidneys, lungs etc.

This book reviews the latest developments in membrane systems for bioartificial organs and regenerative medicine, investigates how membrane technology can improve the quality and efficiency of biomedical devices, and highlights the design procedures for membrane materials covering the preparation, characterization, and sterilization steps as well as transport phenomena. The different strategies pursued for the development of membrane bioartifi cial organs, including crucial issues related to blood/cell-membrane interactions are described with the aim of opening new and exciting frontiers in the coming decades.

The book is a valuable tool for tissue engineers, clinicians, biomaterials scientists, membranologists as well as biologists and biotechnologists. It is also a source of reference for students, academic and industrial researchers in the topic of biotechnology, biomedical engineering, materials science and medicine.

1Natural and synthetic membranes

1.1Biomedical polymers used for membranes

Several compounds can be used for preparing membranes. They can be classified on the basis of: (i) the source of natural or synthetic materials, if they are from nature or synthesized in the laboratory, respectively; (ii) the chemical nature as organic, inorganic, and composite; (iii) degradation properties as biodegradable and nonbiodegradable. The most widely-used compounds are organic polymers or macromolecules. Synthetic polymers have a wide variety of properties and uses and are produced commercially. In particular, biodegradable synthetic polymers have become increasingly popular for use in biomedical applications. Man-made polymers that react to their surroundings are known as smart polymers, or stimulus-responsive polymers, and can also be applied for a variety of purposes in technology and biomedicine.

The first and most important attribute of a polymer is the identity of the monomer residues (repeat units), on which depends the nomenclature. Polymers that contain only a single type of repeat unit are known as homopolymers, while polymers containing a mixture of repeat units are known as copolymers [1]. An example of a homopolymer is poly(styrene), which is composed only of styrene monomer residues, whereas ethylene-vinyl acetate that contains more than one variety of repeat units is a copolymer. Some biological polymers such as polynucleotides (e.g., DNA) are composed of a variety of nucleotide subunits, which are different but structurally related to monomer residues.

In a polymer the number of monomers determines the molecular weight: the larger the number of monomers, the larger the molecular weight. A common means of expressing the length of a chain is the degree of polymerization, which quantifies the number of monomers incorporated into the chain [2, 3]. Synthetic methods include step-growth polymerization, chain-growth polymerization, and plasma polymerization. In chain growth polymerization, monomers are added to the chain one at a time only (e.g., polyethylene), whereas in step-growth polymerization chains of monomers may combine with one another directly (e.g., polyester) [4]. In plasma polymerization monomers are activated by a gas discharge generated by plasma sources in order to initiate polymerization. Synthetic polymerization reactions may be carried out with or without a catalyst. Since synthetic polymerization techniques typically yield a polymer product in a range of molecular weights, the weight is often expressed statistically to describe the distribution of chain lengths present. Common examples are the number average molecular weight and weight average molecular weight. The ratio of these two values is the polydispersity index, commonly used to express the “width” of the molecular weight distribution [5].

The physical properties of a polymer are strongly dependent on the size or length of the polymer chains. For example, as chain length is increased, melting and boiling temperatures increase quickly [6]. Impact resistance also tends to increase with chain length, as does the viscosity, or resistance to flow, of the polymer in its melt state. Chain length is related to melt viscosity: a tenfold increase in polymer chain length results in a viscosity increase of over 1000 times. The increase of chain length decreases chain mobility and increases strength, toughness, and the glass transition temperature (Tg) as a result of the increase in chain interactions such as van der Waals attractions and entanglements. These interactions tend to fix the individual chains more strongly in position and resist deformations.

The microstructure of a polymer (sometimes called configuration) relates to the physical arrangement of monomer residues along the backbone of the chain. These are the elements of polymer structure that require the breaking of a covalent bond in order to change. Structure has a strong influence on the other properties of a polymer. For example, two samples of natural rubber may exhibit different durability, even though their molecules comprise the same monomers.

An important microstructural feature of a polymer is its architecture; the polymer can be either linear or branched [3]. A branched polymer molecule is composed of a main chain with one or more substituent side chains or branches. A polymer’s architecture affects many of its physical properties including, but not limited to, solution viscosity, melt viscosity, solubility in various solvents, glass transition temperature, and the size of individual polymer coils in solution.

The typical states of polymers are the glassy, rubbery, and semicrystalline state. The threshold temperature below the polymer is glassy (hard and brittle) and above the polymer becomes rubbery (elastic and flexible) is the glass transition temperature or Tg. In the glassy state the mobility of the polymeric chains is very restricted since the segment cannot rotate freely around the main chain bonds. In the rubbery state the segment can rotate freely along the main chain bonds, implying a high degree of chain mobility. The state of the polymer is important for its mechanical, chemical, thermal, and permeation properties. In the case of porous membranes the choice of polymer has an effect on chemical and thermal stability, on wettability as well as on the surface characteristics of adsorption and interactions. In the case of dense nonporous membranes the state of polymer has a strong effect on the permeability of gases and vapors through the membrane. The permeability is generally much lower in the glassy state than in the rubbery state [7].

Semicrystalline materials such as polyamides do not exhibit a clear Tg or “rubbery” region. For these polymers the main transition occurs at Tm when the crystalline regions break down. Some chain rotation in the amorphous regions will occur below Tm, giving some impact resistance at these temperatures. Values of Tg and Tm for a number of polymers are given in Table 1.1. Tg values are affected by the chemical structure of polymers. For example polymers with a main chain characterized by –C–C– bond are more flexible and have low Tg with respect to those having –C–O– bond. The presence of aromatic groups or unsaturated bonds in the main chain increases the Tg values. In the case of polymers that contains alternating unsaturated and saturated bonds the Tg value does not change significantly, since the rotation around the saturated bonds –C–C– compensates the stiffness of unsatura...

Table of contents

• Cover
• Title page
• Copyright
• Preface
• Contents
• 1 Natural and synthetic membranes
• 2 Basic issues in membrane separation for biomedical devices
• 3 Artificial organs
• 4 Blood-membrane interactions
• 5 Engineering of membrane bio-hybrid organs
• 6 Cell-membrane interactions
• 7 Membrane bioartificial organs
• 8 Regulatory framework and ethical issues
• Index