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The physicochemical properties of liposomes and rigidity vary with the bilayer
components and phospholipids used [20]. For instance, unsaturated phospholipids,
such as soybean phosphatidylcholine, form permeable liposomes and less stable
bilayer, whereas unsaturated phospholipids with long acyl chains, such as dipalmi-
toylphosphatidylcholine, form liposomes that are rigid and impermeable.
Phospholipids are esters of fatty acids, phosphoric acids, glycerol, and other
alcohols, for example, phosphatidylcholine, phosphatidylethanolamine, phosphati-
dylinositol, and phosphatidylserine. These all consist of fatty acids and esters at one
or two positions of the glycerol molecule with phosphate group esteried at
positionthree.
The rst-generation liposomes, composed of phospholipids and cholesterol,
could reduce toxicity and improve therapeutic index of entrapped drug. However,
these liposomes were prone to rapid engulfment by reticuloendothelial system
(RES). In order to circumvent this issue, stealth liposomes are designed by coating
liposome with polyethylene glycol, i.e., PEGylated liposomes. PEGylated lipo-
somes have increased circulation time and increased stability of drug by non-
engulfment [21], thus maximizing bioavailability of drug.
A variety of therapeutic agents can be loaded in liposomes, such as anticancer
drugs, biologicals, and antimicrobials. The hydrophilic molecules are typically
loaded into the aqueous core, while lipophilic substances are loaded in the bilayer
membrane of liposome [22]. The payload can also be physically or covalently
attached to the surface of liposomes. Table.14.4 represents some of the examples of
commercially available liposomal products for drug delivery.
Niosomes
Niosomes are lamellar structures (nonionic surfactant vesicles) formed by com-
bination of nonionic surfactant with cholesterol [23]. Niosomes are similar to lipo-
somes in terms of morphology; however, instead of phospholipids, nonionic
surfactant is used in niosomes. Consequently, the physicochemical properties of
niosomes differ from that of liposomes, as summarized in Table14.5.
The major components of niosomes are nonionic surfactant and cholesterol; sur-
factant is responsible for formation of lamellar structure, while cholesterol provides
rigidity and stability to the shape of niosome. The nature of surfactants inuences
encapsulation efciency, toxicity, and stability of niosomes. Nonionic surfactants of
the Span® series and Tween® series are generally used for the preparation of nio-
somes [24]. The properties of niosomes, such as size, surface charge, lamellarity,
and trapped volume, can be varied by changing the composition. For instance,
incorporation of dicetyl phosphate, phosphatidic acid etc., in the membrane can
impart negative charge, while stearylamine, cetylpyridinium chloride, etc. can form
positively charged niosomes [24]. The surfactant/lipid and surfactant/water ratios
are critical considerations for formulation designing, since encapsulation efciency
of niosomes is greatly affected by these parameters [25]. A surfactant/lipid ratio of
1/2.5 w/w is typically used for formation of stable niosomes.
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14.3.2 Inorganic Nanoparticles
A variety of inorganic materials have been investigated for biomedical applications.
For some of the inorganic materials, e.g., gold, silver, iron oxide, quantum dots, etc.,
nanonization imparts unique electronic properties compared to their parent bulk
material. These properties have been extensively exploited for diagnosis and thera-
peutic purposes.
Gold nanoparticles and iron oxide nanoparticles are two examples of inorganic
nanoparticles that have a long history of use in biomedical eld. Therefore, these
nanoparticles will be explained in detail in the coming section.
14.3.2.1 Gold Nanoparticles
Gold nanoparticles (AuNPs) have centuries-old history in chemistry and biological
sciences [26]. The experiment proposed by Michael Faraday, around 150years ago,
has been considered a baseline for the modern era of AuNP synthesis. Moreover, in
1971, the British researchers Faulk and Taylor [27] developed antibody conjugation
of salmonella antigen with colloidal gold surface coating for direct electron micros-
copy visualization. Since then, the use of colloidal gold has been greatly increased
in different scientic domains such as chemistry, biology, engineering, and
Table 14.4 Examples of liposomal products available for clinical applications
Brand name Drug Indication
Ambisome™ Amphotericin B Fungal infection
DaunoXome™ Daunorubicin Kaposi’s sarcoma
Doxil™ Doxorubicin Ovarian cancer, AIDS-related Kaposi’s sarcoma, multiple
myeloma
Visudyne® Verteporn Age-related macular degeneration, pathologic myopia, and
ocular histoplasmosis
Myocet® Doxorubicin Recurrent breast cancer
DepoCyt® Cytarabine Neoplastic meningitis and lymphomatous meningitis
Lipoplatin® Cisplatin Epithelial malignancies
DepoDur® Morphine
sulfate
Postoperative pain following major surgery
Table 14.5 Some of the differences between niosomes and liposomes
Niosomes Liposomes
– Primarily composed of nonionic
single-chain surfactant and cholesterol
– Prepared from neutral or charged double-chain
phospholipids
– Economical and stable compared to
liposomes
– Phospholipids are expensive and chemically
unstable. Therefore, liposomes require special storage
condition
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medicines. Lately, AuNP has been applied to a variety of medical applications, for
example, gene delivery, radiation therapy,and diagnosis, etc.
The presence of plasmon absorbance bands and their shape- and size-dependent
properties make gold NPs a versatile material for diverse applications in many elds
(as summarized in Table14.6).
For biomedical applications, Aurasol® was the rst product based on AuNPs
that was developed for the management of rheumatoid arthritis. However, it is no
more used. The antiangiogenic property of AuNPs was later exploited for the devel-
opment of Aurimune® that has recently completed phase 1 trials and has shown
good tolerability and tumor deposition. The paclitaxel-loaded Aurimune® platform
is under preclinical development stage. However, the only AuNP-based product
approved for tumor elimination to date is AuroLase® that uses silica nanoparticles
coated with gold nanoparticles.
14.3.2.2 Iron Oxide Nanoparticles (IONPs)
For the past 90years, materials such as iron oxides, iron hydroxide, or their combi-
nation have been extensively used in biomedical eld due to their unique properties,
such as magnetic properties, superparamagnetic properties, and biodegradability.
IONPs have been used in iron replacement therapy, magnetic resonance imaging
(MRI), biosensors, targeted drug delivery, and cell separation [29].
When the size of iron oxide materials is reduced to nanometer range, it exhibits
superparamagnetic properties. These particles are referred to as superparamagnetic
iron oxide NPs (SPIONs) [30]. In this connection maghemite (γ-Fe
2
O
3
), hematite,
(α-Fe
2
O
3
) and magnetite (Fe
3
O
4
) have proven experimentally to be highly magnetiz-
able when external magnetic eld is applied. However, SPIONs tend to agglomerate
in magnetic eld; therefore, effective surface stabilization is required. Typically,
fatty acids, PEG, poly(N-vinylpyrrolidone), dextran, chitosan, etc. are used as coat-
ing agents [31].
The early parenteral formulation of colloidal iron oxide for anemia was stabi-
lized with dextran. However, due to rapid release of elemental iron, the formulation
raised toxicity concerns [32]. Therefore, thick coating with carbohydrate was intro-
duced to mitigate the rapid release of elemental iron in the circulation, e.g., Imferon®
(Fisons) or Dexferrum®. Later on, low molecular weight dextran, sucrose,
Table 14.6 Application of AuNPs on the basis of their properties, adopted from [28]
Properties Major eld of application
Redox activity Electrochemical sensing and electronic
devices
Surface-enhanced Raman scattering (SERS) Imaging and sensing
Surface plasmon resonance (SPR) Colorimetric analysis and photothermal
therapy
Fluorescence quenching Sensing and material fabrication
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gluconate, etc. were introduced for stabilization of iron oxide formulations, and
consequently, the products were less toxic and exhibited little immunogenic reac-
tion [33].
Modern intravenous iron formulations are iron oxide/hydroxide nanoparticles
stabilized with carbohydrates [34]. A variety of iron oxide-based formulation are
available for treatment of anemia, such as Ferinject® and Injectafer®, containing
ferric carboxymaltose, which is commercialized by Vifor Pharma and Daiichi
Sankyo. Other examples are Venofer® containing iron-sucrose complex manufac-
tured by Vifor Pharma, Monofer® containing iron isomaltoside marketed by
Pharmacosmos in Europe, and Feraheme®/Rienso® containing ferumoxytol as
SPIONs, manufactured by AMAG Pharmaceuticals.
Additionally, SPIONs have been used as contrast agent for MRI.For example,
Feridex® and Endorem® containing ironoxide stabilized with dextran are used for
imaging of liver and spleen lesions. Other examples include Resovist® and
Cliavist® containing ferucarbotran with carboxydextran, marketed by Bayer
Healthcare, used for imaging of liver lesions. Additionally, Ferrotran®, previously
knowns as Combidex based on Ferumoxtran-10, is the only MRI contrast agent that
can detect lymph node metastases as small as 2mm [35].
Besides being used as a diagnostic agent, SPIONs have also recommended for
hyperthermia treatment of cancer. Since SPIONs have high magnetic saturation,
therefore it is also employed in the target site to damage cells locally by increasing
temperature. Based on the aforementioned principles, a new product is introduced
into the market known as NanoTherm® (MagForce), which is also available in
Europe since 2018. These tiny magnetic nanoparticles are introduced either locally
into tumor, which is then heated by external magnetic eld, named as NanoActivator®,
which destroys the cancer cells.
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S. A. Khan (ed.), Essentials of Industrial Pharmacy, AAPS Advances in the
Pharmaceutical Sciences Series 46, https://doi.org/10.1007/978-3-030-84977-1_15
Chapter 15
Polymer forBiomedical Applications
IqraRiasat, MuhammadNaeem, MuhammadUmarAslamKhan,
SyedBabarJamal, AtifAliKhanKhalil, SajjadHaider, andAdnanHaider
Abstract The structureof a polymer depends entirely on itsmonomer. Polymersare
broadly classied into two main groups, natural and synthetic, depending on the
source from which they are derived. Polymershave made a name for themselvesin
all areasof science and especiallyin the biomedical eld due to their wide range of
applications. Among the numerous polymers, biopolymershave attracted the atten-
tion of the scientic community mainlybecause of their biocompatible and biode-
gradable properties. In this chapter, we have summarized information about
polymers, their classication, and last but not least, their potential biomedical appli-
cations. This will help students to understand the basic concepts related to polymers
and especiallybiopolymers and their potential applicationin the biomedical eld.
Keywords Polymeric scaffolds · Biocomposite · Biodegradable · Biocompatible
I. Riasat
Department of Bioinformatics and Biosciences, Capital University of Science and
Technology (CUST), Islamabad, Pakistan
M. Naeem · S. B. Jamal · A. A. K. Khalil · A. Haider (
*)
Department of Biological Sciences, National University of Medical Sciences, Rawalpindi,
Pakistan
M. U. A. Khan
Department of Polymer Engineering and Technology, University of the Punjab, Lahore,
Pakistan
S. Haider (
*)
Department of Chemical Engineering, College of Engineering, King Saud University,
PO-BOX 800, Riyadh, 11421, Saudi Arabia
e-mail: shaider@ksu.edu.sa
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15.1 Introduction
A polymer is a large molecule made up of small repeating units, called mono-
mers.Monomers are formed into a large molecule by a process called polymeriza-
tion. Polymers (synthetic and natural)have a positive impact on daily lifedue to
their numerous applications in various elds. Thecentral role of polymersis evident
in from the fact thateverything from plastics to proteins and nucleic acids are com-
prised of these natural and synthetic polymers. Polymerization is the process by
which small repeating units are combined/linked in such a way that they stack up
and form long-chain structures called polymers [1].Due to their unique physical
and chemical properties (tensile strength, viscoelasticity, biocompatibility, biode-
gradability, ease of moulding), they areused in almost all elds of science [2, 3].
The word “polymer” is derived from the Greek words polus (means “many) and
meros (means “part”), referring to molecules containing repeating units. Thus, a
polymer is dened as a large molecule made up of small repeating units, called
monomers, by a process called polymerization [1]. Jöns Jacob Berzelius in 1833
used the termalthough his denition was different from the IUPAC system. It was
Hermann Staudinger in 1920 who from his ndings proposed that polymers are
covalently bonded macromolecules [4]. Recently, however, researchershave been
exploringnon-covalently bonded supramolecular polymers [5, 6].
The number of monomers, types of monomers, and their orientation in a polymer
structure may vary from polymer to polymer. The properties of polymer can be
tailored by the polymerization process. The simplest polymer is formed by bonding
just the same type of monomer, and the polymer is called homopolymer. Similarly,
when different types of monomers are combined, the resultant polymer is calledhet-
ero and copolymer. Copolymers can vary in structure, composition, and the mono-
mer ratio, which ultimately dene the chemical and physical properties of the
copolymer. The interlinking of polymer chainsmolecules to one another is called
cross-linking, for instance, rubber bands and polyethylene bags.
15.2 Classication ofPolymers
There are different types of polymer classications. For instance, in 1929 Carothers
classied polymers on the basis of structure: addition polymers and condensation
polymer [7]. In another classication, polymers are simply categorized as linear
polymers, branched polymers, or cross-linked polymers (Fig.15.1).
In the context of this chapter, classication of polymer on the basis of degrad-
ability will be more relevant: biodegradable polymers and nonbiodegradable poly-
mers. However, here the focus is primarily on biodegradable polymers, since these
polymers have more applications in drug delivery.
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15.2.1 Biodegradable Polymers
Previously nonbiodegradable polymers were more common in the biomedical eld.
However, the problem with such polymers was that they would remain in the body
forever and would need surgical removal after the desired therapeutic objective is
achieved. This was the reason thatscientists started focusing on biodegradable poly-
mers, which when introduced to the bodyare degraded (by hydrolysis, enzymatic
degradation, or combination of both).
Another critical property of polymers for biomedical use is biocompatibility. The
polymer itself or its metabolites must not have toxic effects on the body [7].
Polymers for biomedical application are further classied on the basis of their
origin; they are of two types: natural polymers and synthetic polymers.
15.2.1.1
Natural Biodegradable Polymers
Polymers that have a natural origin are known as the natural polymers. These poly-
mers are obtained from plants, animals, or microorganisms and are also called bio-
polymers. These polymers include proteins, polynucleotides, and polysaccharides
[5, 6]. Biopolymers received a lot of attention because of their benecial properties
such as biodegradability, lack of toxicity, availability, low cost, and, most impor-
tantly, biocompatibility.
Biopolymers have wide range of applications in various elds and more speci-
cally in the pharmaceutical industry [8]. In the pharmaceutical industry, biopoly-
mers are used as coating, gelling, emulsifying agents,binder,and disintegrantsin
capsule and tablet formulation.
Some of the common biopolymers are summarized in Table15.1. One of the
most abundant natural polymers is cellulose, which is obtained from plants and
Fig. 15.1 Representation of polymer types based on the linkage of monomers, linear polymer,
branched polymer, and cross-linked polymers
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mostly used in the laboratories for the fabrication of bers, clothes, cosmetics,
paper, pharmaceutical formulations, etc. Some of the semisynthetic forms of cellu-
lose are also prepared in laboratories that include cellulose ethers and cellulose
esters, which like the pristine cellulose have found application in preparation of
llers, binders, compressibility enhancers, and gelling agents [2, 9].
However, biopolymers show relatively higher susceptibility toward microbial
contamination when they are exposed to the external environment [10–13].
Moreover, it is difcult to purify polymers once it gets contaminated with impurities
during extraction [11].
15.2.1.2 Synthetic Biodegradable Polymers
Synthetic polymers are man-made and are considereda relatively new class of poly-
mers,having been introduced only in the early twentieth century. These polymers
are synthesized in the laboratorythroughchemical reactions thatallowsthe fabrica-
tion of a wide variety of polymers [7, 14].
Biocompatible synthetic polymers offer a wide range of applications in the bio-
medical eld,such as drug delivery, hemodialysis, tissue engineering, etc. Polymers
with the ability to respond to certain internal or external stimuli, such as tempera-
ture, pH, and ionic strength, can be used in stimulus-responsive drug delivery sys-
tems [15].
Synthetic biodegradable polymers typically have unstable bond links in the
backbone, usuallya carbonyl bond attached to an oxygen, nitrogen, or sulfur atom
(Fig.15.2). Depending on the attached functional groups, a wide range of polymers
have been synthesized that have shown potentialfor application in the biomedical
eld [16]. Some of the common classes of synthetic biodegradable polymers are
given as follows.
Polyesters The commonly used polymers included polylactic acid (PLA) and
polyglycolic acid (PGA) (structure shown in Fig.15.3). These polymers were ini-
tially used for absorbable sutures.
PLA isan optically activebiodegradablepolymer that can be synthesized in the
laboratory and derived from plant starch, corn, cassava, maize, sugarcane or
Table 15.1 Representation of natural polymers based on their origin
Origin of natural
polymers Polymers
Animal Gelatin, hyaluronan, chitin
Plant Cellulose, starch, hemicellulose, agar, pectin, guar gum, psyllium, gum
acacia, lignin
Microbe Xanthan, gellan, hyaluronan, curdlan
Algae Carrageenan, alginate, agar
Fungus Schizophyllan, cardlan, scleroglucan, pullulan, chitin
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