Carbopol and its Pharmaceutical Significance: A Review

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Avinash H. Hosmani

Avinash H. Hosmani

In the recent decades, there has been considerable interest in using Carbopol
as an excipient in a diverse range of pharmaceutical applications.

Carbopol polymers are polymers of acrylic acid cross-linked with polyalkenyl ethers or divinyl glycol. They are produced from primary polymer particles of about 0.2 to 6.0 micron average diameter. The flocculated agglomerates cannot be broken into the ultimate particles when produced. Each particle can be viewed as a network structure of polymer chains interconnected via cross-linking1.

Carbomers were first prepared and patented in 19572. Since then, a number of extended release tablet formulations, which involve carbomer matrices, have been patented3.

Carbomers readily absorb water, get hydrated and swell. In addition to its hydrophilic nature, its cross-linked structure and its essentially insolubility in water makes Carbopol a potential candidate for use in controlled release drug delivery system.4,5

Chemical nature:

The USP-NF, European Pharmacopoeia, British Pharmacopoeia, United States
Adopted Names Council (USAN), and International Nomenclature for Cosmetic Ingredients
(INCI) have adopted the generic (i.e., non-proprietary) name “carbomer” for
various Carbopol homopolymer polymers. The Japanese Pharmacopoeia states, Carbopol
homopolymers as “carboxyvinyl polymer” and “carboxy polymethylene.” The Italian
Pharmacopoeia also identifies Carbopol 934P as “carboxy polymethylene” and the
Deutschen Artzneibuch calls Carbopol 980NF “polyacrylic acid.” Carbopol copolymers,
such as Carbopol 1342 NF and 1382, have also been named “carbomer” by the USP-NF,
but are considered “Acrylates/C10-C30 Alkyl Acrylates Cross polymer” by the
INCI.

Carbopol polymers are offered as fluffy, white, dry powders (100% effective).
The carboxyl groups provided by the acrylic acid backbone of the polymer are
responsible for many of the product benefits. Carbopol polymers have an average
equivalent weight of 76 per carboxyl group6. The general structure
can be illustrated with fig. No.1

Image

Fig. No. 1 – General Structure of Carbopol Polymers.

Image

Fig No. 2 - Schematic drawing of a molecular segment of a cross-linked 
polyacrylic acid polymer

 

Carbopol polymers are manufactured by cross-linking process. Depending upon the degree of cross-linking and manufacturing conditions, various grades of Carbopol are available. Each grade is having its significance for its usefulness in pharmaceutical dosage forms7.

Carbopol 934 P is cross-linked with allyl sucrose and is polymerized in solvent
benzene.

Carbopol 71G, 971 P, 974 P are cross-linked with allyl penta erythritol and polymerized in ethyl acetate. Polycarbophil is cross-linked polymer in divinyl glycol and polymerized in solvent benzene. All the polymers fabricated in ethyl acetate are neutralized by 1-3% potassium hydroxide. Though Carbopol 971 P and Carbopol 974 P are manufactured by same process under similar conditions, the difference in them is that Carbopol 971 P has slightly lower level of cross-linking agent than Carbopol 974 P. Carbopol 71 G is the granular form Carbopol grade8.

Physical Properties 9:

The three dimensional nature of these polymers confers some unique characteristics, such as biological inertness, not found in similar linear polymers. The Carbopol resins are hydrophilic substances that are not soluble in water. Rather, these polymers swell when dispersed in water forming a colloidal, mucilage-like dispersion.

Carbopol polymers are bearing very good water sorption property. They swell in water up to 1000 times their original volume and 10 times their original diameter to form a gel when exposed to a pH environment above 4.0 to 6.0. Because the pKa of these polymers is 6.0 to 0.5, the carboxylate moiety on the polymer backbone ionize, resulting in repulsion between the native charges, which adds to the swelling of the polymer. The glass transition temperature of Carbopol polymers is 105°C (221°F) in powder form. However, glass transition temperature decreases significantly as the polymer comes into contact of water. The polymer chains start gyrating and radius of gyration becomes increasingly larger. Macroscopically, this phenomenon manifests itself as swelling.

Table No.1 Physical and Chemical Properties of Carbopol 10

Appearance

Fluffy, white, mildly acidic polymer

Bulk Density

Approximately 208 kg/m3 (13 lbs.
ft3) *

Specific gravity

1.41

Moisture content

2.0% maximum

Equilibrium moisture content

8-10% (at 50% relative humidity)

PKa

6.0 ± 0.5

pH of 1.0% water dispersion

2.5 - 3.0

pH of 0.5% water dispersion

2.7 - 3.5

Equivalent weight

76 ± 4

Ash content

0.009 ppm (average) **

Glass transition temperature

100-105C (212-221F)

 

* Polymers produced in co solvent (a cyclohexane / ethyl acetate mixture) have a bulk density of 176 kg/m3 (11 lbs/ft3).

* * Polymers produced in ethyl acetate have an ash content (as potassium sulfate) of 1-3% on average.

Rheological properties :

While the relationships between structure and properties have been of interest both academically and in industry. Different grades of Carbopol polymers exhibit different rheological properties, a reflection of the particle size, molecular weight between crosslinks (Mc), distributions of the Mc, and the fraction of the total units, which occur as terminal, i.e. free chain ends11-18.

The molecular weights between adjacent crosslinks (Mc) are approximately inversely proportional to the crosslinker density. These may be calculated from the functionality of the crosslinking monomer, the relative ratio of acrylic acid to crosslinking monomer, and the efficiency of the crosslinking reaction, assuming negligible chain ends4. Alternatively, the molecular weight can be qualitatively compared to the rheological properties of a swollen gel and/or from the equilibrium-swelling ratio. In simple terms, low viscosity, low rigidity polymer, such as Carbopol 941 and Carbopol 971P, have a higher Mc. Conversely, they have lower crosslinker densities. The higher the crosslinker level, the lower the equilibrium swelling ratio. In the network theory of elasticity, the elastic modulus, G, is inversely proportional to the molecular weight between crosslinks (Mc). There have been attempts to extend the elasticity theory to swollen gels19,20. Based on this approach, Taylor calculated Mc for Carbopol 941 in the order of several million. This number is far too high as compared to the theoretical Mc calculated from the stoichiometry. Carnali and Naser estimated the Mc for Carbopol 941 to be 3,300 monomer units (or 3,300 x 72 = 237,600 gm/mole) derived from a combination of dilute solution viscosity and equilibrium swelling.9 The Mc reported for Carbopol 940 was 1,450 monomer units (or 1,450 x 72 = 104,400 gm/mole)4.

Table No. 2 – Viscosity range of different Carbopol Polymers21,22.

No.

Polymer

Viscosity*

1

Carbopol 934 NF

30500 – 39400

9

Carbopol 934 P NF

29400 – 39400

12

Carbopol 71 G NF

4000 – 11000

* Brookfield RVT Viscosity, cP 0.5 wt % mucilage at pH 7.5, 20 rpm
at 25oC

 

Applications of Carbopol polymers:

The readily water-swellable Carbopol polymers are used in a diverse range of pharmaceutical applications to provide:

· Controlled release in tablets1,23-26.

· Bioadhesion in buccal27,28, ophthalmic29,30, intestinal31, nasal32, vaginal33 and rectal34 applications.

· Thickening at very low concentrations to produce a wide range of viscosities and flow properties in topical, lotions, creams and gels, oral suspensions and transdermal gel reservoirs35,36.

· Permanent suspensions of insoluble ingredients in oral suspensions and topicals37.

·  Emulsifying topical oil-in-water systems permanently, even at elevated
temperatures, with essentially no need for irritating surfactants.

Several properties of Carbopol make it potentially valuable as a pharmaceutical
excipient in numerous applications such as:

(1) Controlled release & solid dosage forms 22,23 38-44:

Carbopol is being used in the controlled release solid dosage formulations
since last four decades. The numbers of manufacturers commercializing controlled
release tablets using Carbomers are increasing considerably in recent period
of development. Tablet formulations using Carbopol polymers have demonstrated
zero-order and near zero-order release kinetics. These polymers are effective
at low concentrations (less than 10%). Still they show extremely rapid and efficient
swelling characteristics in both simulated gastric fluid (SGF) and simulated
intestinal fluid (SIF). The Carbopol polymers produce tablets of excellent hardness
and low friability. These polymers can be successfully formulated into a variety
of different tablet forms, including the traditional swallowable tablets, chewable
tablets, buccal tablets, sublingual tablets, effervescent tablets, and suppositories;
providing controlled-release properties as well as good binding characteristics.
Carbomers show larger dissolution times at lower concentrations than other excipients.
Because of these factors Carbopol polymers have greater extent in formulating
dosage forms. Because Carbopol polymers swell rapidly in water and absorb great
quantities, to avoid the use of flammable solvents, roller compaction is being
used as the method to prepare a new form of Carbopol polymer 71G NF. Carbopol
polymer 71G NF is a useful and versatile controlled-release additive for tablet
formulations in direct compression.

Drug Dissolution Mechanism from Carbopol Polymers

In the dry state, the drug is trapped in a glassy core. As the external surface of the tablet is hydrated, it also forms a gelatinous layer upon hydration, however, this gel layer is significantly different structurally from the traditional matrix tablet. The hydrogels are not entangled chains of polymer, but discrete microgels made up of many polymer particles, in which the drug is dispersed. The crosslink network enables the entrapment of drugs in the hydrogel domains. Since these hydrogels are not water soluble, they do not dissolve, and erosion in the manner of linear polymers does not occur. Rather, when the hydrogel is fully hydrated, osmotic pressure from within works to break up the structure, essentially by sloughing off discrete pieces of the hydrogel. It is postulated that as the concentration of the drug becomes high within the gel matrix and its thermodynamic activity or chemical potential increases, the gel layer around the tablet core actually acts almost like a rate-controlling membrane, resulting in linear release of the drug.Because of this structure, drug dissolution rates are affected by subtle differences in rates of hydration and swelling of the individual polymer hydrogels, which are dependent on the molecular structure of the polymers, including crosslink density, chain entanglement, and crystallinity of the polymer matrix. The magnitude and rate of swelling is also dependent on the pH of the dissolution medium. The channels which form between the polymer hydrogels are influenced by the concentration of the polymer, as well as the degree of swelling. Increasing the amount of polymer will decrease the size of the channels, as does an increase in swelling degree. All of these factors must be taken into account to describe the mechanism for release control in tablets formulated with Carbopol polymers.

Benefits in Solid Dosage Application

· Efficient controlled release agents for matrix tablets.

· Improve bioavailability of certain drugs

· Efficient binders in dry as well as wet granulation processes.

· Only granular polymer (Carbopol 71G NF) available for direct compression formulation.

(2) Oral Suspension Applications 8,36,37,45,46:

For many years, Carbopol polymers have been widely used in oral suspensions
to thicken, modify flow properties, suspend insoluble ingredients and provide
bioadhesion. The significance of these polymers is that they eliminate the settling
problem even at low concentrations. As Carbopol polymers swell when hydrated
and neutralized, they form colloidal dispersion.

Carbopol 934 P has been used since mid 1960s. Carbopol 974 P has similar rheological
properties to Carbopol 934 P, as both are highly cross-linked polymers that
produce mucilage with very short flow rheology. Carbopol 971 P provides very
low viscosities and excellent yield values at low usage levels. Suspensions
formed with Carbopol 971 P will have longer rheology. Carbopol 71 G polymers
will give same viscosities and rheology as Carbopol 971 P, but it is easier
to handle and disperse due to its granular nature.

Benefits in Oral Suspension Applications:

  • Long Term Stability of Suspensions over a wide pH range.
  • Highly efficient at low use level.
  • Taste masking of some bitter drugs.
  • Build viscosity and yield value for “non-spill” pediatric formulations.

(3) Bioadhesive Applications 48-59:

Bioadhesion is a surface phenomena in which a material may be of natural or synthetic origin, adheres or stick to biological surface, usually mucus membrane. The concept of bioadhesion is emerging as a potential application in drug delivery due to its applicability for bioavailability enhancement, prolongation of residence time for drug in GIT and better contact between drug and absorbing surface.

Many hydrophilic polymers adhere to mucosal surfaces as they attract water from the mucus gel layer adherent to the epithelial surface. This is the simplest mechanism of adhesion and has been defined as “adhesion by hydration” Various kinds of adhesive force, e.g. hydrogen bonding between the adherent polymer and the substrate, i.e. mucus, are involved in mucoadhesion at the molecular level. Carbopol polymers have been demonstrated to create a tenacious bond with the mucus membrane resulting in strong bioadhesion.

Many commercial oral and topical products available today and under investigation have been formulated with Carbopol polymers, as they provide numerous benefits in bioadhesive formulations.

Benefits in Bioadhesive Applications

  • Improve bioavailability of certain drugs.
  • Enhance patient compliance (fewer doses are needed per day)
  • Lower concentrations of the active ingredients can be used.
  • Provide excellent adhesion forces.

(4) Topical Applications60-66:

Carbomers are very well suited to aqueous formulations of the topical dosage
forms. Many commercial topical products available today have been formulated
with these polymers, as they provide the following numerous benefits to topical
formulations:

  • Safe & Effective — Carbopol polymers have a long history of safe and effective use in topical gels, creams, lotions, and ointments. They are also supported by extensive toxicology studies.
  • Non-Sensitizing — Carbopol polymers have been shown to have extremely low irritancy properties and are non-sensitizing with repeat usage.
  • No Effect on the Biological Activity of the Drug — Carbopol polymers provide an excellent vehicle for drug delivery. Due to their extremely high molecular weight, they cannot penetrate the skin or affect the activity of the drug.
  • Excellent Thickening, Suspending, & Emulsification Properties for Topical Formulations

Products with a wide range of viscosities and flow properties have been successfully formulated and commercialized. Carbopol polymers are used to permanently suspend the active ingredients in transdermal reservoirs as well as in topical gels and creams. Pemulen polymeric emulsifiers can be used to prepare stable emulsions, such as turpentine liniment, without the use of surfactants. Carbopol polymers and Pemulen polymeric emulsifiers are often the thickener and emulsifier of choice in topical lotions.

(5) Oral Care Applications 67-70:

Carbopol polymers impart several desirable characteristics to toothpaste formulations like Viscosity, Yield Value, Low thixotropy and Clarity.

Imparting viscosity at very low concentrations to thicken a system is a primary function of the polymers. Suspending abrasives and solid actives is accomplished through the build of yield value at low polymer concentrations. The combination of Carbopol polymers’ ability to build yield value with low thixotropy provides for a clean, non-stringing ribbon of toothpaste. From aesthetic and practical perspectives this means that Carbopol toothpaste formulations are pumpable, leave minimal solids residue on the tube rim, stand up well on the brush, and can be used in clear formulations.

Benefits in Oral care Applications –

  • Efficient co-binders at low usage levels.
  • Suspending agents for non-soluble actives or excipients.
  • Thicken peroxide gel systems while maintaining product stability.
  • Compatible with commonly used formulation ingredients.

Toxicological studies 71 :

The Carbopol, like other high molecular weight polymers, demonstrate a low
toxic and irritation potential based on their physical and chemical properties.
Accordingly, such cross-linked, high molecular weight acrylic acid polymers
have been found safe for use in a wide variety of cosmetics, detergents and
pharmaceuticals by appropriate regulatory and non-regulatory bodies concerned
with such products. Acute oral studies with rats, guinea pigs, mice and dogs
showed that Carbomers- 910, -934, -940 and –941 have low toxicities when ingested.
The inhalation LC50 of Carbomers 910 in albino rats was 1.71mg/l.
the dermal LD50 of rats exposed to Carbopol 910 was greater than
3.0 g/kg. No mortalities occurred in rabbits injected intravenously with 1%,
2% or 3% Carbopol 934 in aqueous solution at a dose of 5 ml/kg. Rabbits showed
minimal skin irritation when tested with 100% Carbopol 910 or –934, and zero
to moderate eye irritation when tested with Carbomers 910, -934, -940, -941
and/or their various salts at concentrations of 0.20-100%. When Carbopol 934
P was fed orally to dogs and rats, there was no significant effect on body weight,
food consumption, mortality, behavior, and blood chemistry. The CIR Expert Panel
called attention to the presence of benzene as an impurity in the Carbopol polymers
and recommended efforts to reduce it to the lowest possible level.

 

Conclusion

The large variety of applications as well as the steadily increasing number of research workers engaged in studies of Carbopol polymers due to their unique properties, have made significant contributions to many types of formulations and suggest that the potential of Carbopol as novel and versatile polymer will be even more significant in future.

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About Authors

Hosmani A.H.*, Thorat Y.S., Kasture
P.V.

Affiliation: Govt. College of Pharmacy, Karad. India.

Avinash H. Hosmani Image

Mr. Avinash H. Hosmani is working as a Senior Lecturer in Govt.
College of Pharmacy, Karad. He is associated with Pharmaceutics Department. He
earned his postgraduate degree from Pune University. Prior to joining Govt. College
of Pharmacy, Karad, he was working with Emcure Pharma as a research scientist.
His research experience includes conventional and controlled drug delivery, mucoadhesive
and nasal drug delivery. He has several papers and presentation abstracts published
in national and international conferences in this area.

Address: Govt. College of Pharmacy, Karad. Vidyanagar, Tal- Karad,
Dist- Satara, Maharashtra State, Pin – 415 124.  India.

Phone       : +919823154242 Fax   
:  +912164 271196

E-Mail     : havinash29@rediffmail.com

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Comments

deepak5008's picture

hello sir, i have read your article, i found it very useful. i have some query's. 1- can you detail me the difference between different grades of carbopol. 2- what are those properties which makes it more acceptable than other polymer available in market? thanks and regard Deepak sharma