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pH-Sensitive Smart Poly-meric nanofibers

The composition of pH sensitive polymeric nanofibers contain pendant acidic ( e.g, carboxylic and sulfonic

Acids) or basic (e.g, ammonium salts) groups that either accept or release protons in response to changes

To environmental pH. Polyacrylic acids, poly(2-ethylacrylic acid), poly(2-propylacrylic acid) and

Polysulphonamides(derivatives of p-aminobenzenesulfonamide) are few examples of polyacids (anionic

polymers). Similarly, poly(N, N-diakyl aminoethyl methacrylates), poly(lysine), poly(ethylenimine), and

Chitosan represent cationic polyelectrolytes. pH sensitive macromolecules dissociate to give polymeric

Ions after dissolving on ionizing solvent. Repulsion between similar charges on the polymeric chains

results in chain expansion when ionized in a suitable solvent. When unionized in the case of a poor solvent

and hydrophobic polymer they collapse into globules and also precipitate from solution.

They interplay between hydrophobic surface energy and electrostatic repulsion between charges dictates

the behavior of the polyelectrolytes. Since the degree of an aqueous medium, smart polymers dramatically

change conformation in response to minute changes in the pH of aqueous environment.

Various works have been performed on PMMA based polymers. Microparticles prepared of poly(methacrylic

Acid-g-ethylene glycol) P(MAA-g-EG) loaded with insulin exhibited unique pH-responsive characteristics in

Which interpolymer complexs were formed in acidic media and dissociated in neutral/basic environments.

Consequently, insulin release from the gel was significantly retarded in acidic media while rapid release oc-curred under neutral/basic conditions. Copolymer networks of poly(methacrylic acid)grafted with

Poly(ethylene glycol) with reversible pH-dependent swelling behavior, due to the formation of interpolymer

Complexs between protonated pendant acid groups and the etheric groups on the graft chains, has been

Developed. Gels containing equimolar amounts of MAA/EG exhibited lesser swelling at lower pH. The pH of

The pH of the swelling solution affected the average network mesh size. In vitro release of insulin from

P(MAA-g-EG)gels containing PEG grafts indicated a significant release of insulin as the gel decomplexed.

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Composite membranes from nanoparticles of poly(N-isopropylacrylamide-co-methacrylic acid)of various NIPAAm:MAA ratios dispersed in a matrix of a hydrophobic polymer have been investigated.

Temperature Responsive Polymeric Fibers

Polymers sensitive to temperature changes are the most studied class of environmentally sensitive polymers as they

have potential applications in the biomedical field. This type of systems exhibit a critical solution temperature

(typically in water) at which the phase of polymer and solution is changed in accordance with their composition.

Those systems exhibiting one phase above certain temperature and phase separation below it possess an upper

critical solution temperature (UCST). On the other hand, Polymer solution that appear as monophasic below a

specific temperature and biphasic above it generally exhibit the so-called lower critical solution temperature(LCST).

LCST are the type of polymers having number of applications. The typical example is poly(N-isopropylacrylamide)

(PNIPAAm)that presents a LCST at 32 in water solution.

Examples thermal responsive poly(4-vinylbenzyl chloride)-block-poly(glycidyl methacrylate)-garft-poly(N-

Isopropylacrylamide) (PVBC-b-PGMA-g-PNIPAM) nanofibers by the combined technology of reversible addition-

fragmentation chain transfer (RAFT) polymerization atom transfer radical polymerization(ATRP) ,electrospinning, and

“click chemistry”. PVBC-b-PGMA-g-PNIPAM nanofibers exhibit a good resistance to solvents and thermal-responsive

charater to the environment, having a hydrophobic surface at 45 (warer contact angle ~ 140)and having a hy-drophilic

surface at 20 (warer contact angle ~30). Below that temperature the polymer is soluble as the hydrophilic interac-tions,

due to hydrogen bonding, are predominant, whereas a phase separation occurs above the LCST(cloud point) due to

predomination of hydrophobic interactions. LCST values of polymers are found to be controlled by the alkyl group

bound to the nitrogen atom.Other type of temperature sensitivity is based on the intermolecular association as in the

Case of Pluronics or Poloxamers, where hydrophobic associations of PPO blocks lead to the formation of micelle

Structure above CMT(critical micelle temperature).

N-alkylacrylamide monomers are mostly employed to obtain temperature snsitive polymers. These monomers are

Synthesized by nucleophilic substitution reaction of acryl chloride with the suitable amine.

Electrospinning of nanofiber: An Overview


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In recent years, polymers are processed by various techniques such as drawing(1),tempelate synthesis(2), phase

Separation(3), self-assembly(4), electrospinning(5), etc. to prepare polymernanofibers from micron to nanometer

Scale. The main idea is to bring reduced diameter of polymeric fibers so as to achieve outstanding characteristics

Such as very large surface area to volume ratio, flexibilities in surface functionalities, and superior mechanical

properties.

(1)The drawing-in process is similar to dry spinning. Drawing method can be used to make one to one and very long

Single nanofibers. However, this method is possible only for viscoelastic polymers that can undergo strong

deformations while being cohesive enough to support the stresses developed during pulling. Similarly in the case of

Template synthesis, a nanoporous membrane is used as a template to make nanofiber of solid(a fibril) or hollow(a

tubule) shape. By this method nanometer tubules and fibrils of various raw materials such as electronically

conducting polymers, metals, semiconductors, and carbons can be fabricated. But this method cannot one-by-one

continuous nanofibers.

(2) Phase separation is complex process consists of dissolution, gelatin, and extraction using a different solvent,

freezing, and drying resulting in nanoscale porous foam. This process takes longer time to transfer the solid polymer

into the nano-porous foam.

(3) On the other hand, self-assembly is a process in which individual pre-existing

components organize themselves into desired patterns and functions. However, the self-assembly is also a time

Consuming process similar to that as phase separation/

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INTRODUCTION

Response to stimulus is the basic principle of living systems. As life is made up of biopolymers such

as carbohydrate, proteins and neuclic acids, functions of the living cells are regulated by these

biopolymers which form the basis around for natural processes. Based on the lessons from nature,

polymer scientists have been designing useful materials that respond to external stimuli such as

light, temperature, PH, chemicals and electric field. Polymeric materials which respond by large

changes due to small changes in the environments are referred as ‘stimuli respond polymers’ or

‘smart polymer’ or ‘intelligent polymers’ or ‘environment sensitive polymer’. Accordingly smart

polymers have promising applications in the biomedical field as delivery systems of therapeutic

agent, tissue engineering scaffolds, cell culture supports, bioseparation devices, sensors or

actuators system. However, the goal of scientists is not only to mimic biological process but also to

create novel species and discover new process. The environmental triggers behind these transitions

could be change in temperature or PH shift, increase in ionic strength, presence of certain metabolic

chemicals, addition of an oppositely charged polymer and magnetic field, and light or radiation

forces have also been reported as stimuli for these polymers. Among the above, pH and

temperature responsive polymers are main focus of polymeric scientists.

Due to the very small diameter (nanometer scale) of the polymeric nanofibers obtained by

electrospinning, the surface area to volume ratio of the fibers is significantly high. Also, the

nonwoven mats formed from such nanofibers have very small pore sizes.

However, the total porosity of the mats will still be very high. Hence, smart polymeric nanofibers can

provide better cell penetration and growth so as to promote cell growth.


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