Lecture 6: Hydrogels & Smart Polymers

hydrogels

materials that are composed of hydrophilic, cross-linked polymer chains

crosslinks

covalent bonds, physical entanglement, hydrogen bonds or strong van der Waals, crystallites that bind to two or more polymer chains

classification of hydrogels

• homopolymer hydrogels

• copolymer hydrogels

• interpenetrating polymeric hydrogels

preparation of hydrogels

• synthesize in dry state and put in water

• synthesize in aqueous solution and further swell in water

mechanisms of reactions

• chemical cross-linking

• photopolymerization: UV

• irradiative cross linking: electron beam, gamma, x-rays

volume degree of swilling (Q)

volume of the swollen state/dry state

weight degree of swelling (q)

weight of the swollen state/dry state

highly swollen

allows more solute to diffuse in and out of structure (ex: cellulose derivative, poly(vinyl alcohol, poly(ethylene glycol))

moderately and poorly swollen

ex: PHEMA and derivatives

importance of degree of swelling

• solute diffusion

• surface properties and surface mobility

• optical properties

• mechanical properties

M_c

average molecular weight between two consecutive junctions (cross-links)

advantages of hydrogels

• closely mimic mechanical properties of soft tissues

• may be polymerized into any desired geometry

• free diffusion of cellular nutrients and waste products

• in situ polymerization possible

• mechanisms of polymerization allow incorporation of bioactive signals or bio-responsive domains

application of hydrogels

biocompatible and blood compatible with biomaterials, wound healing adhesive, cartilage, skin, nerve repair, carrier for cell transplantation pharma

smart materials

environmentally responsive, show drastic change in swelling due to changes in pH, temperature, ionic strength, nature and composition of swelling agent, enzymatic chemical reaction, electrical and magnetic stimuli

pH sensitive

containing acidic or basic pendant groups → which ionize at appropriate pH or ionic strength → increased swelling

temperature sensitive

change in temperature → change in polymer-swelling agent compatibility → change in swelling

LCST

(34.3 deg C) low critical solution temperature, below which polymer is soluble, above which, hydrophobic, don’t swell in water

PNIPAAm

a. At 25 degrees Celsius (below LCST), material is hydrophilic and water droplets wet the surface.
b. Above LCST, polymer gains energy, changes confirmation to become hydrophobic, and water droplet stays intact

smart polymer responding to bio-stimuli

glucose oxidation, glucose concentration high → gluconic acid and hydrogen peroxide → permeability change → insulin release

conductive polymers

class of polymers that can conduct electricity by forming a super-conjugation around the backbone which allows the sharing of electrons; alternation of pi bonds

electrically controlled release of drug from conducting polymer

inject electron into polymer, loses + charge, - ions and drug released into solution

can have materials that are hydrophobic at a low pH and hydrophilic at a high pH?

smart HG loaded with drug won't get released while in the stomach because the pH is low; once it passes the stomach and the pH increases, the smart HG undergoes pH controlled swelling, and diffuses drug out