Chitosan Hydrogels: Swelling and Synthesis

Swelling

• Two models describe the swelling of chitosan hydrogels.
• The first model describes the swelling fraction increasing linearly with the square root of time.
• This model is only valid for smaller swelling fractions.
• The second model focuses on the relationship between stress gradient and swelling/network expansion.
• These models lack the ability to describe sigmoidal swelling curves.
• Sigmoidal swelling curves often represent non-Fickian diffusion behavior.
• Fickian diffusion correctly describes sigmoidal swelling behavior when considering the movement of the gel surface.

Chitosan Hydrogels Synthesis

• Chitosan hydrogels can exist in different shapes including beads, films, and discs.
• Uncrosslinked chitosan hydrogels are synthesized by precipitating chitosan (CS) in sodium hydroxide (NaOH).
• The chitosan and acetic acid mixture forms beads in a precipitation bath.
• In a petridish, the mixture dries to form a film.
• Glutaraldehyde is used as a crosslinker for CS in disc shaped hydrogels.
• CS can be crosslinked using other methodologies such as irradiation techniques.

Chitosan Hydrogel Swelling Ability

• Chitosan beads display a swelling of 100-200% at pH 7.
• Chitosan beads show lower swelling in alkaline pH.
• Chitosan is soluble below its pKa of 6-6.5 and insoluble above it.
• Chitosan films exhibit higher swelling than beads.
• Uncrosslinked chitosan film swelling is about 3000% at pH 7.
• This film is susceptible to dissolution in acidic conditions, resulting in a 3D tubular shape in neutral condition.

Modeling of Water Absorption in Hydrogels

• Peleg's absorption model is used to describe water absorption.
• This model is expressed using the following equation:

  S = So ± k1/(1 + k2*t)
  

• This equation uses two parameters, k1 and k2 which represent kinetic and characteristic constants.
• The "+" sign is used for water absorption and "-" is used for desorption.
• The Exponential association model is used for describing exponential growth and decay.
• This model is expressed by the following equation:

  S = [1−exp(-kR2t)]
  

• The kinetic constant in this model is kR2.

Diffusion Coefficient Equation

• The diffusion coefficient for cylindrical hydrogels can be calculated using the following equation:

  D = k1/πr^2 (4/n)
  

• The equation is derived from Fick's second law of diffusion.
• D represents the diffusion coefficient (m^2/s)
• r represents the radius of the swollen hydrogel (meter)
• k1 is kinetic constant and n is the diffusion exponent.

Thermogravimetric Analysis of CS hydrogels

• Thermal properties of CS hydrogels are assessed using thermogravimetric analysis (TGA).
• Three stages of weight loss are observed during thermal decomposition.
• The first stage mainly consists of water evaporation from the hydrogels network.
• The second stage involves decomposition of CS and conversion of complex molecules into simpler ones like acetic acid and lower fatty acids.
• The third stage continues the decomposition of remaining fragments.
• Chemical modification generally leads to a decrease in thermal stability of CS.

Kinetics of Water Absorption

• Two kinetic models are used to describe water absorption: First order and second order kinetics.
• The R2 values (correlation coefficient) for both models are given in Tables 2 and 3.
• The kinetic constants k1, k2 and R2 values are presented for hydrogels at different pH values, temperatures, and in the presence of different salts.

Diffusion Exponent

• The diffusion exponents values are given in Table 4.
• These values provide insights into the diffusion mechanism of water in the hydrogels.