Hydrogel defintion
Polymer structures swollen with water
How are hydrogels held together?
(1) primary covalent cross-links;
(2) ionic forces;
(3) hydrogen bonds;
(4) affinity or “bio-recognition” interactions;
(5) hydrophobic interactions;
(6) polymer crystallites;
(7) physical entanglements of individual polymer chains; (8) a combination of two or more of the above interactions.
Amorphous Hydrogels
Macromolecular chains are arranged randomly.
Semicrystalline Hydrogels
Characterized by self-assembled regions of ordered macromolecular chains (crystallites).
Complexation Hydrogels
Held together by specific types of secondary forces. These include hydrogen bonds, hydrophobic group associations, and affinity “complexes” [e.g., (1) heterodimers (peptide/peptide interactions called “coil– coil”); (2) biotin/streptavidin; (3) antibody/antigen; (4) conA/ glucose; (5) poly(d-lactic acid)/poly(l-lactic acid) (PDLA/ PLLA) stereocomplexes; and (6) cyclodextrin (CD) inclusion complexes].
Structural Evaluation
- Ideal (indicating tetra-functional cross-links (junctions) produced by covalent bonds) networks are only rarely observed.
- Instead, multifunctional junctions or physical molecular entanglements commonly play the role of semipermanent junctions.
‘Cross-link’
- Indicates the covalent or secondary connection points of several chains (chemical).
- May be carbon atoms, but they are usually small chemical bridges.
- A cross-link may be crystallites or other secondary interactions of a permanent or semipermanent nature.
- –> They have a finite size and contribute to the physical properties during biomedical applications.
Forming hydrogels
- Cross-linking: Usually based on free radical reactions
- Ionic forces
- Hydrophobic interactions
Physical Behaviour –> properties
- The physical behavior of biomedical hydrogels is dependent on their dynamic swelling and equilibrium in water and in aqueous solutions.
- Influenced by material (hydrophilic, polar), how it is cross-linked and molecular interactions forming hydrogel (ability for water to move in).
Methods to form a hydrogel: Covalent cross-linking
Covalent cross-linking formation of hydrogels occurs through functional groups present within a polymer. The functional groups can react and form covalent bonds, linking the polymer chains together to form a network.
- Eg. PVA functionalised with methacrylate groups, where a methacrylate group can form a covalent bond with another methacrylate group in the presence of an initiator and UV light.
Variables in this process include the number of functional groups (i.e. methacrylate groups) along a polymer chain.
- The number of functional groups effects the properties of the hydrogel via the facilitation of cross-linking.
- –> More cross-linking would result in a change (increase) in mechanical properties.
- –> More cross-linking would also alter the mesh size of the hydrogel which would alter the size of molecules that can diffuse in and out.
Methods to form a hydrogel: Hydrophobic interactions.
Hydrogel formation via hydrophobic interactions typically occurs in block or graft copolymers that contain both a hydrophilic and hydrophobic component. When these polymers are in solution, the hydrophilic and hydrophobic components of the polymers repel each other.
Variables: there can be ABA or BAB polymers (A = hydrophilic, B = hydrophobic).
Specifically, ABA/BAB copolymers can assemble into ‘micelles’ in an aqueous environment with a
hydrophobic centre and a hydrophilic outer shell interfacing with water. These hydrogels can be used in drug delivery applications for the delivery of drugs.
Methods to form a hydrogel: Physical cross-linking
Formation of hydrogels via physical cross-linking methods is most commonly facilitated by freeze-thaw cycle preparatory methods. It involves the casting of aqueous solutions of polymer (most commonly PVA), cooling to −20 °C and thawing
back to room-temperature several times. Each repeated procedure is referred to as a freeze-thaw
cycle (FT). This technique produces stable hydrogels that are physically cross-linked by the presence
of crystalline regions.
Swelling
- Swelling is the process of water ingress / going into the polymer.
- Swelling capacity can be determined via the swelling ratio in terms of volume or mass. Measuring via mass is usually easiest.
- Swelling ratio = Q = (Vpolymer + Vsolvent) / V polymer
- Swelling ratio = Q = (W2-W1) / W1 where W2 = weight of gel after swelling, W1 = weight of gel before swelling
