Hello Shima Parviz
Alginate forms a gel through ionic crosslinking with divalent cations like Ca²⁺. PBS, contains phosphate ions which can chelate calcium ions, meaning they can effectively bind to and remove Ca²⁺ from the alginate gel structure. The loss of these crosslinking ions weakens the alginate gel, leading to its dissolution and degradation.
Also, borax, interacts with the hydroxyl groups of PVA to form crosslinks. However, borate-PVA complexes are reversible, particularly in aqueous solutions and with changes in pH levels. In solutions, changes in concentration or pH can break or form these crosslinks, affecting the material's physical state, with increased pH (more borate ions) promoting gelation and decreased pH sometimes acting to break the network. This reversibility means that the crosslinks can break down, reducing the scaffold's integrity and mechanical strength. Because of theses weakening crosslinks rapid degradation and dissolution of the printed scaffold in PBS does occur.
You may solve this problem by exploring agents that form more stable covalent bonds or are less susceptible to ion exchange in PBS. Try modifying the alginate structure like consider using partially oxidized alginate or alginate with modified functional groups to promote more stable crosslinking. You may also optimize the proportions of PVA and alginate in the scaffold to potentially enhance mechanical properties and stability.
Best,
The scaffold disintegrates quickly in PBS due to the reversible, non-covalent nature of its crosslinks, which are unstable in a saline environment.
The borax crosslinks holding the PVA network together are dynamic and reversible. In PBS, the high ionic strength disrupts these bonds and the hydrogen bonding in PVA, causing it to dissolve.
Simultaneously, the monovalent sodium ions (Na⁺) in PBS displace the divalent calcium ions (Ca²⁺) crosslinking the alginate in a process called ion exchange. This breaks the alginate's "egg-box" structure, causing it to swell and dissolve.
In essence, the saline environment of PBS rapidly attacks both the weak PVA-borax matrix and the ionic alginate crosslinks, leading to the scaffold's complete dissolution within a day. To improve stability, permanent covalent crosslinking strategies (e.g., using glutaraldehyde or citric acid for PVA) are required.
