In case that the drug can leach out over time, the drug will create a pore inside hydrogels while being released, which may influence the overall hydrogel properties. Generated pores (especially interconnected) may easily dissipate energy under a rheology testing, resulting in an increase in loss modulus and in tangent of delta (damping capability).

Drug may became more viscous (Increase in loss modulus) due to interconnected pores.

What if the drugs may be acting as a bio-plasticizer and augmenting the viscous portion of the viscoelastic mixture, thereby raising its loss modulus.

Bae Hoon Lee The generation of the pores I believe is as a result of the interactions between the polymer and the drug. I also noticed that below a certain concentration of the drug, although these interactions do occur, they do not change the G' (storage modulus) or the G" (loss modulus). However, above the critical drug concentration, I observe the increase in loss moduli only. I wonder if we can also describe (in addition to the dissipation of energy from pore size increment) that the stronger the interactions between my polymer and drug (due very high drug concentration), the higher chances we have the formation of complexes (due to electrostatic interactions). These complexes, do not take part in gel formation, but since there are still complexes (solid), that is why we still have a high G'. Since a hydrogel's mechanical strength is also defined by G'/G", there is therefore a clear reduction in overall mechanical strength.

Joshua Omambala can you please elaborate more on how a drug can influence only the viscous portion of a viscoelastic hydrogel?

Muhammad Bilal The change in the properties of the drug itself is not being assessed in the current project. Rather, there is greater emphasis on how exactly the drug influences the rheological properties of the polymeric hydrogel....

From only the rheological measurements would be difficult to have deep information about what is really happening at molecular level - interactions between the drug and the polymer.

If your G' doesn't change significantly at least you can be confident that the interactions between the polymer chains responsible to the network formation and elastic properties of it are not changing significantly.

What is the polymer and what is the drug molecular weight?

Most probably the drug dispersed withing the network is contributing to the general molecular mobility and does to increase the energy dissipated during the mechanical solicitation. So even if the drug didn't leach out of the gel, the effect would be similar to that pointed out above by Bae Hoon Lee

José A. Lopes da Silva , indeed very true. I also performed CD spectroscopy, DLS/ zeta potential measurements, EMSA and confocal microscopy using lower concentrations of the polymer and drug. From these, I was able to state at least that they do interact and that electrostatic interactions were predominant. This was not surprising considering that the two macromolecules are cationic and anionic in nature.

The molecular weight of the drug is around 1400 Da whereas that of the polymer (composed of self-assemble anionic strands each around 1.2 KDa) is 60000 Da.

This is a very good point (the fact that the interactions responsible for network formation are not significantly changing) and I am very grateful for this statement as I didn't discuss this because I was so focused on the change in G" only. But I am also worried about stating only this as the reason because from the above-mentioned experiments I performed, when the drug concentration was high (from 15/1 drug:polymer molar ratio), there was a drastic increase in size of nanoparticles formed (for the lower concentration polymer and drug mixtures) and complete retardation of the polymer for my EMSA experiment. These two observations were due to the formation of larger aggregates or complexes between my macromolecules....

Do you think it is hence possible that this might also be the case in my gel? That at this drug concentration, some polymer chains (likely not completely cross-linked) may exist in smaller aggregates accounting in part to the increase in energy dissipated and contributing to the general molecular mobility within the gel? Or is it impossible and not applicable in this system?

Suppose the overall number of the physical crosslinking points are the same (G') in the system; extra molecules (not being involved in the physical crosslinking) above a certain concentration might contribute to noticeable energy dissipation in the rheology of polymer-drug mixtures. Probably, their time-dependant properties (e.g., stability, release profile, and mechanical properties) need to be checked further.

Bae Hoon Lee noted! I did perform dug release and mechanical properties of the formulation but have not performed stability between the control and the drug loaded gel. I will try and do this in the coming months, thanks for the suggestion.