You could also look at light diffraction. It is a direct non-contact technique which is fast. Some hydrogels due to the formation of crosslinking point induce a change in the way they diffract the light. Gellan gum is one of them, due to the formation of an helix. I don't know about your polyamine crosslinker neither your exact formulation so just a suggestion.

http://ac.els-cdn.com/S0144861796000744/1-s2.0-S0144861796000744-main.pdf?_tid=7d5c8054-7f87-11e4-9ff2-00000aab0f27&acdnat=1418118243_e16b3af36ac3562edcb5d5ff96788949

For the gelling time you can use a rheometer, more sophisticated than the machine you implied but you also get the chance to get much more information. Depending on the geometry, you can get away with even less than 1-2 mL

Thanks Prof. Luxenhofer for your answer. My only concern with a rheometer is that colleagues have told me it can take several minutes to set up the sample. Is this correct, or are there ways of adapting it so that it can start measuring the gelation time immediately? Some of the gels I will work with will have very short gelation times, perhaps ranging from 15 seconds up to 10 minutes.

Well, that is true, for such rapid gelling samples it would be quite challenging, but you´ll probably also run into problems with the viscosimeter, although I haven´t worked with such rotational viscosimeter since being an undergrad. What is suitable for you also depends how you have to mix or how you induce gelation...

Thanks for confirming that. I'm going to be working with gellan gum (polysaccharide) hydrogels for tissue engineering applications. They are cross-linked using a polyamine. Polymerisation begins instantly when the two components are mixed and the gelation time depends on the concentration of each component. Mixing will be done simply using a pipette.

I need to optimise the gelation time to be approximately 5 minutes – enough time to handle them but not long enough for encapsulated cells to sediment. Therefore a rheometer may not be suitable. It is true that a viscometer may also be challenging to work with but as far as I can tell, it should be possible to begin the measurement as soon as mixing begins.

You could also look at light diffraction. It is a direct non-contact technique which is fast. Some hydrogels due to the formation of crosslinking point induce a change in the way they diffract the light. Gellan gum is one of them, due to the formation of an helix. I don't know about your polyamine crosslinker neither your exact formulation so just a suggestion.

http://ac.els-cdn.com/S0144861796000744/1-s2.0-S0144861796000744-main.pdf?_tid=7d5c8054-7f87-11e4-9ff2-00000aab0f27&acdnat=1418118243_e16b3af36ac3562edcb5d5ff96788949

Hi Nick,

I've looked at gellan gum gelation times myself and would have to agree with Robert that rheology is the best method - if you have the time to get the sample on the stage and start the test. 

Aurelien (above) is on the right track too, I've examined gelation of gellan gum using circular dichrosim spectroscopy. We had an instrument that allowed you to inject polymer solution and cross-linker solution into the cuvette and monitor simultaneously. See if you have got something like that available to you.

I would actually recommend trying to make the rheology method work though. Try the following:

1) Have the rheometer set-up and ready to go for measuring. Lower the geometry (recommend cone and plate for small sample volume) down to just enough space to fit a syringe tip between them. This cuts down the time taken to lower the geometry to measurement height.

2) Have your gel solution and cross-linker solution ready to go. Then using a single syringe, draw up the gel solution, then a small plug of air, then draw up the cross-linker solution. Hopefully the plug of air will stop the solutions from mixing inside the syringe.

3) Immediately dispense the syringe contents in between the cone and plate, pull the tip out and hit the run measurement button. The motion of the rheometer ought to facilitate even mixing of the two solutions for you.

With practice, I think you can get that time down to less than one second.

As for the rheology experiment, go with a cone and plate geometry; oscillating at 1% strain; angular frequency of 10 rad.s-1; if you have it, use temperature control to 37C too. Keep an eye on the storage modulus to monitor the gelation time.

Dear all, I´d like to make a further suggestion to Damians excellent work flow. You could use a double barrel syringe with the mounted mixer. Such, you should have pretty decent mixing of the solution in a very short time frame. However, if gelation is too fast (< 1 sec), it still might give you problems. I don't endorse any company, but see attached link if you are not sure what I mean.

http://www.medmix.ch/F-SYSTEM.html

Dear Aurélien, Damian and Robert,

Thank you all for your very helpful suggestions.

If possible, I'd like to measure the gelation more directly than basing it on light diffraction, but it's good to know there is another method available if necessary.

Having spoken to one of the companies that sells viscometers and rheometers, I've been advised that even more advanced rotational viscometers are unsuitable, since the hydrogels will be relatively compliant and the high torque of the motor will mean the instrument lacks sensitivity to the relatively low viscosity.

They also advised me that cone and plate rheometry would be suitable even for measuring gels with very quick gelation. Unfortunately the rheometers we have here are not suitable but I am now looking into sourcing one or gaining access to one.

The suggestion regarding using a dual syringe is useful and something I was considering as I was reading point 2 in Damian's answer.

Thanks again for all your useful advice!

We've used NMR to monitor gelation times, but this would require that your system have a signal that you can easily monitor.  If you have a good internal standard you can get some good qualitative data from this technique.

Viscosity-dependent fluorescent probes are the best choice for real-time monitoring the gelation process. You just put very low concentration of the probe molecule into the sol. Then record (take photo or video) the emission enhancemnt at different time. The experiment can be done in an NMR tube or a thinner/smaller transparent pipe (let UV-light penetrate). This kind of fluorescent probe is faintly emissive in dilute solution but becomes efficiently emissive in gel. I have such kind luminescent molecules, including water soluble, amphiphilic, and hydrophobic species. I can send you sample if you give me exact mail address and tell me the solvent.  

Hi Jingzhi,

You're technique sounds very interesting - Can you explain a little more about how it works or link me to an article on it?

minimise the cross linker or initiator concentration