In coaxial  Electrospinning of mixed solutions of natural and synthetic polymers one of the most important factors is adjusted folw rate of core and shell solutions, Won, J. J et al shows that applied voltage is the most important factor in electrospinning of PVA/Albumin. it depend on the type of polymers, parameters such as concentration, flow rate and applied voltage.

There are no particular difficulties ... 

Do you have some protocol??? I'm using alginate as core and PCL as shell. I obtain some fibres but i'm not sure if are core-shell fibres.

I dont have experience with fibers but can suggest a way to check if you have core-shell or not.

1. Water contact angle (WCA): Check contact angles for fiber mat made of pure alginate, pure PCL and the supposedly core-shell fibers. Make sure all fibers including the pure ones are made using the core-shell needle in order to keep the thickness of fibers constant. A coparision WCA of these mats should give an idea.

2. Surface charge meaurement: Other option will be conductometric titration to measure the surface charge of fibers. Aginate should be anionic (carboxyls titrated with NaOH) so a core-shell structure will obviosly show up in surface charge measurments.

Best wishes

If you fibers are of substantial diameter, 3~5 microns or more, you can try using optical microscope. The light coming through will be refracted at a different angle compare to light coming more the side edges which contain a single component.

Having said that, they best way to know  core-shell architecture is to stain your fibers and do a TEM. Its the most conclusive method    

You can easily find out if you have a core-shell structure or not with TEM 

check Oshima Masahiro lab process engineering at Kyoto university, , plenty of work done with elctro spinning, you may find papers related to the subject, if you need link let me now

Hi Juliana, please see this article which details the use of a coaxial extrusion needle for alginate core, pcl sheath printing, including testing with cells in the core.

http://www.ncbi.nlm.nih.gov/pubmed/24658021

In coaxial  Electrospinning of mixed solutions of natural and synthetic polymers one of the most important factors is adjusted folw rate of core and shell solutions, Won, J. J et al shows that applied voltage is the most important factor in electrospinning of PVA/Albumin. it depend on the type of polymers, parameters such as concentration, flow rate and applied voltage.