How about modifying the surface of the silica particles before your polymerization step? You could attach vinyl groups to the surface using vinyltrimethoxysilane. Firstly, heat the particles at 150 deg C for 4 hours to remove the hydration layer on the silica surface. Second, disperse the particles into anhydrous toluene and add about 1% v/v of silane and let the reaction run at 70 deg C for an hour. After the reaction rinse the particles with methanol making sure that during this step you don't allow the particles to dry because a thick silane film will form.

This method will attach vinyl groups to the surface which in turn will aid in attachment of your polymerizable material. 

Thank you for your response David. I actually aminated the particles with (3-aminopropyl)triethoxysilane (APTES). I figured this would also aid in the attachment of the PEGDA. The hydrogel is linking to the surface properly, but the layer is too thin. I am wondering if it is possible to coat more PEGDA on to an already coated and polymerized surface.

How thick of a layer are you currently depositing? Also, how would an amine covalently react with the PEGDA in your polymerization scheme?

I am not exactly sure how thick the layer is but it is no more than 50 microns, probably even less than that.

The PEGDA is first reacted with the aminated particles through the Michael addition reaction. The C=C bond on one of the acrylate end groups gets one of the hydrogen's from the NH2 on the particle surface. The particles with a PEGDA surface are then polymerized in suspension.

Thanks for the clarification. What is you ultimate goal in polymer thickness? I assume you have studied conditions regarding the amount of monomer in solution and polymer thickness that is achieved after polymerization.  Have you noticed flocculation of the particles during polymerization? If so, how about increasing the amount of monomer and decreasing the amount of particles to allow for an increase in film thickness with a supposed decrease in flocculation?

I haven't really set a goal for the thickness of the hydrogel layer yet. As of now, I really just want to increase the layer from where it is now. I have been studying the relation between monomer concentration and polymer thickness recently and I intend to run some experiments in the coming weeks.

For the flocculation part...I actually polymerize the particles in a suspension of mineral oil with surfactant so they do not flocculate. It has worked well for all my samples.

I really appreciate the time you are putting into this David.