SHOE DOCTOR OPTI-GEL SLING BACK GRIPS. These gel heel grips help prevent blisters on heels and provide extra grip on strappy shoes. Fabrication: Premium Grade Silicone Opti-Gel Gel with Recyclable Packaging. Shoe Doctor Opti-Gel Heel Grips offer invisible protection, help prevent blisters on heels and provide extra grip. Blister prevention aid. Re-usable from shoe to shoe. 2 x VALUE PAIR PACK.