This paper may help you......

http://www.hindawi.com/journals/ijap/2014/379379/

These materials are somewhat expensive, so if your frequency is not that high (lower than 8 GHz) Id suggest using the normal available PCB material (FR4), which is cheaper. 

according to your frequency range of operation and the application these papers may help you

1. Textile Materials for the Design of Wearable Antennas

A Survey

http://www.mdpi.com/1424-8220/12/11/15841/pdf

2.Development of an Antenna Material Based on Rubber

that has Flexibility and High Impact Resistance

https://www.google.com.eg/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwih54za2o3KAhUBORoKHY4EAVAQFggrMAE&url=http%3A%2F%2Fwww.ntn.co.jp%2Fenglish%2Fproducts%2Freview%2Fpdf%2FNTN_TR76_en_p058_063.pdf&usg=AFQjCNFMBciaGJj2uAcSZ7v5tkvXIFbW3Q&sig2=rg491TyI7PAuz2poaEkX0g&bvm=bv.110151844,d.ZWU

3.https://www.google.com.eg/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwih54za2o3KAhUBORoKHY4EAVAQFgg1MAI&url=http%3A%2F%2Fcdn.intechopen.com%2Fpdfs-wm%2F37715.pdf&usg=AFQjCNHLvz-IbD9YgNlMCLm5OlfVoBHHyg&sig2=27qoI_rBTRI7zfFUxS5jUQ&bvm=bv.110151844,d.ZWU

4.https://www.google.com.eg/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=0ahUKEwih54za2o3KAhUBORoKHY4EAVAQFghJMAQ&url=http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F233379748_Flexible_Substrate_Antennas&usg=AFQjCNH9P6oYP8Ot76gfJam6_S-FSC0iYw&sig2=CRk9X43tQ-dZhIaXT776AQ&bvm=bv.110151844,d.ZWU

In general, lower dielectric constants can radiate energy better than higher dielectric constant. Check Rogers Corp. ACS youtube chennal, they gave a lot of practical information.

Substrate material depends upon the type of function you want to perform with Antenna this paper will help you about the flexible material as substrate:

https://www.researchgate.net/publication/283245871_Dielectric_studies_of_environmentally_friendly_and_flexible_lignocelluloses_fibrils_for_miniaturization_of_patch_antenna

Article Dielectric studies of environmentally friendly and flexible ...