A side seam thickness of the Breastmilk Storage Bag easy link is 5 mm. with double layered Plastic, durable and tough. The inner material made with LDPE Plastic which is highly resilient, prevent from breaking and leak proof. It suitable for freezing in frosty cold temperature. The outer material made with PET which prevent stretching and shrinking while freezing and thawing. It can protect the pen ink, chemical which can seep to the storage bag and block out the air into the breastmilk. The opaque bag is to a good preserve of nutrient-rich breastmilk. Produced by food Grade processes and materials. The bag can store up to 8 ounces. The record tab is easy to write, clearly and colorful design.