Smartwool. With all of the features that the PhD® Run Ultra Light Micro offers, your newest technology may be in your sock drawer. Indestructawool™ technology provides you with enhanced durability, while a 4 Degree™ elite fit system makes for a performance oriented fit. And Shred Shield™ technology reduces wear and tear from your toes. It's breathable, comfortable, and built to help take you farther.