Polyester multifilament yarn

Multifilament Polypropylene

Polyester multifilament yarn and the metal i dunno!

monofilament

There are no hollow core squash racket strings, though they do exist for tennis.

There are two main types of string design in strings used for squash: monofilament & multifilament.

Monofilament strings are a single solid material while multifilament strings are braided and have no central core. Monofilament strings tend to be more durable but typically transfer more shock to the arm while multifilament strings have a softer feel at impact and are closer in playability to natural gut.