What can replace Stainless Steel parts when bioinertness is necessary?

Several things can replace stainless steel parts in bioinert situations. If you are looking to continue using metal, then in some cases titanium is an acceptable alternative. However, titanium is brittle and very difficult to cut, and in the case of tubing, it is very expensive.

For other purposes, almost universally, PEEK is a fantastic alternative to stainless steel. PEEK, or polyetheretherketone, can be used in high-pressure situations like stainless steel, and it is compatible with most chemicals used in HPLC. PEEK tubing is less expensive than stainless steel, easier to cut and more flexible. PEEK fittings are appropriate to be used with most all instruments. PEEK fittings offer high-pressure holding capabilities, and typically only need to be tightened finger tight, minimizing the need for special tools.