Chicago private equity firm Water Street Healthcare Partners said Friday that it has purchased a Massachusetts maker of infusion therapy products and supplies.

Water Street, which did not disclose a purchase price, said it will commit “up to $75 million of equity to invest in and grow” Stoughton, Mass.-based Medical Specialties Distributors.

The deal brings Water Street’s portfolio of health care companies to 12, the company said.

Water Street said Medical Specialties has more than 4,000 customers, including specialty pharmacies owned by Walgreens, Accredo Health Group and Coram Specialty Infusion Services. The business of specialty pharmacy such as home infusion is fast-growing given the need for more aging baby boomers to get care in their homes.