As filed with the Securities and Exchange Commission on December 7, 2023. UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Amendment No. 4 to FORM S-1 REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933 FibroBiologics, Inc. (Exact name of registrant as specified in its charter) Registration No. 333-275361 (State or other jurisdiction of incorporation or organization) (Primary Standard Industrial Classification Code Number) (I.R.S. Employer Identification Number) 455 E. Medical Cen ...