*Review* **Biologically Inspired Collagen**/**Apatite Composite Biomaterials for Potential Use in Bone Tissue Regeneration—A Review**

### **Barbara Kołodziejska, Agnieszka Kaflak and Joanna Kolmas \***

Chair of Analytical Chemistry and Biomaterials, Department of Analytical Chemistry, Medical University of Warsaw, ul. Banacha 1, 02-097 Warsaw, Poland; barbara.kolodziejska@wum.edu.pl (B.K.); agnieszka.kaflak@wum.edu.pl (A.K.)

**\*** Correspondence: joanna.kolmas@wum.edu.pl; Tel.: +48-22-5720755

Received: 7 March 2020; Accepted: 7 April 2020; Published: 9 April 2020

**Abstract:** Type I collagen and nanocrystalline-substituted hydroxyapatite are the major components of a natural composite—bone tissue. Both of these materials also play a significant role in orthopedic surgery and implantology; however, their separate uses are limited; apatite is quite fragile, while collagen's mechanical strength is very poor. Therefore, in biomaterial engineering, a combination of collagen and hydroxyapatite is used, which provides good mechanical properties with high biocompatibility and osteoinduction. In addition, the porous structure of the composites enables their use not only as bone defect fillers, but also as a drug release system providing controlled release of drugs directly to the bone. This feature makes biomimetic collagen–apatite composites a subject of research in many scientific centers. The review focuses on summarizing studies on biological activity, tested in vitro and *in vivo*.

**Keywords:** collagen; hydroxyapatite; biomimetic material; scaffold; bone regeneration; biocomposite
