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Synthetic hydroxyapatite (HAp) has been successfully produced with the aim of obtaining biomaterials that meet the biomechanical requirements for bone tissue engineering, while being compatible with the surrounding biochemical and cellular environment. Combining proteins with HAp can generate improved composite biomaterials, which are environmentally friendly, renewable, and biocompatible. In this context, HAp/protein-based composite materials have been widely exploited since the late 20th century to the present day. In this review, we explore the biomedical relevance of the association of HAp with several proteins of interest such as fibroin, sericin, fibrin, and keratin. The processing strategies for their synthesis and effect on the obtained shape and physicochemical, mechanical, and biological performance are herein discussed. This work can provide useful information for the design of HAp-based biomaterials with specific emphasis on bone tissue regeneration characteristics for biomedical applications.

Calcium phosphates (CaPs) have been widely used in the field of biomedical engineering as bone graft substitutes or as carriers for drug delivery applications. Recent developments have focused on combining CaPs with proteins to obtain functional biomaterials that accommodate a broader spectrum of functional requirements. Silk sericin was considered an unutilized protein by-product from the textile industry, generating tons of residues every year. However, much effort has been dedicated to its recovery after being associated with numerous biological properties such as antioxidant, antibacterial, anti-coagulation and regenerative activities. In the past years, sericin has also demonstrated to be suitable as a template for CaP mineralization. The present review focuses on the recent developments for the production of sericin/CaP composites, exploring their potential applications in bioengineering and opening new avenues in other research fields such as in the cosmetic, food and environmental sectors. In addition, this paper can also be useful as a guideline to design future research based on sericin/CaP biomaterials.

In the last decade, three-dimensional (3D) extrusion bioprinting has been on the top trend for innovative technologies in the field of biomedical engineering. In particular, protein-based bioinks such as collagen, gelatin, silk fibroin, elastic, fibrin and protein complexes based on decellularized extracellular matrix (dECM) are receiving increasing attention. This current interest is the result of protein’s tunable properties, biocompatibility, environmentally friendly nature and possibility to provide cells with the adequate cues, mimicking the extracellular matrix’s function. In this review we describe the most relevant stages of the development of a protein-driven bioink. The most popular formulations, molecular weights and extraction methods are covered. The different crosslinking methods used in protein bioinks, the formulation with other polymeric systems or molecules of interest as well as the bioprinting settings are herein highlighted. The cell embedding procedures, the in vitro, in vivo, in situ studies and final applications are also discussed. Finally, we approach the development and optimization of bioinks from a sequential perspective, discussing the relevance of each parameter during the pre-processing, processing, and post-processing stages of technological development. Through this approach the present review expects to provide, in a sequential manner, helpful methodological guidelines for the development of novel bioinks.

BACKGROUND: Sericin (SS) induces nucleation of bone-like hydroxyapatite (HAp) when used as an organic matrix. HAp/SS nanocomposites have been conventionally synthesized through precipitation in stirred tank reactors (STs). Despite its simplicity, this process is time consuming and presents difficulties in scale-up. In our study, HAp/SS nanocomposites were successfully synthesized in a ST and in a meso-oscillatory flow reactor (meso-OFR), to compare the efficiency of both reactors and to study HAp mineralization using SS as a template. RESULTS: The production of stable HAp, indicated by pH stabilization, was achieved after 180 min in the ST and after 30 min in the meso-OFR. X-ray diffraction and Fourier transform infrared analyses showed that the particles obtained in both reactors are HAp/SS nanocomposites with low crystallinity. Scanning electron microscopy evidenced the formation of rod-and plateshaped nanoparticles and revealed that the presence of SS led to the production of larger particles. The latter observation was confirmed by laser diffraction. Additionally, increasing SS concentration resulted in the formation of more plate-like particles. CONCLUSIONS: Precipitation is more efficient in the meso-OFR, HAp/SS being obtained four times faster. The presence and concentration of SS led to differences in the size and morphology of the synthesized particles, suggesting a critical role of SS in the mineralization process. This work reports a new approach for the manufacture of high-added-value nanocomposites with similar characteristics to biological bone and the results of a study of the influence of SS as an organic component in HAp nucleation. Further, the use of this protein and technology leads to significant waste minimization.

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Calcium plays an important role in barrier function repair and skin homeostasis. In particular, calcium phosphates (CaPs) are well established materials for biomedical engineering due to their biocompatibility. To generate biomaterials with a more complete set of biological properties, previously discarded silk sericin (SS) has been recovered and used as a template to grow CaPs. Crucial characteristics for skin applications, such as antibacterial activity, can be further enhanced by doping CaPs with cerium (Ce) ions. The effectiveness of cell attachment and growth on the materials highly depends on their morphology, particle size distribution, and chemical composition. These characteristics can be tailored through the application of oscillatory flow technology, which provides precise mixing control of the reaction medium. Thus, in the present work, CaP/SS and CaP/SS/Ce particles were fabricated for the first time using a modular oscillatory flow plate reactor (MOFPR) in a continuous mode. Furthermore, the biological behavior of both these composites and of previously produced pure CaPs was assessed using human dermal fibroblasts (HDFs). It was demonstrated that both CaP based with plate-shaped nanoparticles and CaP-SS-based composites significantly improved cell viability and proliferation over time. The results obtained represent a first step towards the reinvention of CaPs for skin engineering.