NOVEL DEVELOPMENTS IN ADVANCED MATERIALS FIELDS: POROUS AND NON-POROUS BIOMATERIALS USED IN REGENERATIVE MEDICINE AND TISSUE ENGINEERING

In this brief review, porous and non-porous biomaterials used as scaffolds in regenerative medicine and tissue engineering and new innovative techniques to obtain biomaterials were discussed. Various methods have been presented to obtain advanced materials used as scaffolds, such as (i) 3D printed biomineral composites obtained with bacteria-loaded ink (bactoInk), (ii) the use of vegetable waste, such as rice husks, parsley, spinach or cocoa in the development of bioplastics, (iii) the use of natural biological materials of animal origin (such as bovine bones, corals, snail shells or eggshells) from waste, or (iv) the creation of new biomaterials that can reduce or combat the infection of scaffolds after implantation.


INTRODUCTION
The synthetic or natural solid and sometimes liquid materials are used as biomaterials to construct a medical device for a total/partial replacement of different parts of human components (bones, skin, cartilage, etc) [1, 2, 3, 4, 5], to suport different body structures and organs or used for diagnosis, therapy [6] (drug delivery for example) [7] or reconstruction surgery [8,9].
The repair of bone fractures or reconstruction of bone cracks is a very complex process of bone regeneration. Over time, the researchers in the field developed starting materials (3D scaffolds) to support different types of cells, from osteoblasts, osteoclasts, or osteocyte cells to bone lining cells [10,[17][18][19]. Porous structures with interconnected porosity or open cells allow the transport of body fluid respectively the growth and regeneration of bone in the pore areas, and thus a viscoelastic biomaterial with a remarkable regeneration capacity is obtained [20]. By developing the porous scaffolds, it is necessary to have some essential requirements (characteristics) [21][22][23][24][25][26], briefly presented below in the   [62], heart tissue [63] or in hard tissues such as bone and dentin [35,44,45,64]. For soft tissues, scaffolds are made of polymer-based materials, and for hard tissues, metals, ceramics, and metal-or ceramic-based composites can mostly be used. Bone tissue has a relatively tough and flexible collagen structure compared to other body tissues and it is also reinforced by calcium phosphate nanocrystals. When developing traditional and new biomaterials, the specific characteristics of the main categories of biomaterials used must be taken into account [23,47,48,51,[65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82].
The advantages and disadvantages of the main types of materials used in regenerative medicine and tissue engineering are briefly presented in Figure 2. Hydrogels are 3D network of natural or synthetic polymers and can serve as a support to ensure the structural integrity of tissues, control the delivery of drugs and proteins to tissues and cultures, and also serve as adhesives or barriers between tissue and material surfaces [10,59,97,98]. Hydrogels can be obtained from natural polymers known for their application, for example in corneal defects due to their very aqueous environment, biocompatibility, and very transparent nature Mo, AISI 316L, Pt,etc) bioactive ( tantalum, Co-based alloy/ FA nano-composite), or biodegradable(AZ 31, AZ 91, Zn-xCu (x=1-4 wt%), Zn-3Cu-xMg (x=0.1, 0.5 and 1.0 wt.%), Fe-35Mn, Fe-10Mn-1Pd, Fe -30Mn-6Si; Tungsten, etc.), which over time have been intensively studied [99].
.   [122][123] and has the advantages of high porosity (45%-80%), interconnected pores, appropriate pore size (200-500 μm) and good elasticity and compressive strength [43] (viii) 3D bioprinting techniques [62,[124][125][126] and so on. The 3D bioprinting techniques gives the material fast fabrication of composite structure, high porosity, low temperature process and complex structures. To build complex 3D biological models, we can use, as novel developments in advanced materials fields, three-dimensional (3D) printing. This allows a controlled deposition of cells, biomaterials, and biological compounds (e.g. bioInks). The development of responsive biomaterials as bioInks and their bioprinting technique are today in the attention of researchers, due to their controllable properties of the material (the response to external or internal stimuli induced by printing can be controlled). Recently, a novel porous biomaterials was developed by 3D printing by a team of researchers from Switzerland and the United Kingdom [22] They developed an innovative technique for obtaining bone biocomposites, using an ink loaded with bacteria. One of the advantages of this technique consists in obtaining mineral-based materials with a 3D structure, identical to the initially printed polymeric scaffold, with a high hardness and rigidity and much lighter compared to CaCO3 (93% of the weight of CaCO3). The secret of the success of researchers Hirsch et. al.

NOVEL DEVELOPMENTS IN ADVANCED MATERIALS USED AS SCAFFOLDS
[22] from the École Polytechnique Fédérale de Lausanne and the University of Cambridge consists in the fact that, inspired by nature, they designed an easy and versatile process based on bacteria, to produce porous composites based on CaCO3, composites composed exclusively of materials derived from nature and similar to the structure of human trabecular bone. This is achieved by the manufacture of gelatin microgels containing ureolytic bacteria [22], (eg

Figure 4. Fabrication of 3D printed biomineral composites (production of bacteria-loaded microgels and biomineralization of 3D printable granular bioink (BactoInk) through microbially-induced CaCO3 precipitation
For the success of making these bone biocomposites, it is necessary that the microgels be biocompatible, solidifies under conditions compatible with bacteria, and be sufficiently concentrated to allow 3D printing. Nowadays, in the context of increasing resistance of bacteria to antibiotics and to prevent implant infections, it determines the need for alternative solutions, such as antibacterial implant coating, that combining antimicrobial peptides (AMPs) and silver nanoparticles (AgNPs) for dual impact [127]. AMPs , nanoparticles, metal ions, cationic polymers release antimicrobial agents, induce killing by contact or prevent bacterial adhesion and fouling.
A non-traditional way to obtain biomimetic mineralized hybrid scaffolds through mineralization was followed by Ye and his collaborators [128]. They developed by solgel method, the mineralized scaffolds which are characterized by the fact that they can kill different pathogens by contact, such as Escherichia coli, Streptococcus gordonii as well as cytocompatible human bone marrow-derived mesenchymal stromal cells. In Figure 5 are represented schematic the intrafibrillarmineralized collagen with antimicrobial peptides coatings [128]. hydrophobicity. The obtained scaffold is a highly hydrophilic mineralized collagen scaffold that which gives an ideal substrate to form a dense and stable coating of the antimicrobial peptides. To develop better implants for biomedical applications, the sol-gel technique has allowed the creation of new generations of bioactive glasses with great potential. Avram et. al.
[24] made by the sol-gel method three different types of calcium phosphate glasses from ternary system SiO2-CaO-P2O2: the reference sample P (Fig. 6 a) not doped with metals, which is the basic composition, (Table 1 ), the second sample, PA type is doped with silver (Fig. 6 b) and the third sample PC, is doped with copper (Fig. 6 c) [24]. They determined the minimum bactericidal dose of each type of glass on two strains of bacteria with high pathogenic potential.  They concluded that bioglasses doped with silver (PA sample) have a more effective antimicrobial activity than those doped with copper (PC), but in the long term, during the exploitation period of the implant, which can last from a few weeks to a few years, this difference in the microbial activity of the two compared samples becomes insignificant.
In the case of using bone supports doped with silver, they can also reduce the incidence of infection or even fight it. It is known that implanted scaffolds are treated as "foreign" objects by the body's immune system and can be colonized by bacteria, leading to infection. To give the scaffolds inherent antibacterial properties, the team of Sánchez-Salcedo et al., [26] from Universidad Complutense de Madrid and CIBER de Bioingeniería, Biomateriales y Nanomedicina incorporated silver nanoparticles (AgNPs), which have well-known antibacterial properties, in their scaffold matrix.
They used an innovative one-pot sol-gel method to produce mesoporous bioactive glass (MBG) matrices based on SiO2-CaO-P2O5 doped with metallic AgNPs and combined this with rapid prototyping (RP), which creates structures with ultra-large dimensions. As a result of the antimicrobial tests performed on these new materials, it was shown that the growth of Staphylococcus aureus and Escherichia coli is reduced (inhibited) due to the presence of AgNPs in the mesoporous bioactive glass matrix.
Because it is difficult to obtain synthetic bone grafts that mimic the compositional content of bone, it is preferable to produce bone graft materials from natural materials.
To imitate the structure, morphology, and mechanical properties of native bone tissues, as natural biocomposites, different methods are used, the newest of which are those that use biological or of vegetable or animal origin from agricultural wastes as raw material. For example, natural materials like bovine bones, deep-sea snails, coral seashells, eggshells etc., are sources of natural hydroxyapatite. Unal et al. [23] in their study they used bovine femoral and tibial cortical bones, as the best bulky parts of the bones for the extraction of hydroxyapatite bioceramic material. The processing route for obtained synthetized and bioinspired calcium phosphate composites is schematically presented in Figure 7. The use of food waste such as eggshells in the development of biomaterials has the advantages of bone mineralization and growth, treatment of osteoporosis, and therefore is used as a bone graft.
In order to improve the quality of life of patients with bone diseases, the team of Russian researchers led by Choudhary [129] have synthesized a bioactive polymerceramic composite from eggshells, for fixing implants and restoring bone defects of the skull. Synthesis process of (a) magnesium-based silicate bioceramic called diopside, of (b) calcium silicate wollastonite (CaSiO3) and (c) forsterite (Mg2SiO4) are presented in Figure 8.

CONCLUSIONS AND FUTURE PERSPECTIVES
Innovation in bone regeneration is essential to ensure fixation of artificial joints/implants and dental fixations (eg fiber reinforced cement improves stabilization of dental implants in bone [83].In this brief review, the main characteristics of an ideal porous biomaterials used as scaffolds were presented and the characteristics of bioceramics, biopolymers metallic biomaterials and biocomposites used for medicine and tissue engineering were discussed. The requirements and main applications of metallic biomaterials and new developments of advanced materials used as scaffolds, such as 3D printed biomineral composites with with bacteria-loaded ink (bactoInk),the use of plant or animal raw materials or waste, and the creation of new biomaterials that can reduce or combat the infection of scaffolds after implantation, have been also discussed. In the future, researchers are concerned with developing new strategies to design successful advanced polymeric biomaterials, such as smart (polymeric) biomaterials with self-healing and shape memory properties and other innovative, advanced type of biomaterials for regenerative medicine and tissue engineering.