The Influence of Scaffold Interfaces Containing Natural Bone Elements on Bone Tissue Engineering Applications

Abstract

Bone has the capacity to repair damage and reproduce itself, but if the defect is too large, a scaffold is needed to promote regeneration. Recently, researchers have developed numerous approaches to promote bone reformation, such as the direct delivery of bioactive molecules, guided tissue regeneration membranes, and creating osteoinduction/osteoconduction surfaces. The surface of a medical device is the first contact area for a biological system; therefore, the interactions between biological fluids and the surface of the implant determine the performance of the implant. Well-designed surface physical and chemical properties, such as topography, net charge, components, and hydrophilicity, enhance cell attachment and proliferation. Various surface modification technologies and methods have been studied to enhance cellular expression. This review selects scaffold materials that are FDA-approved and have been widely used in the clinic and focuses on recent studies of surface modification with hydroxyapatites and collagen, which are the main components of the bone matrix, for the enhancement of bone regrowth.

1. Introduction

Small bone defects can recover through the healing process, but large defects, such as those resulting from cancer and trauma, et al., are generally extensive and therapeutically challenging [1]. Autografts, allografts, xenografts, and artificial bone-graft materials are current treatment approaches. Limited supply and donor-site morbidity are the drawbacks of autograft transplantation, and the major disadvantages of allograft or xenograft transplantation are host rejection and the disease xenozoonosis [2]. One tissue engineering approach is when scaffold materials with or without growth factors are precisely embedded into the target sites, and they establish in situ tissue formation to connect the degenerated and scaffold materials. Scaffolds, cells, and growth factors are well known as triangles of tissue engineering [3,4], but with advancements in materials science, it has been disputed whether growth factors are fundamental for bone tissue engineering. A collection of biomaterials has been investigated and used clinically for bone repair as well as regeneration. Degradation of implantation is often accomplished with an unexpected reduction in mechanical properties. However, if the degradation rate can be controlled, the loads will be transmitted from the implants to the host tissue and soft tissues to prevent the stress concentration effect [1,5]. Advancements in the materials used in biodegradable rods, plates, pins, and screws, including stitch anchors, have occurred in recent years. In the past two decades, interest has developed in providing tissue regeneration in the craniofacial region and periodontal regions [6].

Bone tissue is a complex nanocomposite composed of 70% inorganic phase and 30% organic phase in which the mineral phase is intimately embedded into organic matrices. The inorganic phase of bone is, poorly, microcrystalline hydroxyapatite ([Ca₅(PO₄)₃OH], HAp) with an atomic Ca/P ratio of 1.67. The microcrystalline nature of HAp is a specific feature of biological systems, and it causes HAp adsorption in the human body to maintain the metabolic balance of calcium and phosphate. The organic phase of bone comprises type I collagen (90% w/w) and noncollagenous proteins such as osteocalcin, osteopontin, and bone sialoprotein, which are mainly synthesized and organized into specific architectures by osteoblasts. These cells become osteocytes after bone matrix formation. This means that attracting osteoblasts attached to or anchored to the implant surface is the critical step in enhancing bone formation around the implant and integrating the implant with host tissue. Calcium phosphate-derived ceramics, particularly HAp, have attracted tremendous attention from tissue engineering researchers. This is because synthesized HAp is similar to the mineral constituent of natural bone in its chemical and structural properties [7]. Synthetic HAp is an osteoconductive material that enables bone cell attachment and growth at the bone–implant interface and is a functional material to connect with natural bones [8]. Unlike proteins, HAp does not have significant immune responses and exhibits high biocompatibility [9]. Therefore, the formation of a layer of biological apatite on the bone graft surface or the enhancement of proteins within blood and tissue fluids adsorbed on the surface after implantation are major approaches used for promoting cell attachments on the implant surface.

To promote the bone remodeling process, biomaterials acting as temporary scaffolds are generally divided into three categories according to their function: osteoconduction, osteoinduction, and osteogenesis. Osteogenesis means that implants achieve bone formation by incorporating live osteoprogenitor cells or inducible osteogenic precursor cells. These cells enhance the connection between the implants and the host tissue in an early stage of therapy. Additionally, encapsulated cells form a bone matrix inside the implant toward the outside, which shortens the bone damage repair process. However, maintaining the biological activity of entrapped cells in the long term and the cell source are key challenges of this method. Moreover, providing a temporary/biodegradable scaffold to support cell regrowth instead of providing permanent implants to supplement biomechanical strength is an essential requirement of the osteogenesis approach. Therefore, the review does not discuss scaffolds used in osteogenesis applications. In the past several decades, one type of biomaterial has exhibited the capability to activate bone formation while implanted in heterotopic sites, a capability known as osteoinduction. Such osteoinductive biomaterials have extreme potential for the advancement of new therapies in bone regeneration. Osteoinduction means that the materials have the potential to attract cells and stimulate cells toward preosteoblasts [9]. This implies which basic, identical, and pluripotent cells are turned on to promote the bone-forming cell lineage [10]. Although a collection of well-characterized osteoinductive biomaterials have been proclaimed in the literature, scientists thus far have lacked a full understanding of the biological mechanisms behind this phenomenon [11]. Another approach is the incorporation of bioactive factors into the osteoconductive matrix to improve the efficiency of osseointegration. Osteoconduction is widely defined as the potential of materials to assist bone growth [12]. This process is essential to the field of dentistry, as it is fundamental for implant replacement; however, for osseointegration, establishing long-term interactions between living bone and implants is demanding [13]. Bone regrowth processes cannot be activated by osteoconductive material, which instead serves as a template for nearby bone cells that are growing and remodeling. Osteoconductive materials require the material surface to ease osteoblast lineage cell adherence, proliferation, and maturation. Regardless of the material types used as scaffolds, the degree of cell attachment at an early time is a critical parameter that determines the duration of repair. Therefore, preparing an osseointegration surface as a substratum for attached cells is a key feature of the interface for most osteoconductive materials.

Nanomaterials with close proximities in scale to the scale of natural bone composites have led many scientists to devote themselves to the development of nanomaterials for bone tissue engineering applications. If the amino acid sequence related to cell attachment on surfaces is designed in nanometer topography, cell adhesion levels can be promoted. The adsorption of bioactive molecules to create surface toughness is a general method; for example, adsorption-enhancing osteoblasts and bone-forming cell function biomolecules fibronectin and vitronectin [14] can significantly decrease the grain sizes of the scaffold surface to less than 100 nm, and this is essential for consecutive bone cell functions (e.g., proliferation and differentiation). Additionally, it has also been shown that the unfolding of these proteins leading to bone cell adhesion and function can be modulated with calcium-mediated cell protein adsorption on nanophase materials [15]. Nanos and microscale pore sizes have also been shown to promote bone cell differentiation [16] and osteogenesis [17]. The successful use of nanomaterials for better osteointegration of orthopedic implants and bone tissue engineering approaches has been extensively summarized in several recent reviews [16,17,18,19,20] and will not be reviewed here. This review focuses on the influences of surface properties altered by key components of bone matrix, HAp and collagen, on bone tissue engineering applications, especially poly(ε-caprolactone) (PCL), poly(lactic-co-glycolic acid, PLGA), and chitosan, which have been widely used in clinical studies. Moreover, this review describes fabrication techniques used to prepare scaffolds, which can be modified to promote bone regeneration.

2. Influences of Key Inorganic Components of Bone Matrix Nanohydroxyapatite

Each cell type has a specific binding motif on the extracellular matrix, and these interaction spots are nanometers in size; therefore, the physical properties of a surface such as roughness and topography are generally designed in nanoscales. In vitro, osteoblast adhesion, proliferation and differentiation, and mineralization are enhanced for nanomaterials with grain sizes less than 100 nm [21,22]. The adherence of osteoblasts on the surface covered with titanium nanoparticles is threefold that on the conventional titanium-particle-coated surface [23]. It becomes clear that anchorage-dependent bone-forming cells on rough scaffold surfaces show higher attachment, proliferation, and differentiation efficiency. Galindo et al. [24] reported that a new method accumulates nHAp on a polymer matrix to produce composites with better physical properties and higher biocompatibility and bioactivity. To establish new biomedical applications, the interfacial interactions between polymer and HAp in biofusion should be investigated. Simple coating techniques have been used to establish nHAp/polymer blending interfaces for in vitro studies of interfacial interactions between proteins and cells, which reveal better biocompatibility, mechanical properties, and biodegradability. The cells cultured with the spherical nHAp suspension show better cellular responses including adhesion, proliferation, and cell–matrix interactions, which implies that spherical nHAp particles are more useful than acicular (needle-like)-shaped particles. The studies demonstrate that nHAp with a particle size of 20 nm increases the viability and proliferation of bone marrow mesenchymal stem cells (BMSCs) and represses the growth of osteosarcoma cells. Nano-HAp particles with diameters of approximately 50 nm exhibit apoptotic action of the hepatoma cell line. Studies of interfacial interactions formed between biomolecules and nHAp were performed, and the distribution of these interactions by appropriate protein preadsorption was also summarized. In addition, compared with pure HAp, doping strontium in HAp can improve solubility and promote the differentiation and mineralization of osteoblast-like cells, therefore, Sr–HAp as a substrate for bone cell culture scaffolds have attracted increasing attention in recent years [25,26,27]. Moreover, some studies have proven that bioglass (BG) can lead to the formation of a hydroxycarbonate apatite layer on the surface when BG contacts with body fluid, which then enhances bone regeneration [28]. Rizwan et al. [29] reported a low-pressure spark plasma sintering (SPS) procedure to prepare HAp–bioglass (BG) composite scaffold materials with BG contents up to 30 wt.%. When compared with traditional sintering, the delicate processing of the SPS procedure with compaction pressure and sintering time provided HAp–BG composite scaffolds without inordinate reactions among constituents, which also prohibited the crystallization of the BG. Increasing the BG content not only strengthens the physical properties, such as the relative density, bulk density, and hardness, but also demonstrates the enhanced bioactivity of the composite samples in vitro.

2.1. Poly(ε-caprolactone)

In bone engineering applications, poly(ε-caprolactone) (PCL) can be classified among materials that enable biocompatibility with biodegradable polymers, as it is used to treat bone defects and has the effects of increasing bone ingrowth and regeneration [30,31]. Moreover, its good flexibility makes it an attractive candidate for accompanying tissue-guided regeneration (GTR) as a membrane for the treatment of periodontal diseases [32,33]. Nyberg et al. [34] reported the first successful 3D-printed composites with porous PCL mineral scaffolds and differentiated their relative capacities to drive osteoinduction in adipose-derived stem cells. Herein, the 3D-printing system PCL–Bio-Oss (BO) along with PCL–decellularized bone matrix (DCB) displayed more osteoinduction than the synthetic materials PCL–hydroxyapatite (HAp) or PCL–tricalcium phosphate (TCP). However, their results demonstrate that doping 3D-printed PCL scaffolds with DCB, BO, or synthetic particles provides different degrees of bone healing support in vivo.

Cho et al. [35] reported that the PCL–hydroxyapatite nanoparticle (nHAp) composite structure and the exposure of nHAp enhanced the mechanical properties as well as the bone-regeneration capability of the material-extrusion scaffold. In their study, a PCL–nHAp composite scaffold with a crystalline structure was assembled using a material extrusion method, indicating that mechanical properties were enhanced not only by the composition but also by the structure. Meanwhile, the mechanical properties and in vitro cell response were enhanced by increasing the weight ratio of the resolved nHAp. The electrospinning technique is a useful method to fabricate a mimic extracellular matrix (ECM) nanofibrous structure scaffold. The gelatin/PCL electrospun matrix surface coated with nHAp not only increased the roughness of the surface to enhance cell adhesion but also established a biomimetic environment to promote cell growth [36]. Naudot et al. [37] evaluated the efficiency of bone regeneration in a 3D cylindrical structure composite created from electrostatic template-excited deposition through the alternate deposition of electrospun PCL nanofibers, including electrosprayed hydroxyapatite nanoparticles (nHAp), to produce a honeycomb micropatterned substrate. They first established the cytocompatibility of this honeycomb PCL–nHAp scaffold in culture with BMSCs. The scaffold was then embedded (with or without seeded BMSCs) in a rat critical-sized calvarial defect model for 2 months. Microcomputed tomography was used every 2 weeks to observe new bone formation in situ. The images showed that the honeycomb PCL–nHAp scaffold was osteoconductive. In addition, the growth of the scaffold containing BMSCs was combined with remarkably greater bone volume during the 2-month experiment. Thus, if the biomimetic honeycomb PCL–nHAp scaffold can be loaded with patient MSCs, it might, therefore, have great potential in maxillofacial applications. Based on the Sr–HAp possessing elevated osteogenic potential relative to pure HAp, Tsai et al. [38] have produced PCL/Sr-substituted hydroxyapatite nanofibers (PCL/SrHANFs) with electrospinning to evaluate the behavior of osteoblasts. The results found that PCL/SrHANFs enhanced the expression level of osteogenic proteins, such as alkaline phosphatase (ALP), osteocalcin (OCN), and bone sialoprotein (BSP), compared to PCL membranes alone. Melt electrospinning writing (MEW) is a newly imminent 3D-printing technique to fabricate more advanced microfibril 3D scaffolds [39]. Wang et al. [40] united MEW with solution electrospinning (SE) technologies and successfully prepared PCL/gelatin micro/nano-fibril scaffolds. The mechanical strength of the scaffold was provided by the PCL microfibers and induced cell orientation, whereas the long-term hydrophilicity of the scaffold was increased by the inclusion of gelatin nanofibers and induced cell adhesion and proliferation. After 7 days of culture, their results showed that cells could be distributed throughout the whole multilayer dual-scale composite scaffold, which indicated that cells could migrate through the gaps between nanofibers. In contrast with conventional 3D scaffolds, the MEW composite scaffolds exhibited not only good mechanical properties in addition to cell orientation effects but also a cell-identified ECM-mimicking 3D microenvironment.

2.2. Poly(lactic-co-glycolic Acid), PLGA

PLGA has been used in several clinical applications and has been shown to be biocompatible, nontoxic, and noninflammatory [41,42] but rarely used in the orthopedics area owing to its low mechanical strength, ductile characteristics, and absence of osteogenic bioactivity [43]. Generally, the surface chemistry of PLGA does not entirely support cell adhesion for enhancing bone ingrowth and proliferation because of its hydrophobic nature [44]. Organic solvents are commonly used to fabricate polymer/ceramic composite scaffolds; however, they have the potential to destroy cells or tissues. In addition, composite scaffolds fabricated using conventional and traditional methods limit the exposure of ceramic on the scaffold surface. A novel fabrication technique was proposed to expose the ceramic on the surface of the scaffold to enhance the osteoconductivity and bioactivity of the scaffold. PLGA/nHAPF scaffolds were assembled through the gas spraying and particulate leaching (GF/PL) technique without the use of organic solvents [45]. Selective staining of ceramic particles showed that nHAp on the scaffold surface was more numerous on the GF/PL scaffold than on scaffolds prepared with traditional solvent casting and particulate leaching (SC/PL). In vivo evaluation was performed by embedding both scaffolds into critical-size defects in rat skulls for 8 weeks to observe bone regeneration. In defect areas, bone formation was more comprehensive on the GF/PL scaffolds than on the SC/PL scaffolds as examined with histological analyses and microcomputed tomography. Higher exposure of HA nanoparticles on the scaffold surface may be responsible for more bone formation on the GF/PL scaffolds than on the SC/PL scaffolds. Li et al. [46] utilized gelatin as the medium to coat nHA and chitosan on PLLA microspheres. Compared with noncoated PLLA microspheres, the greatest proliferation-promoting effect was observed with the nHA-coated microspheres, and the nHA/chitosan microcarriers enhanced osteogenic expression.

Wang et al. [47] established a biomimetic, ordered nanostructure on the surface of the interior pores of nHAP-coated PLGA scaffolds and studied the effects of the porosity of the PLGA/nHAP on cell behaviors. Both the viability and proliferation rates of cells implanted in the nHAP-coated PLGA scaffolds were greater than those in the PLGA scaffolds alone. In bone defect repairs, the radius defects had, after 12 weeks of implantation of nHAP-coated PLGA scaffolds, fully recovered with undoubtedly better bone formation than that seen with the group of PLGA scaffolds, as shown by X-ray, microcomputerized tomography, and histological experiments. Mao et al. [48] reported the assembly of poly(lactic acid)/ethyl cellulose/hydroxyapatite (PLA/EC/HAp) composite scaffolds for potential applications as weight-bearing bone replacements in tissue engineering. The mixed method used in this study greatly expanded the mechanical properties more than the particulate leaching method. The results show that PLA/EC/HAp scaffolds at the 20 wt.% HAp loading level demonstrated excellent mechanical properties along with the desired porous structure. The porosity, contact angle, compressive yield strength, and percent of weight loss after 56 days met the physiological requirements regulating bone regeneration. These results suggested that the PLA/EC/HAp scaffolds assembled by combining several fabrication techniques such as high-concentration solvent casting, particulate leaching, and room-temperature compression molding have possible applications in bone tissue engineering.

2.3. Naturally Derived Materials

The degradation rate of chitosan mainly depends on the degree of crystallinity and acetylation [49]. Chemical alteration of the chitosan polymer significantly affects the degradation and solubility rate, and the biodegradation of a highly deacetylated form was observed over several months in vivo [50]. In brief, chitosan can be degraded by nontoxic products in vivo, so it has been commonly used in diverse biomedical applications. However, its application in tissue engineering is limited by poor hydrophilicity and cytocompatibility. Acevedo et al. [51] reported that they prepared a new gelatin–chitosan polymeric membrane that incorporated osteoconductive materials, nHAP, and titania nanoparticles. Modulation of the mechanical properties and the biodegradable rate of the nanocomposite membrane was completed with UV-radiation-induced cross-linking. The basic function of gelatin-based materials is to act as a physical barrier, but titania and hydroxyapatite nanoparticles, known as osteoconductive materials, endow the composite with osteogenic potential. The described material is a great alternative to other bone-regeneration membranes and has the potential to be used in oral/orthopedic applications.

Chitosan is often used in a gel form for application; for example, Wang et al. [52] combined nHAp and chitosan to fabricate a drug-containing hydrogel to investigate its efficiency to treat infected bone defects. Their results showed that the infection was eliminated, and the defects were restored faster than those in the control group. Abazar et al. [53] reported that electrospun nanofibrous scaffolds were assembled by applying HAp, polyvinyl alcohol (PVA), chitosan polymers, and platelet-rich plasma (PRP), a bioactive substance isolated from human blood. An in vitro study of scaffold osteoinductivity was performed via osteogenic differentiation of mesenchymal stem cells (MSCs), and an in vivo study of osteoconductivity was performed by implantation into a critical-sized rat calvarial defect. In vitro alkaline phosphatase activity, calcium content, and gene expression assays demonstrated that the scaffolds had good structures and good biocompatibility. The in vivo results showed that the defect was renovated to a good extent in animals implanted with PVA–chitosan–HAp. One of the main challenges for preparing polymer and nHAp composites as bone scaffolds is physicochemical homogeneity. Correia et al. [54] developed a new procedure to fabricate a nanocomposite composed of in situ growth of nHAp coated with alginate (ALG), overcoming the problem. Then, they mixed nHAp/ALG nanocomposites with polyvinyl alcohol (PVA) to obtain nanofiber matrix scaffolds with electrospinning.

Beta-tricalcium phosphate (β-TCP) is an osteoconductive material that has been used in clinical applications for several years, and it has also been found that β-TCP can transform into HAp under stimulation of the osteoclastic degradation environment in vitro [55]. Huang et al. [56] established an organic, solvent-free, injectable, mechanically strong, and biodegradable material by incorporating different concentrations of β-TCP into PCL with the intention of use for medical purposes. Xu. et al. reported [57] that a hydrophobic derivative of agarose, agarose acetate (AGA), was blended with the bioactive material β-TCP to assemble AGA/β-TCP nanofibrous membranes through an electrospinning process. After the assembly of β-TCP, the hydrophilicity and mechanical properties of agarose acetate were significantly improved. rBMSCs expressed higher proliferation and osteogenic differentiation levels on nanofibrous membranes containing the β-TCP matrix. In addition, the nanofibrous membranes showed no accessible inflammatory response in vivo during 4 weeks of implantation.

3. Influences of Key Organic Components of Bone Matrix Collagen (Proteins/Peptides)

Numerous peptides have been shown to assist and trigger a bone healing response and have been recommended as therapeutic carriers for clinical use [58,59]. The objective of this section is to present a summary of the potential roles of proteins/peptides in bone healing and bone regeneration based on osteoinductive materials reported previously. Polypeptides used in bone regeneration combine with acidic poly(amino acids), such as gamma-poly(glutamic acid) (gamma-PGA) and poly(aspartic acid), and basic poly(amino acids), such as polylysine and polyarginine. Hsieh et al. reported [60] that gamma-PGA can be well blended with chitosan to assemble gamma-PGA/chitosan composite porous matrices. The hydrophilicity, including serum protein adsorption capacity, of the composite matrix increased significantly. The addition of gamma-PGA increased the maximum load strengths of the composite matrices, thereby enhancing the mechanical properties. They also found that the cells attached and propagated more effectively on gamma-PGA/chitosan composite matrices than on the chitosan matrix.

Many tissue regeneration studies have been conducted on collagen-based scaffolds because of their native biological properties. Composing a biomimetic structure by covering bioactive molecules is a useful approach to generating a more acceptable cell response; thus, herein, the modification of bulk polymer properties with collagen is addressed. Electrospinning (ES) is a cost-free and simple controlled parameter procedure to produce nanofibrous substrates. General synthetic polymers exhibit limitations, such as hydrophobicity, insufficient cell recognition sites, and osteoconductivity. Many studies have improved these intrinsic properties using a traditional treatment that directly immerses the polymer substrate in collagen solution followed by cross-linking to obtain a collagen layer on polymer surfaces, such as the PCL–ES surface, which is activated by an alkaline treatment followed by EDC/NHS coupling collagen [61]. Recently, an increasing number of researchers have improved ES techniques to prepare natural/synthetic composite ES matrices such as mixed collagen with synthetic polymer solutions to form ES-based membranes rather than surface coatings [62,63], sequential electrospinning of polymers and collagen to make composited ES matrices [64], or preparation of core–shell nanofibers using a coaxial electrospinning process [65]. In comparison with the traditional modified membrane, an almost greater cell response was exhibited. Similarly, the review does not cover the type of modification. 3D printing is the newest technique to fabricate a well-designed structural scaffold; however, the critical point is to improve initial cell attachment instead of passing through the porous matrix during in vitro cell seeding. Ebrahimi et al. [66] used plasma-pretreated 3D-printed PCL scaffolds to improve the hydrophilicity of PCL followed by modification with HAp and COL I. Their results showed that HAp and COL have a synergistic effect on hADSC proliferation and the expression of osteogenesis genes. Tabatabaei et al. [67] used a similar method to coat collagen on 3D-printed PCL/TCP scaffolds. They also demonstrated that collagen-coated scaffolds have higher bioactivity than noncoated samples.

Except for collagen, to enhance the synthetic polymer scaffold’s performance, the polymers were coated with PDA (polydopamine) to enhance the osteogenic differentiation of adipose-derived stem cells (ADSCs) [68] grafted with bioactive molecules such as proteins and peptides or composited with nHAp. Li et al. [69] investigated the efficiency of exosomes derived from osteogenically committed ADSCs in encouraging the osteogenic differentiation of bone marrow MSCs (BMSCs) in bone tissue formation. The exosomes were bound to a PLGA/PDA matrix, which led to a slow and controlled release coordinated through their biodegradation properties to mimic physiological release conditions. After approximately 48 h, BMSCs had almost completely internalized the exosomes that stimulated cell proliferation and migration, including osteogenic differentiation, in vitro. Furthermore, compared to the PLGA/PDA scaffold embedded on a murine critical-sized calvarial bone defect, the PLGA/PDA/exosome constructs were more effective for stem cell migration, homing, and new bone formation.

One simple way to imitate the physiological environment is to functionalize biomaterial surfaces, including ECM-derived peptides capable of engaging stem cells, as well as generating lineage-specific differentiation. Bilem et al. [70] reported that human bone marrow mesenchymal stem cells (hBMSCs) engaged in differentiation are prescribed through bioactive molecules insulated within their extracellular matrix (ECM). They studied the effects of RGD and BMP-2 (bone morphogenetic protein-2) ligand crosstalk on the expansion of hBMSCs’ osteogenic commitment without choosing a differentiation medium. The RGD peptide stimulated cell adhesion to cell transmembrane integrin receptors, while the BMP-2 peptide, analogous to residues BMP-2, was shown to prompt hBMSC osteoblast differentiation. Hu et al. [71] initially prepared core–shell poly(lactide-co-glycolide) (PLGA)/polycaprolactone (PCL)/BMP-2 (PPB) fibrous scaffolds using coaxial electrospinning. 3D scaffolds were established on the PPB fibers with the thermally induced self-agglomeration method. The 3D scaffold increased the osteogenic differentiation of rADSCs, as shown by the PPB group. The synergistic effects of the 3D scaffold along with peptides (BMP-2) significantly upgraded the drift compared to the 2D scaffolds in vitro.

4. Conclusions

Surface properties/energies change with different manufacturing techniques, and this can result in variations in protein adsorption on the material surface, which in turn determines scaffold performance. The present review focuses on recent scientists utilizing HAp or collagen to modify scaffold surfaces using different approaches, and all of them showed positive responses in terms of material properties and cellular responses. Currently, scientists can manipulate materials at the atomic, molecular, and supramolecular levels, and bulk materials and surfaces can be designed with dimensions similar to those of the components of native bone. However, thus far, few new products have been used in clinic, indicating that many challenges still need to be overcome. For example, what is the best way to rationally design a scaffold with a proper bioactivity surface that is suitable for cell adhesion, proliferation, and differentiation so that bulk material properties cannot be represented? Surface properties have been shown to influence cell–biomaterial interactions, and it is important to understand how these interactions are regulated through adsorbed proteins. After all, the absence of standards for describing different surface morphologies has led to convoluted explanations in various studies. Surface area modifications that follow surface topography changes are generally not corrected for, and the unexpected creation of restrained spaces during surface fabrication could occur in parts that act structurally differently from the rest of the material. Furthermore, the many techniques used to assemble different surface compositions also adjust surface chemistry, making it difficult to determine cause and effect. Until now, there has been no consensus on what surface properties are essential for a scaffold to promote bone regrowth. We hope that this review will provide a summary of recent studies on surface modification using HAp and collagen, thereby inspiring more broader developments to meet clinical needs.