Nanostructure materials are those materials which have their dimensionality in the range of nanometers [ 1, 2 ]. Nanomaterials are synthesized using two major approaches: topdown and bottom-up techniques. Self-assembly is spontaneous assembly of constituents to form a complex nanostructure in the absence of significant external intervention. There are two types of self-assembly intermolecular and intramolecular self-assembly. Self-assembly is highly useful because it provides the path for the aggregation of structures, which are very small, to modify individually into the organized patterns that often give various functions to materials. Self-assembly is the result of a combination of weak forces such as noncovalent interactions, hydrogen bonds, electrostatic interactions, pi-pi stacking, hydrophobic forces, van der Waals forces, and chiral dipole-dipole interactions. Selfassembly of nanomaterials is affected by interparticle interactions, particle size, and particle shape [ 3 ]. Selfassembled nanomaterials represent a classic of induced noncovalent interactions [ 4 ]. The cooperative association of disordered nanomaterial building blocks contributes to the spontaneous creation of more ordered (or organized) nanostructured structures [ 5 ]. A highly structural nanoparticle assembly is assessed for application in wiring, superlattice creation, and rings. Several self-assembly systems were created, from copolymers in blocks to three-dimensional (3D) cell culture scaffolds. Because of its applications in optical materials, nanoelectronics, computing, photonics, medical imaging, and diagnostics, the functional material assembly draws attention. Interactions between the materials, particle size, and particle composition affect nanomaterial selfassembly [ 4–6 ].

There are several standard features of the various types of self-assembly developments in nanoscience that allow for the adoption of a conceptual framework in the context of the following three phases (Figure 1).

Spontaneous self-assembly of spherical and nonspherical nanoparticles is distinguished from one another. Polydispersed spherical nanoparticles, 5% self-assembled 2D or 3D compact structures, and nonspherical nanoparticles exhibit different ways of self-assembly [ 8 ].

Significant instances of the self-assembly mechanism are originated in biomaterials, where the combination of different macromolecular components and the coordination of their activities allow for extremely complex functions of biological interest [ 9–11 ]. For instance, folding polypeptide chains within different functional types of proteins or conformation nucleic acids are essential instances of procedures of self-assembly in numerous biological functions [ 12, 13 ]. Proteins are an excellent instrument for modern nanotechnology and serve as building blocks for quaternary frameworks and functional selfassembled nanostructures [ 14 ]. These nanoassemblies were used to generate hierarchical protein nanostructures, counting 1D (tubules/strings/nanowires), 2D (nanorings/networks), and 3D (hydrogels and crystalline frames) [ 15 ]. There are also three main forms of carbon-based nanostructured materials amongst the utmost common organic nanomaterials, which play an important role in the development of modern nanotechnologies, specifically graphene, carbon nanotubes (CNTs), and fullerene (Figure 2). Such nanomaterials are significant nanoplatforms in the creation of emerging nanotechnologies, with good mechanical properties, high aspect ratio, outstanding optical activity, and multifunctional surface properties [ 7, 16, 17 ]. Of particular interest is the use of selfassembling CNTs in biomedical applications. Single-layer carbon nanotubes (SL/CNTs) are capable of transferring imaging agents (radioisotopes or fluorophores) or medicines to certain tumors and provide major recompenses over other methods based on nanoplatforms [ 18, 19 ]. A fullerene-based nanocarrier tool for doxorubicin drugs has recently been designed to treat potential lung cancers [ 20 ]. Fullerene derivatives with well-established functional properties are also capable of nanostructures for bioactive macromolecules that are optimally distributed [ 21 ]. Graphene is used as a matrix that interferes with various cells and biomolecules [ 2, 22 ]. The peculiar characteristics of graphene, such as excellent physicochemical, electrical, large surface area, and biocompatibility, have been revealed in the past decade, leading to ongoing research into the use of graphene nanomaterial in various clinical applications and regenerative medicine [ 22 ]. The graphene-based materials can appear as nanomaterials of the next decade [ 23 ]. Graphene promotes interest in biosensing and bioimaging as two-dimensional, three-dimensional, and hybrid. The researchers should find the use of graphene nanomaterials in different tissue scaffolds as a significant field of interest shortly [ 2, 24–26 ].

The mechanisms of self-assembly of biomolecules to different nanostructures have been extensively studied, and some reviews of the synthesis, design, and applications of self-assembled biomolecular nanomaterials have been recorded [ 27–29 ]. For example, Yang and colleagues presented a summary of the self-assembly of proteins into different supramolecular products, in which the design techniques for the self-assembly of proteins were applied and discussed in detail [ 30 ]. Willner and Willner reviewed the applications of nanostructures and nanomaterials constructed on biomolecules for sensing and nanodevice manufacturing [ 31 ]. After studying these reports, we realized that contributing a review on the self-assembled 3D poly functionalized nanostructures is still valuable for us. Therefore, in this study, application of 3D self-assembly (such as hydrogels, CNTs, graphene oxide, nanodiamonds, and buckminsterfullerene) was explained in gene delivery, small molecule drug delivery, cancer therapy, and tissue engineering. 2. 3D Self-Assembled Nanostructures for

Tissue Engineering Tissue engineering celebrates the ability to reconstruct and remove damaged sections of the body by developments in the medical industry. The significant growing need for organ and tissue transplants has sparked ongoing research into the cell’s rejuvenating properties. It promoted the development of a new tissue engineering approach, as the primary response to tissue and organ destruction. The key factors to be discussed in cell regeneration include the structure and origin of the cells, the scaffolding materials used, scaffolding design, the cell, and the outer tissue forming environment [ 32, 33 ]. Because of the nanoscale structure of human tissues (Figure 3), advances in nanotechnology have led to advances in regenerative medicine with the potential to replicate nanoscale composition and function of human tissues and organs [ 34 ].

Peptide amphiphiles, carbon nanotubes (CNTs), selfassembled peptides, electrospun fibers, and layer structures are among the most widely used nanomaterials [ 21 ]. The development of new nanostructures composed of bioactive molecules capable of directly and reproductively interacting with cell receptors and proteins to control procedures such as cell proliferation, cell differentiation, cell production, and tissue and organ regeneration dedifferentiation has called natural attention to this. Various studies using

Building blocks Initial configuration

IC (r0,t0)

FINAL STATE Self-assembled material

FS (r,t) Stages of Self-assembly Processes

Supramolecular

interaction H H

N

N O N H N

N O O

N

Ru N N

N N

N

O Possible switching interaction by Stimuli

Electronic components diodes/transistors

Drug delivery Carbon nanotube Graphene

Fullerene

New materials (Bio-)Sensors nanostructured materials have shown the feasibility of this method and its use in the regeneration of various tissues (such as the heart, bone, nerve, cartilage, lung, skin, and vascular) by improving the biological properties of cells such as cell adhesion, proliferation, and cell differentiation [ 3, 37–40 ], respectively. In the in vivo framework, the cells are located in three-dimensional (3D) microenvironments, encompassed by certain cells and the extracellular matrix (ECM) whose elements, such as elastin, laminin, and collagen, are organized into nanostructures (i.e., triple helixes, fibers) with various bioactive reasons for cell homeostasis regulation in three-dimensional (3D) microenvironments; the cells are in vivo surrounded by other cells and an extracellular matrix (ECM) whose components, such as

Axon initial segment

Neural tissue

Tropocollagen triple helix ~1.5 nm m 003n Bone tissue collagen, elastin, and laminin, are organized into nanostructures (i.e., fibers, triple helixes) with different bioactive motives regulating the homeostasis cell [ 41, 42 ]. 2.1. 3D Self-Assembled Peptide Hydrogels in Tissue Engineering. Bone tissue is a particularly complex example of such a composite because it contains multiple levels of hierarchical organization. Various nanoscale protein filaments (e.g., peptides) can be inserted into high-aspect nanofibers, which can replicate the in vivo cells’ internal microenvironment. Such nanofibers wrap round body cells that stretch long distances across their surfaces and act as cables that mechanically link and sustain adjacent cells through the creation of three-dimensional networks (Figure 4(a)) [ 43 ]. For example, one form of peptide amphiphiles has been investigated to produce nanofibers for bone tissue engineering via a pH-induced selfassembly process [ 44 ]. These peptide amphiphiles contain several main structural features, including long hydrophobic alkyl tails that accumulate in aqueous solution to drive self-assembly, four consecutive cysteine residues forming disulfide bonds to polymerize self-assembled structures, a bonding region of three glycine residues to provide flexibility for the hydrophilic head group, a single phosphorylated serine residue that strongly interacts with calcium ions to improve mineralization, and an Arg-Gly-Asp peptide ligand to enhance cell adhesion [ 37 ] (Figure 4(b)). Dithiothreitol-treated peptide amphiphiles at pH 8 are soluble in aqueous solution and begin self-assembled at pH 4. Fibers with an average diameter of 7 nm and a duration of up to many micrometers are produced and can be analyzed using electron microscopy with cryotransmission (Figure 4(c)).

In addition, a family of peptide amphiphiles has been shown to self-assemble into nanofiber networks by modifying both the concentration of pH and salt ion (e.g., sodium and potassium) in aqueous solutions. Due to their amphiphilic nature, separate model oligopeptides can also be selfassembled into nanofibers [ 38 ]. The first of the oligopeptides creators, EAK 16-II, a 16-amino acid peptide, was contained in zuotin, a yeast protein that was initially described as binding to left-handed Z-DNA [ 39 ]. This natural peptide had an AEAEAKA amino acid sequence—KAEAEAKAK, which can form a stable β-sheet structure and self-assemble into hydrogels in various shapes when the sodium/potassium concentration in aqueous solution is balanced. These peptide scaffolds can boost the adhesion, proliferation, and differentiation of mammalian cells [ 40 ]. Holmes [ 41 ] researched neuronal cell adhesion and differentiation using AcRADARADARADA-NH2 (RAD16-I).

N SHH

Cell (osteoblast, fibroblast, mesenchymal stem cells,...) Growth factors (BMP-2, CCK-8, Ang-1, TGF-1, bFGF,...)

Calcium source (hydroxyapatite, calcium phosphate,...)

Two small self-assembling peptide hydrogel scaffolds, and Ac-RARADARADADADA-NH2 (RAD16-II), showed that mouse neurons can develop active dendrites on these scaffolds. 2.2. Carbon-Based Nanomaterials in Tissue Engineering. All the carbon nanomaterials on one or more ends are bioactive. Most exhibit high bone tissue engineering capabilities with acceptable mechanical possessions, no osteoblast cytotoxicity, and endogenous antibacterial activity (deprived of the use of exogenous antibiotics) [ 42 ] (Figure 5). 2.2.1. Tissue Engineering by Graphene-Based Nanomaterials. GO-based systems provide a wide variety of applications for engineering bone and tissue regeneration. GO nanomaterials’ remarkable advantages are the wide surface area, adequate wettability, excellent mechanical properties, strong adhesion power, and quick start of stimulation performance. Besides, these materials may solve the weak interaction between bioceramics and biopolymers by incorporating strong electrostatic and p-p stacking interactions [ 46, 47 ]. GO would also definitely start to draw experts from prospective bone regeneration fields and other tissue engineering programs. Below are three items related to the usage of GO for scaffolding bone tissue. First, the existence of GO in natural biopolymer-based scaffolds has more potent stimulating effects on the bone tissue mineralization cycle than synthetic polymers. Second, the presence of GO in the matrix of polymer scaffolds will stimulate the growth and spread of bone cells on both natural and synthetic polymer scaffolding surfaces. However, on the GO synthetic polymer scaffold, the fraction of dead cells was higher than that of the natural biopolymer scaffold from the GO. Third, while the proportion of dead cells on the GO synthetic polymer scaffold was higher than that of the GO natural biopolymer, GO natural biopolymer scaffolds can produce better mechanically resistant bone tissue. Table 1 summarizes the results of GO nanomaterials and their application in bone tissue engineering.

Omidi et al. made a carbon dot/chitosan hydrogel and found it to be biocompatible with Staphylococcus aureus and have antibacterial efficacy [ 56 ]. It has been shown that chitosan hydrogel composites filled with carbon dots composed of citrate-conjugate ammonium hydrogen have enhanced mechanical properties. These nanomaterials were extremely pH-reactive and found to be highly successful for wound cure. Consequently, carbon dots/chitosan nanocomposite both have pH responsiveness and antibacterial rial properties. In tissue engineering, this study has the potential to improve wound healing. GQDs have been used in recent research for regenerative, and stem cell-based uses in tissue engineering. Many researchers have used stem cells to organize them across a variety of categories, utilizing several techniques. GQDs can be used to promote stem cell differentiation in the right conditions. Qiu et al. looked at how GQDs perform an important osteogenic differentiation function [ 57 ]. In particular, GQDs have been shown to affect the early activation of osteogenesis. This nanomaterial increases the abundance of calcium, too. Due to their low toxicity, differentiation, and excellent mechanical properties, these particles are highly valuable in the regenerative medicine field. Tissue engineering’s future depends on the threedimensional (3D) scaffolds created by exciting new biomaterials. Recent advances in tissue engineering for applications in 3D graphene scaffolds are shown in Figure 6 [ 58 ]. 2.2.2. Carbon Nanotubes in Tissue Engineering. Due to its exceptional biocompatibility, carbon nanotubes/nanofibers (CNTs/CNFs) are seen as potential candidates for Ref. [ 48 ] [ 49 ] [ 50 ] [ 51 ] [ 51 ] [ 52 ] [ 53 ] [ 53 ] [ 54 ] [ 55 ] Graphene 3D scaffold in tissue engineering

Scaffold fabrication Graphene sheets

Crosslinking polymer Implantation Growth factor

External physiochemical

stimuli Differentiated tissue/organ cells forms

Graphene 3D scaffold

Stem cells Specific developed

Organs

Macroporous proliferated cells scaffold with applications in bone tissue engineering, electrical and mechanical possessions [ 59 ]. In a new study by Price et al., osteoblast adherence by a diameter of 60 nm CNF significantly improved and, at the same time, diminished competitive cells (smooth muscle cells and fibroblasts) in order to induce sufficient osseointegration [ 60 ].

Sitharaman et al. recently published an in vivo study of the ultrashort SWCNT polymer nanocomposites (singlewalled carbon nanotubes), following up to 12 weeks of implantation in rabbit femoral condyles and subcutaneous pockets. For 4 to 12 weeks, nanocomposites had intense hard and soft tissue reactions [ 61 ]. Hirata et al. studied MWCNTcoated 3D-C scaffolds and checked the adhesion of the cells to MWCNT-coated C sponges. Their study of actin stress fibers showed that the tension in Saos2 cells, which developed on materials covered with CNT, became more apparent after seven days of growth. MWCNT coating gives the cell culture a 3D scaffold, which is more fitting than SWCNT [ 62 ].

The structural and molecular dynamics of the scaffold microenvironments can be discussed in the interaction between different types of CNTs and their effect on bone cell growth and attachment. Because of its wide specific surface area, SWCNTs can give more space for efficient cell adhesion to the scaffold. At the same time, MWCNTs will manage the positive contact between the cells and the scaffold surface due to the more aggregated condition of the MWCNTs. While the cytotoxicity of CNTs continues to be a problem in bone tissue engineering because of the complicated interactions between CNTs and cellular processes, the inclusion of CNTs in the scaffold matrix may enhance cell interactions. Due to the smaller number of oxygen atoms in functional groups of functionalized CNT, the spreading and aggregation of cells within the scaffold microenvironments is less successful than GObased scaffolds. Table 2 summarizes some of the observations regarding the use of CNT-based materials in bone tissue engineering. 2.2.3. Fullerenes in Bone Tissue Engineering. Fullerenes are closed-cage structures consisting of roughly spherical carbon atoms and hybridized by sp2. The fullerenes C60 and C70 are more widespread than the whole of other types. A natural application of the fullerene compounds provides alternative types of development in bone tissue. The spherical molecular structure of the fullerenes allows their use in biomedicine as free radical scavenger agents [ 70 ]. For instance, because neuroprotective agents are HIV particle inhibitors, fullerene materials incorporate new behavior.

As a consequence of findings of enhanced cell adhesion to fullerene biomaterials, advances in bone tissue engineering have drawn significant attention in recent years [ 71 ]. Bacakova et al. developed fullerene-coated carbon nanofibers capable of increasing the adhesion of osteoblastic MG 63 cells and increasing cell proliferation by up to 4.5 times in 7 days [ 72 ]. This work recorded fullerene-based microarrays which were prepared using a metallic “nanomask” to improve the growth and adhesion of MG 63 osteoblast bone cells. Hierarchical surface morphology has played a crucial role in the formation of cells because the cells are almost entirely located between the prominences of the grooves. However, fullerene-based biomaterial did not allow the cells to extend by more than 1 mm in height. That is explained by the hydrophobic nature of the materials fullerene [ 73 ]. In another research, this group proposed that complements and other carbon nanoparticles could be therapeutic agents for arthritic bone diseases. Their consequences showed that fullerene materials were stable and did not cause DNA damage or alter osteoblastic MG 63 and U-2 OS cells morphology. However, they could increase the biological function of bone cells [ 74 ]. 2.3. 3D Self-Assembled Nanostructures for Use in Scaffolds. Scaffolds can be called the tissue engineering field’s “beating heart.” Without proper scaffolding, the cells cannot expand in an artificial setting. Bone cells are perhaps the essential forms among all the separate cells in the human body, providing a well-designed scaffold for constructed living bones. Research has shown that nanostructured materials through desirable cell surface possessions can enable more protein interactions than conventional materials to support more effective new bone development [ 75, 76 ]. 2.3.1. Graphene Family Materials as Scaffold or a Reinforcement Material in Scaffold. In bone tissue engineering, the most common technique is to reproduce the bone remolding and regeneration processes as natural. Biocompatible scaffold, biodegradable, and osteoinductive or osteoconductive techniques must reach three dimensions [ 77 ]. This form of scaffold would provide an ideal microenvironment for imitating the extracellular matrix (ECM) for osteogenic cell binding, division, differentiation and proliferation, and growth factor carriers [ 78 ]. Graphene should make the full surface area of the substratum suitable as a flexible biocompatible scaffold for cell differentiation and osteogenic differentiation [ 79 ]. For example, 3D graphene foams used as substrates for human mesenchymal stem cells (hMSC) have shown their capacity to preserve stem cell viability and facilitate osteogenic differentiation [ 80 ]. In addition, 3D graphene (3DGp) scaffolding and 2D graphene (2DGp) coating have been shown to induce the differentiation of periodontal ligament stem cells (PDLSC) into mature osteoblasts by higher rates of mineralization and graphene-related upregulated bone-related genes and proteins with or without chemical inductors [ 81 ]. GO was placed by Han et al. on Ti scaffolds, modified with polydopamine (PDA). After that, a separate form of gelatin microspheres (GelMS) was encapsulated in BMP-2 and vancomycin. The drugcontaining GelMS were subsequently placed on GO/Ti scaffolds and stabilized by usable GO groups (Figure 7). The novel scaffolds play an essential role in bone recovery and tolerance for bacterial infections [ 82 ]. Substance P (SP) is a neuropeptide containing 11 strictly retained amino acids [ 83 ]. It includes, for example, inflammatory wound healing, control, and angiogenesis in many procedures, and is necessary to facilitate the recruitment of MSC implants [ 84 ]. So apart from the BMP-2, this peptide, SP, was applied to the GO-coated Ti surface by La et al. The dual delivery mechanism of GO-coated Ti has demonstrated the continuous release of BMP-2 and SP, and the ability of SP to activate MSC mi-integration. In vivo, the Ti/SP/GO/BMP-2 group showed more magnificent new bone formation in the mouse calvary relative to implant recruitment by SP for the Ti/GO/BMP-2 population [ 85 ].

A significant factor in the design of tissue engineering scaffolds is the mechanical strength and longevity of the

BMP-2 adsorption Porous GelMS (b)

Van adsorption

PDA deposition

Chitosan coating

BMP-2 loaded CGelMS Chitosan coating

Van loaded CGelMS

GO deposition Pure Ti scaffold

PDA-adlayer on Ti scaffold

GO/Ti scaffold Assembly

Electrostatic interactions (c)

Drug loaded CGelMS-GO/Ti scaffold 100 m 10 m 50 m material. GO-based composites have very porous structures and high mechanical strength, which give strong prospects for regeneration to Liang et al. scaffolds. It is reported that composite scaffolds HAp/collagen (C)/poly(lactic-co-glycolic acid)/GO (nHAp/C/PLGA/GO) facilitate the proliferation of MC3T3-E1 cells (Figure 8), [ 86 ]. For scaffold preparation, they formulated nHAp/C/PLGA/GO nanomaterials with a particular percentage of GO weight and measured the mechanical properties of the scaffold. The findings showed that the dynamics would raise the mechanical strength of the scaffold by 1.5 wt. percent of GO and provide a good cell adhesion and propagation substratum.

One of the critical factors influencing the mechanical possessions of the bone-shaped in tissue engineering is the adhesion of bone cells to the substrate at the center [ 88 ]. A variety of work has focused on this subject over the last few years—for example, Mahmoudi and Simchi. A nanofibrous matrix was developed using electrospun material to improve 37°C, pH = 9,

Stirring

The authors declare that they have no conflicts of interest.