Role of 4-Hexylresorcinol in the Field of Tissue Engineering

: 4-hexylresorcinol (4-HR), as a derivative of phenolic lipids, has biological and pharmacological properties that are beneﬁcial when used with a biomaterial. It has antimicrobial and antiseptic activity and can thus prevent contamination and infection of biomaterials. 4-HR suppresses the nuclear factor kappa B (NF- κ B) signaling pathway related to osteoclast di ﬀ erentiation. The suppression of NF- κ B increases the bone formation marker and contributes to new bone formation. The tumor necrosis factor- α (TNF- α ) is a pro-inﬂammatory cytokine produced by macrophages and suppressed by 4-HR. Suppression of TNF- α decreases osteoclast activity and promotes wound healing. 4-HR increases the vascular endothelial growth factor and has an anti-thrombotic e ﬀ ect. When incorporated into silk vascular patches, it promotes endothelium wound healing. Recently, 4-HR has exhibited biological properties and has been successfully incorporated into various biomaterials. Consequently, it is a useful pharmacological chemical that can be used with biomaterials in the ﬁeld of tissue engineering.


Introduction
4-hexylresorcinol (4-HR) is derivative of phenolic lipid which is naturally synthesized in higher plants [1]. It has antimicrobial and antiparasitic properties, and has been used as an ingredient in topical antiseptics and throat lozenges [2]. 4-HR, as an inhibitor of tyrosinase and polyphenol oxidase, is used as a food additive to prevent fruit browning and shrimp melanosis [3][4][5]. 4-HR is used as an ingredient in anti-aging creams and other cosmetics for its antioxidant property [6]. It exhibits an anti-cancer effect when combined with cisplatin; it inhibits squamous-carcinoma cell proliferation and tumor growth in xenograft models [7].
Recently, 4-HR has been used with various biomaterials, including bone substitutes, silk, dental implants, and polymers [8][9][10]. It exhibits the biological and pharmacological properties of bone, vessels, and soft tissue regeneration [11,12]. 4-HR has a beneficial effect when used with biomaterials with antimicrobial and anti-immunologic activities [13,14]. It contributes to new bone regeneration when incorporated into bone substitute materials [15]. Moreover, it induces the vascular endothelial growth factor (VEGF) and endothelium regeneration [12]. 4-HR inhibits the foreign-body reaction and can contribute to the success of biomaterial implantation in the body [14]. It is known that 4-HR has a similar action to the female sex hormone estrogen; however, a recent study reported that it does not change the expression of the estrogen receptor and has no estrogen-like effect in a rat model [16]. 4-HR has shown favorable results in tissue regeneration when incorporated into several kinds of biomaterial. It has biological and pharmacological effects and contributes to tissue healing and regeneration. Here we reviewed the various properties of 4-HR in tissue engineering and the recent application study of 4-HR as a biomedical agent. We evaluated the clinical efficacy of 4-HR and anticipated its role in the field of tissue engineering.

Antimicrobial Activity
4-HR is known to have antimicrobial and antiparasitic activities [17]. It was used as an antiparasitic and antiseptic agent in the medical field for the treatment of typhoid carriers in the 1920s [18,19]. Recently, it has been used as an ingredient in topical antiseptics for the treatment of sore throats [20]. 4-HR has antioxidant and antigenotoxic properties and inhibits bacterial, fungal, and parasite growth [21]. Moreover, it has bactericidal activity against oropharyngeal organisms related to acute sore throats [2]. Since 4-HR is a derivative of phenolic lipid, its antimicrobial activity is related to the biological activity of phenolic lipids. Phenolic lipids have cytotoxic, antimicrobial, and antiparasitic activity, and interacts with the biological membrane and cellular metabolism. It affects the phospholipid cell membrane and changes the membrane structure and permeability, leading to disturbance of the membrane. Phenolic lipids also affect intercellular proteins and the DNA structure [21].
4-HR exhibits antimicrobial activity when incorporated into biomaterial. 4-HR incorporated into a silk disk and suture material releases from the silk in an aqueous medium and inhibits the growth of seven species of microbes in an anti-bacterial test [13]. Further, when 4-HR is an ingredient in a poly(lactic-co-glycolic acid) (PLGA) film, which is a biodegradable polymer and is used as a drug carrier, the 4-HR-loaded PLGA film exhibits antimicrobial activity against a panel of Gram-negative and Gram-positive bacteria, yeast, and fungi [10]. When 4-HR was incorporated into bovine bone, the bone disc had an antibacterial effect and inhibited the growth of two species of oral pathogens [15].

Suppression of Nuclear Factor Kappa B Signaling Pathway
4-HR suppresses the nuclear factor kappa B (NF-κB) signaling pathway by inhibiting the phosphorylation of NF-κB [22]. The NF-κB pathway is related to the bone metabolism, which is the key pathway in osteoclast differentiation [23]. The suppression of NF-κB increases bone formation markers, such as osteoprotogerin and osteocalcin, and accordingly increases new bone formation [11]. 4-HR suppresses the tumor necrosis factor-α (TNF-α) which negatively affects osteogenic differentiation [24]. TNF-α increases the NF-κB pathway by the phosphorylation of p65, which is the marker of NF-κB activation. Inhibiting NF-κB and TNF-α is beneficial for new bone formation. 4-HR suppresses the TNF-α-induced NF-κB signaling pathway and contributes to new bone formation when used with biomaterials [8,11].

Promotion of Wound Healing by Tumor Necrosis Factor-α Suppression
TNF-α is pro-inflammatory cytokine produced by macrophage, lymphocyte, neutrophils, and mast cells [25]. TNF-α expression induces osteoclast differentiation via the receptor activator of the NF-κB ligand (RANKL) pathway. It is suppressed by the 4-HR application in the RAW264.7 cell [26]. The suppression of TNF-α by 4-HR decreases the osteoclast activity and contributes to new bone formation in vivo [11,24]. TNF-α is expressed in the acute inflammatory phase, and its prolonged expression impairs wound healing. TNF-α is highly expressed in burn wounds, and 4-HR application reduces the TNF-α expression and promotes rapid wound healing [26].

Inhibition of Foreign-Body Reaction and Acceleration of Biodegradation
Any biomaterial induces a foreign-body reaction when it is implanted in the body. Macrophages emerge and form giant cells around the biomaterial, inducing a foreign-body reaction and causing giant-cell granuloma formation [27]. 4-HR inhibits the foreign-body reaction and giant cell formation around silk, and accelerates the degradation of silk while decreasing the inflammatory reaction [13,14]. 4-HR inhibits the expression of diacylglycerol kinase (DAGK) and decreases the giant cell formation in the RAW264.7 cell [14]. DAGK activation is related to phagocytosis and multinuclear cell formation. Moreover, the deficiency of DAGK is related to cell apoptosis and suppression of proliferation [28]. Inhibition of DAGK by 4-HR decreases the foreign-body giant-cell formation, induces macrophage apoptosis, and contributes to graft degradation [14].
Before forming a giant cell, the macrophage undergoes apoptosis in which new macrophage area is recruited, and this may contribute to biodegradation of the grafted material. A 4-HR-treated silk graft shows increased graft degradation and less giant cells [14]. 4-HR increases the expression of the matrix metalloproteinase (MMP), which is the enzyme for proteolysis [29]. MMPs are expressed in the acute inflammatory phase and late remodeling phase and contribute to wound healing [30]. MMPs are mainly produced from macrophages, and have proteolytic enzymes for extracellular matrix protein [31,32]. 4-HR increases the expression of MMP-2, 3, and 9 in the macrophage and contributes to the biodegradation of the silk suture ( Figure 1a). 4-HR treated silk suture material is grafted in the subcutaneous pocket of rat back skin. A 4-HR-treated silk suture group showed higher expression of MMP-2, 3, and 9, and degradation of graft rather than only-silk suture group (Figure 1b) [13]. Further, 4-HR incorporated into bovine bone shows a more rapid bone degradation than untreated bovine bone [15].
Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 11 Any biomaterial induces a foreign-body reaction when it is implanted in the body. Macrophages emerge and form giant cells around the biomaterial, inducing a foreign-body reaction and causing giant-cell granuloma formation [27]. 4-HR inhibits the foreign-body reaction and giant cell formation around silk, and accelerates the degradation of silk while decreasing the inflammatory reaction [13,14]. 4-HR inhibits the expression of diacylglycerol kinase (DAGK) and decreases the giant cell formation in the RAW264.7 cell [14]. DAGK activation is related to phagocytosis and multinuclear cell formation. Moreover, the deficiency of DAGK is related to cell apoptosis and suppression of proliferation [28]. Inhibition of DAGK by 4-HR decreases the foreign-body giant-cell formation, induces macrophage apoptosis, and contributes to graft degradation [14].
Before forming a giant cell, the macrophage undergoes apoptosis in which new macrophage area is recruited, and this may contribute to biodegradation of the grafted material. A 4-HR-treated silk graft shows increased graft degradation and less giant cells [14]. 4-HR increases the expression of the matrix metalloproteinase (MMP), which is the enzyme for proteolysis [29]. MMPs are expressed in the acute inflammatory phase and late remodeling phase and contribute to wound healing [30]. MMPs are mainly produced from macrophages, and have proteolytic enzymes for extracellular matrix protein [31,32]. 4-HR increases the expression of MMP-2, 3, and 9 in the macrophage and contributes to the biodegradation of the silk suture ( Figure 1A). 4-HR treated silk suture material is grafted in the subcutaneous pocket of rat back skin. A 4-HR-treated silk suture group showed higher expression of MMP-2, 3, and 9, and degradation of graft rather than only-silk suture group ( Figure 1B) [13]. Further, 4-HR incorporated into bovine bone shows a more rapid bone degradation than untreated bovine bone [15].

Angiogenesis and Vascular Tissue Healing
Angiogenesis is an important process in wound healing. Pro-angiogenic drugs are necessary to promote wound healing and tissue regeneration [33]. 4-HR has a pro-angiogenic effect by increasing vascular endothelial growth factor (VEGF)-A and C, and the angiogenin expression in vitro ( Figure 2) [34]. 4-HR increases MMP-2, 3, and 9 expressions, and the MMPs are positively correlated with the

Angiogenesis and Vascular Tissue Healing
Angiogenesis is an important process in wound healing. Pro-angiogenic drugs are necessary to promote wound healing and tissue regeneration [33]. 4-HR has a pro-angiogenic effect by increasing vascular endothelial growth factor (VEGF)-A and C, and the angiogenin expression in vitro (Figure 2) [34]. 4-HR increases MMP-2, 3, and 9 expressions, and the MMPs are positively correlated with the VEGF expression [13,35]. The angiogenic effect of 4-HR is mediated by the various MMPs, and the application of MMP inhibitors decreases the 4-HR-induced VEGF-A, VEGF-C, and angiogenin expressions [34].
VEGF expression [13,35]. The angiogenic effect of 4-HR is mediated by the various MMPs, and the application of MMP inhibitors decreases the 4-HR-induced VEGF-A, VEGF-C, and angiogenin expressions [34]. 4-HR has an anti-thrombotic effect and delays blood coagulation by inhibiting thrombus formation. In vitro, the group with 4-HR showed a lower optical density based on spectrometer measurements, which indicates a lower concentration and less blood coagulation [36]. The antithrombotic effect of 4-HR is related to the calcium chelation, and its own anti-inflammatory effect; however, further study of this mechanism is necessary [7,37]. 4-HR, which has a pro-angiogenic and anti-thrombotic effects, can be used as a component of biomaterial in vascular surgery.

4-HR with Bone Graft Substitutes
4-HR has various beneficial properties for use with bone graft substitutes. It has antimicrobial and antiseptic properties, and thus can prevent bacterial contamination and infection of the graft material [15]. It suppresses the NF-κB pathway and TNF-α expression, decreases osteoclast activity, and contributes to new bone formation. It has been used with bone substitutes and shows favorable results in bone formation. 4-HR incorporated into porcine bone promotes new bone formation in rat calvarial defects. 4-HR suppresses the TNF-α-induced NF-κB pathway, and increases bone formation and osteogenic marker expressions, including osteocalcin, osteoprotegerin, and alkaline phosphatase [11].
The biodegradation of biomaterial is an important process for the success of grafted materials. 4-HR can accelerate the biodegradation of silk through MMP activation, and induces rapid degradation of bovine bone grafted in rat calvarial defects [13][14][15]. Degradation of the graft material can be disrupted by the foreign-body reaction and giant-cell formation around the material. 4-HR inhibits giant-cell formation and decreases the phagocytic activity via suppression of the DAGK expression to allow it to contribute to degradation of the graft material [14].

4-HR with Guided Bone Regeneration Membrane
Guided bone regeneration (GBR) is a technique to promote new bone formation in bony defects, using barrier membranes [38]. GBR membranes prevent soft tissue cell infiltration, allow osteogenic cell migration, and promote new bone formation [39]. As a component of the GBR membrane, 4-HR was successfully incorporated into a silk-based membrane and used as an ingredient in the GBR membrane [8]. The antimicrobial property of 4-HR incorporated into the silk membrane does not change after sterilization, including by autoclaving and with ethylene oxide gas [40]. 4-HR incorporated into the silk membrane shows more bone formation than the silk-only membrane in rabbit calvarial 4-HR has an anti-thrombotic effect and delays blood coagulation by inhibiting thrombus formation. In vitro, the group with 4-HR showed a lower optical density based on spectrometer measurements, which indicates a lower concentration and less blood coagulation [36]. The anti-thrombotic effect of 4-HR is related to the calcium chelation, and its own anti-inflammatory effect; however, further study of this mechanism is necessary [7,37]. 4-HR, which has a pro-angiogenic and anti-thrombotic effects, can be used as a component of biomaterial in vascular surgery.

4-HR with Bone Graft Substitutes
4-HR has various beneficial properties for use with bone graft substitutes. It has antimicrobial and antiseptic properties, and thus can prevent bacterial contamination and infection of the graft material [15]. It suppresses the NF-κB pathway and TNF-α expression, decreases osteoclast activity, and contributes to new bone formation. It has been used with bone substitutes and shows favorable results in bone formation. 4-HR incorporated into porcine bone promotes new bone formation in rat calvarial defects. 4-HR suppresses the TNF-α-induced NF-κB pathway, and increases bone formation and osteogenic marker expressions, including osteocalcin, osteoprotegerin, and alkaline phosphatase [11].
The biodegradation of biomaterial is an important process for the success of grafted materials. 4-HR can accelerate the biodegradation of silk through MMP activation, and induces rapid degradation of bovine bone grafted in rat calvarial defects [13][14][15]. Degradation of the graft material can be disrupted by the foreign-body reaction and giant-cell formation around the material. 4-HR inhibits giant-cell formation and decreases the phagocytic activity via suppression of the DAGK expression to allow it to contribute to degradation of the graft material [14].

4-HR with Guided Bone Regeneration Membrane
Guided bone regeneration (GBR) is a technique to promote new bone formation in bony defects, using barrier membranes [38]. GBR membranes prevent soft tissue cell infiltration, allow osteogenic cell migration, and promote new bone formation [39]. As a component of the GBR membrane, 4-HR was successfully incorporated into a silk-based membrane and used as an ingredient in the GBR membrane [8]. The antimicrobial property of 4-HR incorporated into the silk membrane does not change after sterilization, including by autoclaving and with ethylene oxide gas [40]. 4-HR incorporated into the silk membrane shows more bone formation than the silk-only membrane in rabbit calvarial defects (Figure 3). In histomorphometric analysis, 4-HR-incorporating silk and only-silk groups have significantly more new bone formation than the control (Table 1) [41]. 4-HR has an antimicrobial and osteoclast inactivation property, and it can contribute to new bone formation. 4-HR incorporated into the silk membrane yields more new bone formation in the rabbit calvarial defect than the commercial collagen membrane [40,42]. In the peri-implant defect model in rabbit tibia, a 4-HR-incorporated silk membrane was applied to cover the defect and to induce bone regeneration. The implant group using the membrane has more favorable osteointegration and bone-to-implant contact compared with the untreated implant group [8]. As an ingredient in the GBR membrane, 4-HR has been effectively used to promote new bone formation in bony defects.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 11 defects ( Figure 3). In histomorphometric analysis, 4-HR-incorporating silk and only-silk groups have significantly more new bone formation than the control (Table 1) [41]. 4-HR has an antimicrobial and osteoclast inactivation property, and it can contribute to new bone formation. 4-HR incorporated into the silk membrane yields more new bone formation in the rabbit calvarial defect than the commercial collagen membrane [40,42]. In the peri-implant defect model in rabbit tibia, a 4-HR-incorporated silk membrane was applied to cover the defect and to induce bone regeneration. The implant group using the membrane has more favorable osteointegration and bone-to-implant contact compared with the untreated implant group [8]. As an ingredient in the GBR membrane, 4-HR has been effectively used to promote new bone formation in bony defects.

4-HR with Dental Implant and Tooth Movement
4-HR is coated on the surface of dental implants using hydroxyapatite (HA), and it affects the osteointegration of the implant. Dental implants coated with HA and 4-HR have more rapid cellular attachment and spread than HA-coated implants [9]. Moreover, HA and 4-HR implants show increased osteocalcin expression and alkaline phosphatase activity in vitro. Further, implants coated with HA and 4-HR have more favorable osteointegration compared with HA-coated implants in vivo [9].
The administration of 4-HR accelerates the orthodontic tooth movement in ovariectomized rats ( Table 2). In vitro, 4-HR increases the expression of the osteogenic marker in SAos-2 cell. In vivo, the subcutaneous administration of 4-HR in ovariectomized rats increases the bone turnover markers in plasma level. The 4-HR administration accelerates the orthodontic tooth movement by increasing bone formation and resorption. In histological analysis, the expression of bone morphogenic protein-2 (BMP-2) is higher in the alveolar bone than is required for bone deposition. Additionally, the expression of RANKL, which is marker for osteoclasts, is higher on the bone resorption side [43].  [34]. It has angiogenic and anti-thrombotic effects and has been used as a component of vascular patch material in vascular surgery [12,36]. A 4-HR-incorporated silk vascular patch was used in angioplasty, and was comparable as a patch material to the polytetrafluoroethylene (PTFE) commercial product [12]. The 4-HR silk vascular patch was applied in a rat carotid artery and showed stable artery patency and flow in angiography. A histology study showed that a new endothelium was regenerated around the silk mat with no thrombus formation [12]. Moreover, the von Willebrand factor (vWF) was highly expressed in the regenerated endothelium ( Figure 4) [12]. 4-HR with antimicrobial and pro-angiogenic ability promotes endothelium wound healing in carotid artery defects and is successfully used as a component of silk vascular patches. (SFM: silk fabric membrane; *: P < 0.05 compared to uncovered control).

4-HR with Dental Implant and Tooth Movement
4-HR is coated on the surface of dental implants using hydroxyapatite (HA), and it affects the osteointegration of the implant. Dental implants coated with HA and 4-HR have more rapid cellular attachment and spread than HA-coated implants [9]. Moreover, HA and 4-HR implants show increased osteocalcin expression and alkaline phosphatase activity in vitro. Further, implants coated with HA and 4-HR have more favorable osteointegration compared with HA-coated implants in vivo [9].
The administration of 4-HR accelerates the orthodontic tooth movement in ovariectomized rats ( Table 2). In vitro, 4-HR increases the expression of the osteogenic marker in SAos-2 cell. In vivo, the subcutaneous administration of 4-HR in ovariectomized rats increases the bone turnover markers in plasma level. The 4-HR administration accelerates the orthodontic tooth movement by increasing bone formation and resorption. In histological analysis, the expression of bone morphogenic protein-2 (BMP-2) is higher in the alveolar bone than is required for bone deposition. Additionally, the expression of RANKL, which is marker for osteoclasts, is higher on the bone resorption side [43].  [34]. It has angiogenic and antithrombotic effects and has been used as a component of vascular patch material in vascular surgery [12,36]. A 4-HR-incorporated silk vascular patch was used in angioplasty, and was comparable as a patch material to the polytetrafluoroethylene (PTFE) commercial product [12]. The 4-HR silk vascular patch was applied in a rat carotid artery and showed stable artery patency and flow in angiography. A histology study showed that a new endothelium was regenerated around the silk mat with no thrombus formation [12]. Moreover, the von Willebrand factor (vWF) was highly expressed in the regenerated endothelium (Figure 4) [12]. 4-HR with antimicrobial and pro-angiogenic ability promotes endothelium wound healing in carotid artery defects and is successfully used as a component of silk vascular patches.

4-HR induces VEGF and angiogenin expression in macrophages
(a)

Application of 4-HR in Epithelial Regeneration
The 4-HR has been used as ingredient of ointment for the burn wound healing [26]. The 4-HR ointment is applied on the burn wound in rat back skin. Additionally, it exhibits rapid epithelization and collagen regeneration compared to the only ointment group. The 4-HR ointment group shows a smaller denuded area and thicker thickness of epidermis at 14 days after treatment. Additionally, it shows low expression of TNF-α in immunohistochemistry [26].
Sericin protein from silk cocoon has been used as dressing material for the treatment of burn wounds [44]. Sericin and 4-HR combination ointment have been used in the diabetic burn wound model [45]. Additionally, it accelerates the wound healing and shows more epithelial regeneration ( Figure 5). Sericin and 4-HR ointment showed better wound healing than the only-ointment group. However, there is no comparison study of the sericin and 4-HR combination against the only sericin ointment [45]. As an ingredient in ointment, 4-HR accelerates the epithelial and collagen regeneration in burn wounds by suppressing TNF-α expression [26].

Application of 4-HR in Epithelial Regeneration
The 4-HR has been used as ingredient of ointment for the burn wound healing [26]. The 4-HR ointment is applied on the burn wound in rat back skin. Additionally, it exhibits rapid epithelization and collagen regeneration compared to the only ointment group. The 4-HR ointment group shows a smaller denuded area and thicker thickness of epidermis at 14 days after treatment. Additionally, it shows low expression of TNF-α in immunohistochemistry [26].
Sericin protein from silk cocoon has been used as dressing material for the treatment of burn wounds [44]. Sericin and 4-HR combination ointment have been used in the diabetic burn wound model [45]. Additionally, it accelerates the wound healing and shows more epithelial regeneration ( Figure 5). Sericin and 4-HR ointment showed better wound healing than the only-ointment group. However, there is no comparison study of the sericin and 4-HR combination against the only sericin ointment [45]. As an ingredient in ointment, 4-HR accelerates the epithelial and collagen regeneration in burn wounds by suppressing TNF-α expression [26].

Xeno-estrogen Issue and Future Perspectives of 4-HR Application
Synthetic chemicals that bind to the estrogen receptor (ER) have estrogenic activity. Such chemicals are called xeno-estrogen; they affect the endocrine system and have adverse health effects [46]. 4-HR exhibits estrogenic activity and some caution is issued when using it as a food additive [47]. However, in a recent study, 4-HR did not change the expression of ERα, and β, or extracellular signal-regulated

Xeno-Estrogen Issue and Future Perspectives of 4-HR Application
Synthetic chemicals that bind to the estrogen receptor (ER) have estrogenic activity. Such chemicals are called xeno-estrogen; they affect the endocrine system and have adverse health effects [46]. 4-HR exhibits estrogenic activity and some caution is issued when using it as a food additive [47]. However, in a recent study, 4-HR did not change the expression of ERα, and β, or extracellular signal-regulated kinase-1/2 in MCF-7 cells, which were increased in the presence of xeno-estrogen [16].
In the ovariectomized rat model, the 4-HR application group shows low levels of prolactin, ERα, and β expression in the pituitary gland, whereas estradiol application shows high levels. This study concludes that there is no evidence of the 4-HR as xeno-estrogen [16]. 4-HR has been used as an ingredient with various kinds of biomaterial. The 4-HR has been incorporated with silk sutures, membranes, vascular patches, and bone substitutes [12,13,42]. A 3% 4-HR solution has been used for the preparation, but the concentration of 4-HR in biomaterial can be varied and differed [11,15]. A relatively high concentration of 4-HR can induce the cell apoptosis and rapid degradation of biomaterial. The 4-HR concentration range from 10% to 15% in bovine bone induces rapid bone degradation and it does not show new bone formation in rat calvarial defects [15]. The lower-concentration 4-HR solution is required for the incorporation with xenografted bone. The 4-HR can induce the cell apoptosis depending on concentration [48], but the effect on connective tissue cells has not been evaluated [15]. Further study is required to determine the optimal concentration of 4-HR in each kind of biomaterial.

Conclusions
4-HR has been used as an antiparasitic and antiseptic agent owing to its antimicrobial activity. Recently, 4-HR has shown various pharmacological and biomedical properties and has successfully been incorporated into various biomaterials, including bone substitutes, silk, and biodegradable polymers. 4-HR inhibits osteoclast activity by suppression of the NF-κB pathway and contributes to new bone formation. Moreover, it inhibits the foreign-body reaction and accelerates the biodegradation of biomaterials. 4-HR, with its angiogenetic and anti-thrombotic effects, increases VEGF expression and contributes to endothelium regeneration in vascular surgery. Additionally, it contributes the epithelial and collagen regeneration of a burn wound. Consequently, 4-HR has various beneficial properties as an ingredient in biomaterials. It is a useful and effective pharmacological chemical when used with biomaterials in the field of tissue engineering for bone, vessel, and epithelial tissue regeneration.