Curcumin based polyurethane materials and their functional applications: a review

Bio-based polyurethanes are novel material with potential advantages for sustainable development, and their development play significant roles in promoting sustainability. Curcumin, a natural monomer, possesses high biological activity and features a symmetrical chemical structure with various functional groups such as phenolic hydroxyl, carbonyl and benzene ring. The presence of hydroxyl groups in the structure of curcumin provides essential conditions for its involvement in polyurethane synthesis. This review article provides an overview of the applications of curcumin as a chain extender, crosslinking agent and end-capper in polyurethanes, as well as its effects on the chemical structure, mechanical properties, and chemical stability of polyurethanes. Furthermore, the functional applications of curcumin-based polyurethanes in various fields such as medicine, food packaging, and coatings are discussed. Finally, considering the current research status and inherent properties of curcumin, the future prospects of curcumin-based polyurethanes are contemplated.


Introduction
Polyurethanes are important polymers that have attracted attention as universal materials due to their unique properties such as processability, strength, and inherent stability [1,2].These characteristics enable polyurethanes to be applied in a wide range of fields including coatings, inks, wound dressings, adhesives, and more [2].The majority of the properties of polyurethanes are dependent on key factors such as chain segment structure, molecular weight, polymer composition, and aggregation state.Additionally, there exists a clear correlation between the raw materials utilized in the preparation process and the resulting polyurethane structure.For instance, soft-hard phase separation is typical of linear and elastomeric structure on account of introducing chain extenders [3,4].Instead, cross-linking agents are used to obtain cross-linked structures [5], and blowing agents are employed to acquire foamed materials, both linear and crosslinked [6,7].At present, the raw materials for producing polyurethanes, including diisocyanates, polyols, and chain extenders, predominantly come from petroleum [8].However, there has been a development of bio-based polyurethanes in the history of polyurethanes production, which align more with the prevailing concept of sustainable development compared to traditional petroleum-based materials.Especially benefited from rich raw materials, environmental friendliness, and broad application prospects, they have made the bio-based polyurethanes become a major current research trend [9,10].
Currently, studies on bio-based polyurethanes are focused on the development of bio-based diisocyanates, biomass chain extenders, and bio-based polyols [11].These raw materials are derived from plant oils, plant phenols, and polysaccharides [12][13][14].Among them, bio-based phenolic compounds like plant phenols may contain functional groups such as hydroxyl, aromatic, unsaturated bonds, carbonyl, and carboxyl.They can be used as monomers, oligomers, or intermediates to develop polymers with specific properties [15].Common bio-based phenols include eugenol, vanillin, cashew phenol, and curcumin [15].Curcumin, in particular, exhibits high biological activity due to its diketone structure and phenolic hydroxyl group [16].Typically, curcumin has three functional groups (as shown in figure 1), namely, an aromatic ortho-methoxyphenol group, an α,β-unsaturated β-diketone group, and a seven-carbon linker [17,18].With the presence of two hydroxyl groups, curcumin can act as a chain extender or chain terminator in the synthesis of polyurethanes, thereby achieving functionalization.Moreover, the existence of intramolecular hydrogen atom transfer along the diketone chain of curcumin leads to an equilibrium between the keto and enol isomers (as displayed in figure 2) [19].This isomerization allows for the presence of three hydroxyl groups in the structure of curcumin, enabling it to act as a crosslinker in polymerization reactions [20].The different forms of curcumin under various conditions indicate that curcumin mainly exists as the keto tautomeric form in acidic/neutral aqueous solutions and cell membranes, while the enol form of the heptadienone chain is more dominant in alkaline media [21,22].
The structure of curcumin and its keto-enol tautomerism equilibrium play a crucial role in conferring its antibacterial, anticancer, anti-inflammatory, antioxidant, and UV-protective functionalities [23,24].However, curcumin suffers from drawbacks such as low water solubility, instability, and poor bioavailability [25,26].The introduction of curcumin into polyurethanes can address these limitations and enhance its efficacy [27,28].A conventional approach involves embedding curcumin into polyurethane materials.Markovi et al [29] utilized a swelling-embedding-shrinking method to prepare curcumin/polyurethane nanocomposites, which  demonstrated excellent antibacterial properties against Staphylococcus aureus and Escherichia coli.Nevertheless, materials prepared using this physical embedding method often lack stability, leading to potential aggregation or leakage of small molecules and reduced sustainability of efficacy.Except for the aforementioned functions, curcumin also plays a role in modulating the mechanical properties.Feng et al [30] have adopted a chemical bonding method to incorporate curcumin into polyurethane, resulting in improved mechanical properties for polyurethane and more effectively preserving and enhancing the antibacterial and anticancer functionalities of curcumin.
Despite the abundance of existing reviews on curcumin, there remains a scarcity of reports on functional polyurethanes based on curcumin.Numerous studies have indicated the enormous potential, significance, and prospects of curcumin in the field of polyurethanes.As the development of curcumin-based polyurethanes gradually becomes a research hotspot [31][32][33][34], it is imperative to summarize and present a review of the recent research on curcumin-based polyurethanes.This holds crucial importance in further advancing the development of the polyurethane fields.This paper integrates the latest research on curcumin, summarizing its use as a chain extender, cross-linking agent, and end-capper in the synthesis of polyurethanes.It elucidates the applications of curcumin-based polyurethanes in the medical, food packaging, coatings, and other fields, while also providing future prospects.It is expected to serve as a reference for the development of bio-based polyurethanes.

Biobased alternative feedstocks: chain extenders, crosslinking agents, and others
Polyurethanes are a type of polymers with urethane groups (-NHCOO-) along the main chain, primarily synthesized through the reaction of diisocyanates, polyols, and chain extenders (such as diols or diamines).Typically, the rigid segments in their structure are formed by the reaction between diisocyanates and chain extenders, while the soft segments consist of the molecular chains of high-molecular-weight diols [35].

Curcumin as chain extenders in polyurethanes
Curcumin has excellent biological activity to be commonly used as a drug [16].Given the outstanding biocompatibility, polyurethanes are often selected as carriers for curcumin to achieve drug release [36,37].The easiest way is blending or encapsulation with curcumin.Unfortunately, it is subjected to aggregation and small molecule overflow, which may affect sustained release effect.Apart from that, curcumin can serve as a chain extender or crosslinking agent in polyurethanes.However, it appears poor diffusion ability compared to simple encapsulation method as the constraints imposed by covalent bonds.Nevertheless, if polyurethane exhibits degradation behavior, curcumin bonded on polyurethanes generates continuous drug release at long time scale [30].
Curcumin, with its symmetrical chemical structure, possesses two hydroxyl groups that can act as reactive functional groups for polymerization.Therefore, curcumin is a natural material that can be used as a chain extender in the synthesis of polyurethanes [38].When employed as a chain extender, the two terminal hydroxyl groups of curcumin react with isocyanate groups (-NCO) to form urethane linkages, thereby creating bridge between polymer chains and elongating the polymer molecular chains.This structural modification induced by curcumin leads to alterations in the hydrophilicity, thermal properties, mechanical performance, and other characteristics of polyurethanes (figure 3).For instance, the benzene ring structure within curcumin restricts the movement of polymer chains, thereby exerting a significant influence on the glass transition temperature (Tg) and melting temperature (Tm) of polyurethanes.Additionally, the dipole-dipole interactions among curcumin molecular chains also result in noticeable changes in the crystallization temperature (Tc) [39].The impact of curcumin as a chain extender on the morphology and properties of polyurethane films has also been investigated as a research hotspot.Mahmood et al [38] found that the regularity of the surface of polyurethane films prepared using curcumin as a chain extender depends more on the use of multi-isocyanates, such as methylene diphenyl diisocyanate, to obtain a more regular structure, while films made with hexamethylene diisocyanate exhibit an amorphous state.Further studies by Zhang et al [40] indicated that curcumin as a chain extender can disrupt the crystallinity of the film, restrict the movement of polymer chains, and increase the regularity and density of the film.The crystallinity and hydrophobicity of curcumin itself can cause a decrease in the surface energy of the film.The changes in structural and performance properties induced by curcumin as a chain extender depend on the amount of curcumin used.Abdollahi et al [39] demonstrated that as the degree of curcumin substitution for 1,4-butanediol increases, polyurethane exhibits enhanced thermal properties and improved mechanical performance, accompanied by a significant increase in molecular weight.This suggests that compared to the traditional chain extender 1,4-butanediol, curcumin demonstrates superior chain-extending capabilities, thus offering promising prospects for applications.Zhang et al [40] also confirmed this point and further pointed out that excessive curcumin may cause a decrease in mechanical performance and make polyurethane brittle due to the presence of a rigid ring structure in the curcumin molecule, thus suggesting a limit on the amount of curcumin used as a chain extender.Meanwhile, the team also mentioned that active functional groups such as benzene rings and ketone carbonyl groups can be retained during curcumin chain extension to give functionality to the polymer.
In general, curcumin as a chain extender offers the advantages of being natural, readily available, and renewable.Compared to petroleum-based polyether polyols, diamines, and alcohol amines as chain extenders, curcumin is more environmentally friendly and safe, with a stronger ability to adjust thermal and mechanical properties.Additionally, curcumin contains abundant functional groups such as hydroxyl and carbonyl groups, making it an excellent choice for polyurethane chain extension.

Curcumin as crosslinking agents in polyurethanes
Polyurethanes can be transformed from a linear or branched structure to a three-dimensional network structure through crosslinking, resulting in higher molecular weight.By appropriately controlling the crosslinking modification of polyurethanes, improvements in mechanical properties, physical properties, and chemical stability can be achieved (figure 4).The chemical structure of curcumin contains a β-diketone group that is prone to undergo keto-enol tautomerism.In an alkaline aqueous solution, the enol form dominates [41,42].Curcumin contains three hydroxyl functional groups that can react with isocyanates, making it suitable for use as a crosslinking agent in the synthesis of polyurethanes.In addition, the autoxidation of curcumin in a buffered solution leads to the formation of hexanedione as a chemical degradation product.Hexanedione also possesses three hydroxyl groups.Furthermore, the hydroxyl content of curcumin can be increased through direct modification, providing an additional source for curcumin-based crosslinkers.
Structurally, the enol tautomeric form of curcumin exhibits excellent crosslinking performance.However, in practical applications, Divakaran et al [20] pointed out that the coexistence of curcumin-based crosslinkers and crosslinkers with higher OH-activity may result in the entrapment of curcumin, where a portion of curcumin exists in an encapsulated form without participating in the crosslinking reaction.Furthermore, Feng et al [30] specifically pointed out that due to the similar reactivity of the three hydroxyl groups on the enol tautomeric form of curcumin towards isocyanates, during the synthesis of polyurethanes, curcumin is involved in end-capping, chain extension, and crosslinking simultaneously.However, some curcumin molecules still remain encapsulated, indicating that this phenomenon is attributed to the inherent structural characteristics of  curcumin itself.The team also noted that curcumin as a crosslinker exhibits dose-dependency.Typically, the crosslinking effectiveness of curcumin used in crosslinking tends to weaken or even lose its crosslinking capability with increasing dosage, ultimately impacting the mechanical properties.Directly modified curcuminbased crosslinkers do not exhibit the aforementioned phenomenon.Song et al [34] synthesized curcumin dioxime, which contains four OH-active functional groups, and prepared polyurethanes with well-controlled crosslinking networks.Curcumin as a crosslinker is a research focus regarding the changes it brings to the structure of polyurethane networks.In terms of crystallinity, Feng et al [30] pointed out that the presence of enol-curcumin impedes the crystallization of PEG.Similarly, Divakaran et al [20] mentioned that as the mass ratio of enol-curcumin increases from 0.5 wt% to 1.5 wt%, the corresponding crystalline diffraction peak of PEG exhibits a decreasing trend.Additionally, in terms of thermal properties, both enol-curcumin and curcumin dioxime contain benzene rings, which increase the rigidity of polymer chains and reduce the degree of microphase separation, resulting in an upward trend in Tg.When curcumin is used as a crosslinker to prepare hydrogels, whether it will affect the rebuilding capability of the hydrogel is a research hotspot.For example, Divakaran et al [20] proposed that the coexistence of curcumin crosslinking and encapsulation lead to the heterogeneity of the structure, thereby decreasing the dissipativity of the hydrogel.Furthermore, the research results of Feng et al [30] indicated that this decrease in dissipativity is caused by the increased π-π stacking interaction of the benzene rings from chemically linked curcumin and physically mixed curcumin.A similar phenomenon is also observed in polyurethanes prepared using modified curcumin dioxime.Song.[34] suggested that in the crosslinked network, curcumin induces a synergistic effect between dynamic imine and hydrogen bonds, endowing polyurethanes with rapid self-healing capabilities.
Based on the above analysis, curcumin has potential as a crosslinker for polyurethane materials, as it can regulate the crosslinking network structure and physical-mechanical properties.However, research in this field is still limited.It is worth noting that modified curcumin-based crosslinkers exhibit more stable crosslinking ability and higher adjustability than curcumin itself.In addition, there have been no reports on using curcumin degradation products as crosslinkers.

Multifaceted role of curcumin in polyurethanes
As mentioned earlier, when curcumin is involved in the polyurethanes chain extension or crosslinking process, due to the similar reactivity of the hydroxyl groups on curcumin molecules, it is possible that only one hydroxyl group on some curcumin molecules will react with isocyanate, acting as chain terminators.This can hinder the chain extension or crosslinking process or directly become part of the side chains in polyurethanes.The presence of curcumin as chain terminators or side chains in polyurethanes has been found to have a significant impact on the mechanical properties, stability, and crystallinity of the polymer.Firstly, the influence of curcumin as chain terminators needs to be considered in terms of its dosage dependence.Feng et al [30] pointed out that an excess of curcumin can act as chain terminators to cap the isocyanate and reduce crosslinking.Additionally, the addition of curcumin typically leads to a decrease in the degree of microphase separation of polymer chains, implying that when curcumin acts as chain terminators and its content further increases, it may result in a decrease in the mechanical properties of polyurethanes due to the hindrance of crosslinking (or chain extension) by curcumin and the damage to the polymer chains.Furthermore, when some curcumin acts as polyurethane side chains without contributing to chain extension or crosslinking, it partially contributes to the decline in the mechanical properties of polyurethanes.Due to the low bioavailability of curcumin itself, improving stability has become a research focus for curcumin-based polymers.Regardless of the form in which curcumin exists, whether as terminators, side chains, etc., hydrogen bond interactions exist between curcumin and polyurethane molecules, which can enhance the stability of the connection between curcumin and the polymer chain.For example, Mahmood et al [19] pointed out that for the enol tautomeric form of curcumin, hydrogen bond interactions exist between the hydroxyl group of the curcumin enol form and the amino group of polyurethane, making the interaction between curcumin and polyurethane more stable, with higher stability than 1,4-butane diol-based polyurethane.Furthermore, this hydrogen bonding interaction can also have an impact on crystallinity.In other studies, Mahmood et al [43] mentioned that the stable keto-enol structure of curcumin and the intermolecular hydrogen bonding interaction with polyurethane contribute to its higher crystallinity.

Functionality and application of curcumin-based polyurethanes
Curcumin, as a monomer with high biological activity but low bioavailability, can be introduced into the polymer chain of polyurethanes to enhance their bioavailability and fully exert their effects.Compared to traditional petroleum-based monomeric raw materials for polyurethanes, curcumin possesses various functional properties.For instance, the phenyl ring in its structure can alter the degree of microphase separation between polymer chains, while the phenolic hydroxyl groups in its structure may engage in hydrogen bonding interactions with other functional groups in the polymer chains.Additionally, changes in the wetting properties of polyurethanes can be attributed to the hydrophobic nature of curcumin, and these properties alterations are also related to the position of curcumin incorporation.The high biological activity of curcumin is attributed to functional groups such as hydroxyl, double bond, phenyl ring, and diketone in its structure.There have been numerous reports on functionalizing polyurethanes by introducing curcumin into the polymer chain while retaining its functional groups to achieve functionalization of polyurethanes (figure 5).

Curcumin-based polyurethanes for medical use
Medical polyurethanes have developed rapidly over the past decades and have become an important category in the medical material field [44][45][46].As a natural bioactive substance, curcumin is widely used in traditional and modern medicine, but the hydrophobicity of curcumin as well as its rapid metabolism, physicochemical and biological instability can bring about its poor bioavailability [41].Adding medical polyurethanes to curcumin can solve inherent curcumin instability and improve its bioavailability [47,48], as well as endow medical polyurethanes with more functionalities [49,50].We mentioned in the previous section that curcumin can be introduced into polyurethanes by encapsulation, chain extension, cross-linking, etc.In fact, curcumin introduced into polyurethane as a drug in different forms presents different properties, among which covalent bonding means will give rise to curcumin diffusion reduction.Besides, the value of curcumin in medical polyurethanes is not just confined to the medical aspects like antibacterial and anti-inflammatory, and can enhance mechanical properties and thermal stability for synthetic polyurethane materials.For example, Hu et al [51] prepared carboxymethylcellulose sodium polyurethane microspheres containing curcumin, which were pH-responsive, ensuring the release and full absorption of curcumin in the alkaline intestinal environment, and also improving the retention time of curcumin in the body's bloodstream, thereby increasing the in vivo utilization rate of curcumin.Zia et al [52] further used chitosan/curcumin blends as chain extenders to develop the chitosan/curcumin polyurethane.The results showed that compared with the traditional curcumin-only or chitosan-only polyurethane materials, this blend-extended polyurethane exhibited higher antimicrobial activity, superior thermal stability and tensile strength of the polyurethane.For in vitro therapeutic applications, curcumin-loaded polyurethane films have attracted much attention for wound dressings.For example, Sagitha et al [53] developed pH-stimulated drug release nanofiber membranes composed of dextran doped polyurethane/curcumin.The nanofiber membranes showed pH-controlled drug release potency, excellent hemostatic, good antiplatelet adhesion, and synergistic antibacterial activity against gram-positive bacteria, which have the potential to be used as wound dressings.Shah et al [50] incorporated curcumin into the polymer chain by bonding during pre-polymerization to prepare polyurethane urea elastomers.With the increase in the molar amount of curcumin, the thermal properties of polyurethanes rose, the modulus of elasticity increased dramatically, and they showed highly antimicrobial properties against Escherichia coli and Staphylococcus aureus, which is expected to be extensively used in wound dressings.Feng et al [30] further prepared curcuminpolyurethane hydrogels using an in situ copolymerization method.Curcumin exists in both bonded and encapsulated forms in the hydrogel.This team not only achieved long-lasting drug delivery through the hydrophobicity of curcumin, but also overcame insufficient mechanical properties of traditional bio-based hydrogels through the cross-linking effect of curcumin.In addition, the effective retention of antimicrobial, antioxidant and antitumor capabilities makes this hydrogel the same high potential use in wound dressings and even tumor isolation membranes.Properly regulating the mechanical properties of curcumin-based polyurethane hydrogels can demonstrate even more applications in the medical field.Divakaran et al [20] synthesized and characterized curcumin-doped polyethylene glycol-polyurethane hydrogels by one-pot method.Curcumin was also presented in the form of bonding and embedding, and the hydrogel had a lamellar structure that allowed for stable release of curcumin.The addition of curcumin brought the minimum compression ratio of the hydrogel up to 63% and the maximum compression fracture strength to 2.7 MPa, while also exhibited antimicrobial and anti-inflammatory abilities.This material has great prospects in vascular transplantation materials and tissue engineering applications.

Curcumin-based polyurethanes for medical use
Food packaging materials need to ensure food safety and transportation security.Microbial contamination and ultraviolet oxidation are common causes of food spoilage.Polyurethanes are common materials in the food packaging field, with heat insulation, sealing and shock absorption.Curcumin has excellent antibacterial and antioxidant properties, while the benzene ring and ketone carbonyl group in its structure give it ultraviolet shielding effect.Through the synergistic effect of conjugated π electrons and adjacent carbon-carbon double bonds, it can effectively absorb a wide range of ultraviolet light.The introduction of curcumin into polyurethanes can not only impart the aforementioned functional properties, but also adjust its mechanical performance.For instance, Zhang et al [40] synthesized polyurethane films using curcumin as a chain extender.
When the curcumin content reached 6 wt%, the films exhibited nearly 100% absorption of ultraviolet radiation A and ultraviolet radiation B, leading to enhanced mechanical properties.However, excessive addition of curcumin resulted in increased film brittleness, as indicated by the data.Fu et al [32] improved the toughness of the film by introducing curcumin into castor oil-based polyurethane and prepared blend film with polylactic acid.The obtained blended film has high ultraviolet absorption and also has good comprehensive properties such as thermal stability, humidity resistance, flexibility, and oxidation resistance, which is potentially promising for application in food packaging films.Furthermore, Zhang et al [54] identified a challenge in early curcumin-based polyurethanes, characterized by high curcumin content but low practical utilization.To address this issue, the team synthesized curcumin-based waterborne polyurethane using curcumin as a chain extender, and compounded it with gelatin to produce composite films with high transparency, UV-blocking capability, and antioxidant activity.The tensile strength of the composite film increased from 3.91 MPa to 11.07 MPa, while the elongation at break rose from 43.8% to 93.8%, overcoming the brittleness associated with curcumin addition.This approach offers a novel avenue for developing food packaging materials by combining natural bioactive small molecules with polymers.The preparation of food packaging materials also requires a more comprehensive consideration of their safety.The migration rate is used to test how much mass can migrate from food-contact materials into the food, which is one of the most critical aspects to establish the safety of a material for food packaging [55,56].Since isocyanates in polyurethane raw materials are toxic, the safety of polyurethane packaging materials in terms of migration rate is more favorable for the application of polyurethane materials in food packaging.This was considered in a recent study by Ruiz et al [57], where the team used curcumin and chitosan as additives bonded in the network structure of polyurethane.Through testing, they found that although the molecular weight of curcumin was lower than that of chitosan, the physical interaction between curcumin and polyurethane exhibited better compatibility, leading to a reduction of overall migration.Formulations containing curcumin but without chitosan were found to be suitable for food packaging.

Curcumin-based polyurethanes for food packaging
In addition to the medical and packaging material fields, curcumin-based polyurethanes also have broad application prospects.For example, in the field of coatings, Song et al [34] used a modified curcumin dioxime as a crosslinking agent to bond in the polyurethane chain, constructing a bio-based covalent adaptable network consisting entirely of curcumin dioxime and castor oil.The mechanical properties of the polyurethane can be adjusted by changing the curcumin dioxime/castor oil ratio, and the biobased poly (urethane urea) covalent adaptable networks exhibit excellent elongation, reprocessability, and welding performance.In particular, at 90 °C, all of the biobased polyurethane coatings achieved >71% self-healing efficiency within 1 min.This is attributed to the synergistic effect of multiple dynamic bonding transformations, including dynamic imine and hydrogen bonding.This material can be used in fields such as leather and protective coatings.
Curcumin-based polyurethanes can also be developed into foam materials for furniture, building materials, sports equipment, and other fields.For example, Sienkiewicz et al [58] successfully prepared polyurethane foam using curcumin as a natural compound.Curcumin can adjust the properties of the foam.Specifically, the addition of less than 2 wt% curcumin polyurethane foams result in smaller and fewer regular cells.Within the limit of 2 wt%, the compressive strength, flexural strength, and thermal performance of the polyurethane foam increase with the increase in curcumin content, while the water absorption decreases.Additionally, curcumin significantly enhances the stability of the polymer when added at 1, 2, and 5 wt%, with 5 wt% exhibiting superior antibacterial properties.This tunable performance of polyurethane foams demonstrates application prospects in multiple fields.
In the textile industry, the use of traditional formaldehyde-based finishing agents has both functional benefits and toxic effects on the human body.If curcumin, as a natural monomer, can be developed as a finishing agent, it can solve this problem.Arshad et al [59] utilized curcumin-based waterborne polyurethane on fabrics.Testing results showed that the use of this material as a finishing agent significantly improved the colorfastness (dyeing, color change), rubbing fastness (dry, wet), and perspiration fastness (acidic, alkaline) of dyed and printed polyester-cotton fabrics.Moreover, the mechanical strength of the fabrics was also enhanced.In the following year, Arshad et al [59] incorporated chitosan into the raw materials and prepared waterborne polyurethane dispersions with a blend of synthetic chitosan and curcumin.In addition to the basic functions of a finishing agent, these dispersions exhibited synergistic antibacterial capabilities.Fabrics treated with this finishing agent hold potential for use in the medical field.
Flexible electronic materials and wearable sensors are cutting-edge areas in material development.The pHresponsive curcumin enables the development of curcumin-based polyurethane sensors.Maeng et al [60] utilized the electrospinning technique to incorporate both the flexibility of polyurethane and the chromatic characteristics of curcumin into various nanofibers that compose a cushion-type sensor.This fibrous mat exhibits a noticeable orange-red color change response when exposed to ammonia gas.The calculated lowest detection limit of the fibrous mat is 11.43 ppm.After optimization, the polyurethane sensor with a curcumin concentration of 30% achieves a high tensile strength of 6.9 MPa and an elongation capability of 458.93% under external physical stimuli.Furthermore, even after undergoing over 1400 cycles of repeated stretching and compression, the sensor remains undamaged, indicating the feasibility of this fabrication method for application in soft robotics and wearable device sensors, enabling effective detection of external environmental conditions.

Conclusion
In recent years, with the prevalence of the concept of sustainable development, there has been a growing interest in natural monomers and biobased polymers.Polyphenols are natural compounds widely distributed in plants, such as vanillin, cashew phenol, and curcumin, which exhibit various biological activities, including antibacterial and antioxidant properties.Among them, curcumin is a natural phenol extracted from plants belonging to the ginger family.It possesses an α,β-unsaturated β-diketone moiety, showing keto-enol tautomerism and exhibiting multiple functionalities such as antibacterial, anti-inflammatory, antioxidant, and UV protection.Studies have shown that curcumin can be incorporated into polyurethane matrices as a chain extender, crosslinking agent, or end-capper.This not only overcomes the limitations of curcumin's poor stability, insolubility in water, and rapid metabolism but also allows for the modulation of the mechanical properties and imparting functionality to polyurethanes.These advantages have led to the widespread application of curcumin-based polyurethanes in various fields, including medical, food packaging, coatings, textiles, sensors, etc.And it has shown application prospects in other fields.

Future perspectives
Although curcumin has been widely studied and applied in the field of polyurethanes, there are still several research areas waiting to be explored.Firstly, when curcumin and its derivatives are introduced as crosslinkers, both bonding and encapsulation forms typically coexist.However, a scheme for accurately controlling the ratio of bonding/encapsulation has not been proposed.Furthermore, exploring the development of new curcuminbased crosslinkers, such as the chemical degradation product of curcumin, bicyclopentadione, could be a viable option.Secondly, as curcumin is a naturally occurring compound with multiple hydroxyl groups, although there have been studies on using it as a chain extender, crosslinker, and end-capper, research on modifying curcumin, polymerizing it, and using it as a soft segment in polyurethanes is relatively scarce.Further exploration in this area could significantly advance the substitution of petroleum-based materials with biobased materials.Additionally, with the growing awareness of environmental protection, future research on polyurethane materials will place greater emphasis on sustainability and environmental performance.Furthermore, polymer-nanoparticle composite materials represent a focal point of research.[61] However, there has been relatively limited investigation into curcumin-based polyurethanes in this domain.Exploring this avenue could catalyze further expansion of the application scope of curcumin-based polyurethanes.Leveraging the unstable and easily degradable nature of curcumin, exploring the preparation of biodegradable polyurethane materials holds significant importance for environmental protection.
Furthermore, in terms of applied research, it is possible to continue exploring and fully utilizing the characteristics of curcumin in practical situations.For example, based on the pH sensitivity of curcumin, pHresponsive food packaging could be designed to monitor food safety in real time.The impact of curcumin on the physical and mechanical properties of polyurethane materials, especially polyurethane foams, also requires further investigation.When curcumin-based polyurethanes are used as coatings, solutions need to be developed to overcome the high coloration issue associated with curcumin.
Overall, the utilization of curcumin in polyurethanes opens up numerous opportunities for innovation and advancement across various domains.Through ongoing research and development efforts, curcumin can assume a pivotal role in enhancing the quality and performance of polyurethane products.

Figure 4 .
Figure 4. Schematic illustration of the crosslinking effects of curcumin.