**1. Introduction**

The road network in China has significantly improved in recent years. According to the latest statistical bulletin of the Ministry of Transport of China, the total mileage of roadways in China has occupied first place globally, reaching 5.198 million kilometers. Asphalt pavement provides outstanding performance and ride comfort, accounting for more than 90% of the highways [1,2]. Nevertheless, as the mileage of asphalt pavements that must be maintained grows, the disease and disposal issues of asphalt pavement have drawn people's attention. During the service life, the pavement will be impacted by loading, temperature, water, and other factors [3], causing the asphalt to undergo physical and chemical changes such as volatilization, oxidation, decomposition, and polymerization [4]. The performance of asphalt will deteriorate as a result of these consequences, the stress relaxation capability of the asphalt pavement declines as stiffness rises, and the asphalt pavement is susceptible to shrinkage fractures at low temperatures. The pavement quality will worsen if the cracks are not fixed or treated promptly, resulting in mesh cracks, cracks, rutting, and other more significant pavement diseases [5]. When the load interval or temperature rises, some microcracks will mend spontaneously (self-healing performance of asphalt), but other cracks that cannot be healed will spread due to vehicle load and form larger cracks. Water in the road environment, such as rainwater, will seep into the

**Citation:** Wu, J.; Liu, Q.; Wu, S. Bionic Synthesis of Mussel-like Adhesive L-DMA and Its Effects on Asphalt Properties. *Materials* **2022**, *15*, 5351. https://doi.org/10.3390/ ma15155351

Academic Editor: Simon Hesp

Received: 29 June 2022 Accepted: 2 August 2022 Published: 3 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

cracks and then gradually infiltrate into the bottom of the road under vehicle load, which will increase the water content of the road base and even the subgrade and diminish the strength.

There are several treatments for different kinds of cracks [6], such as slurry seal, microsurfacing, overlay, joint grouting, seam sticking, etc. Among these treatment approaches, joint grouting is the most extensively utilized due to its easy operation and high cost efficiency. At present, the asphalt pavement crack repair materials utilized in China generally comprise thermal asphalt, cold asphalt, and organic chemical specialist materials [7–9]. Polymer-modified asphalt, polymer-modified emulsified asphalt, and resin are widely employed as joint grouting materials in practical engineering applications [10]. However, from the standpoint of practical use, they have the faults of weak adhesion between grouting materials and fracture interface under water circumstances. To tackle this issue, a novel class of asphalt-based bionic material with good water resistance and strong adhesion was developed in this research.

Bionic design is an effective technique to emulate certain life traits and accomplish the intelligent design of materials. Mussels are a form of a marine creature with excellent adhesive capacity. The exceptional waterproof adhesion and universal adhesive qualities of mussels are particularly appealing. It is observed that mussels secrete various proteins via the Byssus, which are engaged in the adhesion process and are connected to the adhesion capacity and are solidified quickly after contact with saltwater to create an adhesive plate and securely bond to the substrate. The research discovered that the primary component of mucus is mussel adhesion protein (MAP) [11,12]. One of its most distinctive structural properties is the amino acid containing DOPA (dihydroxyphenylalanine). The catechol group (also known as catechol) in DOPA possesses chemical adaptability, versatility, and affinity diversity, which is the key to the super-strong adhesion property of mussels [13,14].

Due to the chemical versatility and affinity diversity of catechol functional groups, DOPA exhibits unique chemical properties. Thus, DOPA can boost the adhesion and cohesion of mussel adhesive proteins [15–17]. Through its chemical versatility, DOPA can undergo different reactions to achieve bonding and cross-link curing [18,19]. As shown in Figure 1, DOPA can be oxidized to Dopaquinone under certain conditions. After that, Dopaquinone can be rearranged and dehydrogenated to form Dehydro DOPA, which results in further cross-linking; DOPA quinone can also undergo Michael addition and Schiff base formation with amino and thiol groups; it can also form Carboxylated dihydroxyindole through intramolecular cyclization, and finally form cross-links. At the same time, Dopaquinone and DOPA can also be disproportionated to form free radicals, and finally coupled to tannins (Quinone tanning).

The magical adhesion properties of mussel adhesive proteins make them have unlimited potential applications in biomedical, national defense, marine engineering, and other fields. However, the ultra-low preparation quantity and expensive price limit the application of this excellent adhesive material. In order to obtain the adhesion ability equivalent to that of mussel adhesion protein in a wet state, people began to develop adhesion materials based on mussel adhesion protein. The study of adhesion materials using mussel adhesion protein was a gradual transition from natural mussel extraction to artificial synthesis. The first made use of the traditional protein extraction method and recombinant gene cloning method to prepare mussel adhesive materials. In the late 1980s, BioPolymers. Ltc successfully prepared mussel adhesive protein from mussel byssus glands by traditional extraction methods and developed a 'Cell-Tak' superadhesive [20,21]. 'Cell-Tak' has also been successfully used to enhance the adhesion of biomedical slides, plastics, or metal surfaces to cellular tissue samples. Subsequently, Hwang et al. recombined, expressed and purified the genes of Mgfp-5 and Mgfp-3A mussel adhesion proteins by recombination technology to obtain a superadhesive like 'Cell-Tak' [22,23]. The recombinant proteins with adhesion properties similar to those of 'Cell-Tak' were obtained. However, the cost of recombinant gene technology is also surprising due to the fact that the regulatory mechanism is still unclear. Lv et al. also tried to perform some work on the extraction of natural protein

and genetic engineering preparation of mussel adhesive materials [24]. Although mussel adhesion protein can be obtained by direct extraction or gene recombination, the separation and purification of mussel adhesion protein from mussels is an extremely complex process with a meager yield. This makes it very challenging to directly apply this adhesive to commercial applications, however, it also further highlights the urgency of developing synthetic mussel bionic adhesives with strong waterproof bonding properties. According to the intrinsic adhesion mechanism of mussel adhesive protein (DOPA, the key factor of adhesion performance), this paper aims to prepare polymeric adhesives to mimic the effect of natural mussel adhesion proteins.

**Figure 1.** Possible cross-linking pathways of DOPA.

Although biomimetic materials have been used in asphalt materials, this work marks the first time the bionic mussel adhesion protein has been applied to asphalt materials. By analyzing its impact on the overall performance of asphalt, this study aims to investigate the viability of using bionic mussel adhesion protein in asphalt. L-Dopa Methacrylic anhydride (L-DMA) was synthesized based on the features of bionic mussel adhesion protein in this study and used to modify 70# base asphalt with varied quantities to prepare L-DMA-modified asphalt (L-DMA-A). The thermal stability and structural composition of L-DMA were investigated. In addition, the high-temperature rutting resistance and low-temperature cracking resistance of L-DMA-modified asphalt were examined by DSR and BBR. Finally, the modification mechanism of L-DMA-modified asphalt was analyzed by FTIR. The flowchart of the experiment design is shown in Figure 2.

**Figure 2.** Flowchart of the experiment design.

#### **2. Materials and Experiments**

#### *2.1. Raw Materials*

In this study, the chemicals used to prepare L-DMA included sodium tetraborate (Na2B4O7•10H2O), sodium bicarbonate (NaHCO3), Levodopa (L-DOPA), methacrylic anhydride (MAA), magnesium sulfate (MgSO4), ethyl acetate (EAc), tetrahydrofuran (THF), and cyclohexane. The first four chemicals were purchased from Aladdin Ltd. (Shanghai Aladdin Biochemical Technology Co. Ltd., Shanghai, China) and the latter three were purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. All chemicals were chemically pure and used without further purification. The asphalt used, 70# base asphalt, was from Hubei Guochuang Hi-tech Materials Co. Ltd., of China (Wuhan), and the basic properties are shown in Table 1.

**Table 1.** Properties of the 70# base asphalt.


#### *2.2. Synthesis of L-DMA*

The mechanism of the synthesis process of L-DMA [25] is shown in Figure 3. An amount of 24 g of Na2B4O7•10H2O and 10 g of NaHCO<sup>3</sup> were accurately weighed and dissolved in 200 mL deionized water and aerated with Ar for 20 min, then 10.4 g of L-DOPA (52.8 mmol) was added and continued to be bubbled with Ar. After 5 min, 9.5 g of MAA (58.1 mmol) was dissolved in 50 mL of THF and slowly dropped to the above solution. During the dropwise addition, 1 mol/L NaOH solution was used to keep the pH of the solution above 8. The reaction mixture continued to be stirred for 18 h at room temperature and protected from light.

The reaction mixture was filtered by extraction to remove the precipitate from the bottom of the flask, and then the pH was adjusted to less than 2 with 4 mol/L HCl solution, followed by extraction with 100 mL EAc. The organic phase was collected after three extractions with 100 mL of EAc, and the aqueous phase was removed. An amount of 4 g of anhydrous magnesium sulfate was added to the organic phase, stirred for 12 h, and filtered to remove the residual water. Finally, the organic phase was concentrated by a rotary evaporator to 50 mL.

Finally, the concentrated ethyl acetate solution was slowly dropped into 400 mL of cyclohexane with high-speed stirring (at 2000 rpm). After the dropwise addition, the solution was stirred for 30 min, and then placed at 4 ◦C for 18 h to allow complete recrystallization. Finally, the solid was filtered out and dried in a vacuum oven. The synthesis process is shown in Figure 4.

**Figure 3.** Mechanism of the synthesis of L-DMA.
