The uniform liquid phase is separated into two or more immiscible liquid phases with different components dominated by charge-charge, hydrogen bonds, or π-π stacking interaction, which is called liquid-liquid phase separation (LLPS). [1] With the development of cell biology, it has been found that the interior of cells is not homogenous but contains many unique regions and compartments with distinct compositions and functions, including membraneless organelles. Up to now, it has been confirmed that LLPS induced the formation of membraneless organelles in cells. [2,3] The associative LLPS refers to the interaction between two or more soluble molecules, which will eventually appear in the same compact phase. This compact phase, named as complex coacervate droplet, is rich in high solute concentration but still contains at least 50% water content. [4] The coacervate droplets with crowded internal have lower dielectric constant than the surrounding water continuous phase, which concentrates solutes from the external and serves as a reactor for specific metabolism. It is the natural selection for living organisms to isolate and protect intracellular active substances. [[5], [6], [7]] Therefore, coacervate droplets are a promising platform for imitating membraneless organelles and serving as a convenient and efficient packaging strategy for various payloads in the aqueous phase, instead of relying on organic solvents.

Nevertheless, the biological application of coacervate droplets still faces challenges. Under gradual increase of ionic strength, the substance may undergo a transition from solid aggregates to coacervate droplets and eventually to uniform solution. Once the critical salt concentration is at that threshold, the coacervate droplets transform into uniform solution. [8] Despite coacervate droplets being stable at low salt concentrations, they will be dissolved while transferring to a solution above the critical salt concentration. Unfortunately, the critical salt concentration of ordinary coacervate droplets is lower than the physiological conditions, which limits their application. [9] Moreover, the inherent membraneless characteristics of coacervate droplets lead to instability such as self-fusion and wall cling property. Some researchers combined semi-permeable membranes based on fatty acids, phospholipids, and amphiphilic polymers with coacervate droplets to develop artificial protocells. [[10], [11], [12]] The construction of artificial cells mainly depends on giant unilamellar vesicles at present. [13] However, the cavities of giant unilamellar vesicles have no crowded internal structure of substance concentration, which is the crucial feature of cytoplasmic and membraneless organelles. [14] On the contrary, coacervate-based artificial protocells (ArtPC) exhibit excellent characteristics such as spontaneous assembly and concentration of payloads with different physiochemical properties. The semi-permeable membrane makes them have selective permeability to a certain extent, effectively partitioning contents, and improving the stability and dispersion of coacervate droplets. Consequently, the development of the ArtPC with physiological stability holds promise for advancing the fields of synthetic biology and pharmaceuticals.

In recent years, the overall prevalence of hyperuricemia has greatly increased, reaching 17.4% in China, 20.1% in America, and 26.8% in Japan. [15,16] Hyperuricemia has been listed as an independent risk factor for cardiovascular and cerebrovascular diseases, kidney diseases, diabetes, and gout. The clinical practice guidelines recommend that all patients with hyperuricemia always control their blood uric acid levels within the ideal range. [[17], [18], [19], [20]] Uricase (Uri) is a specific active protein that can degrade uric acid efficiently. However, mutations and deletions of related genes in primates such as humans lead to the non-expression of Uri in vivo. [21,22] Recombinant Uri (Rasburicase) effectively degrades uric acid, but it is readily hydrolyzed in vivo with a short half-life and strong immunogenicity. [23,24] The pharmacokinetic of modified Uri (Krystexxa) is significantly improved with low immunogenicity. [25] However, the responsiveness is decreased after repeated administration in patients, and the exorbitant price makes it unable to become a clinically effective and commonly used drug. [26] In addition, uric acid is decomposed by Uri to produce allantoin, carbon dioxide, and H2O2. Excessive H2O2 in the blood may induce methemoglobinemia and life-threatening hemolysis. [27,28] It is of great significance to improve the therapeutic effect that can protect the activity of Uri and effectively eliminate H2O2 to treat hyperuricemia and its complications.

Hence, polylysine-polynucleotide complex coacervate droplets were first formulated to efficiently and facilely concentrate small molecules, biomacromolecules, and nanoparticles with favorable stability at physiological pH and salt concentration. To improve biocompatibility, the PEGylated phospholipid membrane was further coated on the surface of the coacervate droplets to prepare ArtPC, which demonstrated the membrane-like and cytoplasm-like structures. It could also reduce the recognition of the endothelial reticular system, thus prolonging systemic circulation time and effectively protecting the activity of enzymes. To apply urate-lowering therapy, the cyclic catalytic system of Uri and catalase (Cat) was confined inside ArtPC. This catalytic system could not only efficiently degrade uric acid into allantoin and H2O2, but also convert toxic intermediates into O2 and H2O. The generated O2 would continue to promote the catalytic degradation of uric acid. This biofunctional ArtPC could participate in metabolizing for the treatment of persistent hyperuricemia in vivo (Scheme 1).