I. INTRODUCTION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTION <<II. VESICLE FUSION METHODIII. THIN-FILM HYDRATION ...IV. SOLVENT EXCHANGE METH...V. COMPARISON AND SELECTI...VI. CONCLUSION AND FUTURE...REFERENCESWith the development of nanotechnology, more nanomaterials are constructed and explored as drug carriers, including polymeric nanoparticles, dendrimers, liposomes, solid lipid nanoparticles, and magnetic and gold nanoparticles.1–51. S. Kumar, V. Lather, and D. Pandita, Int. J. Biol. Macromol. 84, 380 (2016). https://doi.org/10.1016/j.ijbiomac.2015.12.0362. A. Espinosa, R. Di Corato, J. Kolosnjaj-Tabi, P. Flaud, T. Pellegrino, and C. Wilhelm, ACS Nano 10, 2436 (2016). https://doi.org/10.1021/acsnano.5b072493. K. Madaan, V. Lather, and D. Pandita, Drug Delivery 23, 254 (2016). https://doi.org/10.3109/10717544.2014.9105644. M. Gogoi, M. K. Jaiswal, H. D. Sarma, D. Bahadur, and R. Banerjee, Integr. Biol. 9, 555 (2017). https://doi.org/10.1039/C6IB00234J5. D. Pandita, S. Kumar, N. Poonia, and V. Lather, Food Res. Int. 62, 1165 (2014). https://doi.org/10.1016/j.foodres.2014.05.059 Compared with traditional “free” drugs, drugs loaded in the nanocarriers can improve their own problems to some extent and, finally, potentiate therapeutic efficacy.66. K. S. Butler, P. N. Durfee, C. Theron, C. E. Ashley, E. C. Carnes, and C. J. Brinker, Small 12, 2173 (2016). https://doi.org/10.1002/smll.201502119 Among the various nanomaterials for drug delivery applications, mesoporous silica nanoparticles (MSNs) are recognized as attractive carriers, capable of hosting multiple therapeutic agents and easily endowed with various abilities, including targeting or imaging capabilities.7,87. R. R. Castillo, D. Lozano, B. Gonz lez, M. Manzano, I. Izquierdo-Barba, and M. A. Vallet-Reg, Expert Opin. Drug Delivery 16, 415 (2019). https://doi.org/10.1080/17425247.2019.15983758. F. Farjadian, A. Roointan, S. Mohammadi-Samani, and M. Hosseini, Chem. Eng. J. 359, 684 (2019). https://doi.org/10.1016/j.cej.2018.11.156 Mesoporous silica was first reported by Japanese scientists in 199099. T. Yanagisawa, T. Shimizu, K. Kuroda, and C. Kato, Bull. Chem. Soc. Jpn. 63, 988 (1990). https://doi.org/10.1246/bcsj.63.988 and, concurrently, synthesized through the solgel technique by the Mobil Corporation.10,1110. J. S. Beck et al., J. Am. Chem. Soc. 114, 10834 (1992). https://doi.org/10.1021/ja00053a02011. M. E. Leonowicz, W. J. Roth, C. T. Kresge, J. C. Vartuli, and J. S. Beck, Nature 359, 710 (1992). https://doi.org/10.1038/359710a0 Then, the first uniform MSNs were reported by Lu et al.1212. Y. Lu, H. Fan, A. Stump, T. L. Ward, T. Rieker, and J. Brinker, Nature 398, 223 (1999). https://doi.org/10.1038/18410 Later, the mesoporous silica, named MCM-41 (Mobile Crystalline Material-41), was proposed as a drug carrier for the first time in 2001 by Vallet-Regi et al.1313. M. Vallet-Regi, A. Ramila, R. P. Del Real, and J. Rez-Pariente, Chem. Mater. 13, 308 (2001). https://doi.org/10.1021/cm0011559 Since this revolutionary finding, MSNs have been widely employed as drug delivery systems.As drug delivery carriers, MSNs exhibited several beneficial features in comparison to others: (1) large pore volume and high surface area allow high loadings of therapeutic agents; (2) uniform and tunable pore size allows precise adjusting of drug loading and studying release kinetics; (3) tunable particle size allows facile endocytosis by cells with minimal cytotoxicity; (4) two silanol-contained surfaces allow the independent functionalization with different groups; and (5) stable and rigid framework renders MSNs with high chemical, thermal, and mechanical stabilities.14–1614. I. Slowing, J. Viveroescoto, C. Wu, and V. Lin, Adv. Drug Delivery Rev. 60, 1278 (2008). https://doi.org/10.1016/j.addr.2008.03.01215. Y. Zhou, G. Quan, Q. Wu, X. Zhang, B. Niu, B. Wu, Y. Huang, X. Pan, and C. Wu, Acta Pharm. Sin. B 8, 165 (2018). https://doi.org/10.1016/j.apsb.2018.01.00716. Y. Feng, N. Panwar, D. J. H. Tng, S. C. Tjin, K. Wang, and K. Yong, Coord. Chem. Rev. 319, 86 (2016). https://doi.org/10.1016/j.ccr.2016.04.019 However, there is still a long way for the practical application of MSNs, mainly due to their drawbacks, including easy aggregation in physiological environments, uncontrolled and premature release of the loaded drug before reaching the target-site, rapid clearance by the reticuloendothelial system, and the non-negligible hemolytic behavior after injection.17–2217. A. Baeza, M. Colilla, and M. Vallet-Regi, Expert Opin. Drug Delivery 12, 319 (2015). https://doi.org/10.1517/17425247.2014.95305118. L. Wang, L. Wu, S. Lu, L. Chang, I. Teng, C. Yang, and J. A. Ho, ACS Nano 4, 4371 (2010). https://doi.org/10.1021/nn901376h19. K. F. Pirollo and E. H. Chang, Trends Biotechnol. 26, 552 (2008). https://doi.org/10.1016/j.tibtech.2008.06.00720. Q. He, Z. Zhang, F. Gao, Y. Li, and J. Shi, Small 7, 271 (2011). https://doi.org/10.1002/smll.20100145921. L. Pan, Q. He, J. Liu, Y. Chen, M. Ma, L. Zhang, and J. Shi, J. Am. Chem. Soc. 134, 5722 (2012). https://doi.org/10.1021/ja211035w22. X. Huang, L. Li, T. Liu, N. Hao, H. Liu, D. Chen, and F. Tang, ACS Nano 5, 5390 (2011). https://doi.org/10.1021/nn200365aTo counteract the problems mentioned above, surface modification has been developed and applied to change the properties of MSNs. Various functional molecules and ligands were reported to modify the MSNs through covalent bonding or electrostatic attraction, in order to improve their physicochemical properties and broaden their ultimate applications.23,2423. K. Ariga, Y. Yamauchi, G. Rydzek, Q. Ji, Y. Yonamine, K. C. W. Wu, and J. P. Hill, Chem. Lett. 43, 36 (2014). https://doi.org/10.1246/cl.13098724. Q. M. Kainz and O. Reiser, Acc. Chem. Res. 47, 667 (2013). https://doi.org/10.1021/ar400236y Unfortunately, this complicated functionalization process will inevitably affect the drug loading efficiency of MSNs whether the functionalization comes first or last. Different from the aforementioned functionalization strategy, the lipid coating is a more facile and efficient strategy to settle the above problems of MSNs without adversely influencing their drug loading ability.25,2625. J. Zhou, Q. Wang, and C. Zhang, J. Am. Chem. Soc. 135, 2056 (2013). https://doi.org/10.1021/ja311032926. N. Han, Y. Wang, J. Bai, J. Liu, Y. Wang, Y. Gao, T. Jiang, W. Kang, and S. Wang, Microporous Mesoporous Mater. 219, 209 (2016). https://doi.org/10.1016/j.micromeso.2015.08.006 This strategy was inspired by supported lipid bilayers (SLBs) on planar substrates2727. L. K. Tamm and H. M. McConnell, Biophys. J. 47, 105 (1985). https://doi.org/10.1016/S0006-3495(85)83882-0 that were explored as cell membrane mimics for fundamental biophysical studies. Then, SLBs were gradually formed on solid silica particles2828. R. Rapuano and A. M. Carmona-Ribeiro, J. Colloid Interfaces Sci. 193, 104 (1997). https://doi.org/10.1006/jcis.1997.5060 and MSNs,2929. J. Liu, A. Stace-Naughton, X. Jiang, and C. J. Brinker, J. Am. Chem. Soc. 131, 1354 (2009). https://doi.org/10.1021/ja808018y mainly for fundamental studies or drug delivery. This lipid coating may endow the MSNs with some other advantages, such as fluidity.3030. C. E. Ashley et al., Nat. Mater. 10, 389 (2011). https://doi.org/10.1038/nmat2992 The lipid-coated MSNs (LMSNs) can synergistically inherit the merits and overcome the drawbacks of both MSNs and liposomes in one versatile nanocarrier.3131. A. E. LaBauve et al., Sci. Rep. 8, 13990 (2018). For this novel drug carrier, MSNs act as a drug reservoir, supporting the skeleton to accommodate various drug molecules and stabilize the outer lipid layer, respectively. While the lipid layer, in turn, acts as a biocompatible membrane to improve the colloid stability and biocompatibility of MSNs, a drug reservoir to load drugs, a “gatekeeper” to control drug release, and an intermediate to conjugate various targeting molecules.3232. M. M. van Schooneveld et al., Nano Lett. 8, 2517 (2008). https://doi.org/10.1021/nl801596a As expected, LMSNs exhibit properties superior to that of liposomes or MSNs used alone and are considered to be effective carriers for multiple classes of cargo.33–3533. C. E. Ashley, E. C. Carnes, and K. E. Epler, ACS Nano 6, 2174 (2012). https://doi.org/10.1021/nn204102q34. J. Tu, J. Bussmann, G. Du, Y. Gao, J. A. Bouwstra, and A. Kros, Int. J. Pharm. 543, 169 (2018). https://doi.org/10.1016/j.ijpharm.2018.03.03735. E. C. Dengler et al., J. Controlled Release 168, 209 (2013). https://doi.org/10.1016/j.jconrel.2013.03.009The LMSNs were first prepared by Jeffrey Brinker in 2009 through the vesicle fusion method.29,3629. J. Liu, A. Stace-Naughton, X. Jiang, and C. J. Brinker, J. Am. Chem. Soc. 131, 1354 (2009). https://doi.org/10.1021/ja808018y36. J. Liu, X. Jiang, C. Ashley, and C. J. Brinker, J. Am. Chem. Soc. 131, 7567 (2009). https://doi.org/10.1021/ja902039y So far, several methods have been developed to prepare LMSNs, including vesicle fusion, thin-film hydration, and solvent exchange. Although these methods have been widely applied for LMSNs, there is short of elaboration about them, hindering their development. Therefore, in this review, we will systematically and comprehensively elaborate on each method and compare its advantages and limitations. According to this review, we hope these methods will be better understood and applied in the future.

II. VESICLE FUSION METHOD

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. VESICLE FUSION METHOD <<III. THIN-FILM HYDRATION ...IV. SOLVENT EXCHANGE METH...V. COMPARISON AND SELECTI...VI. CONCLUSION AND FUTURE...REFERENCESThe most common and initial method for preparing LMSNs is vesicle fusion, in which liposomes spontaneously fuse to the surface of MSNs upon mixing and are largely driven by electrostatic interactions and van der Waals attractive forces.3737. M. A. Gonzalez Porras, P. N. Durfee, A. M. Gregory, G. C. Sieck, C. J. Brinker, and C. B. Mantilla, J. Neurosci. Method 273, 160 (2016). https://doi.org/10.1016/j.jneumeth.2016.09.003 This strategy is inspired by the vesicle fusion on the planar substrates to form SLBs, where the fusion process proceeds by a fast, two-step mechanism (including adhesion and rupture).28,38–4028. R. Rapuano and A. M. Carmona-Ribeiro, J. Colloid Interfaces Sci. 193, 104 (1997). https://doi.org/10.1006/jcis.1997.506038. T. Cha, A. Guo, and X. Y. Zhu, Biophys. J. 90, 1270 (2006). https://doi.org/10.1529/biophysj.105.06143239. R. Richter, A. Mukhopadhyay, and A. Brisson, Biophys. J. 85, 3035 (2003). https://doi.org/10.1016/S0006-3495(03)74722-540. E. Sackmann, Science 271, 43 (1996). https://doi.org/10.1126/science.271.5245.43 Although for certain conditions one-step SLB formation (including only vesicle rupture) is possible,4141. R. P. Richter, R. Berat, and A. R. Brisson, Langmuir 22, 3497 (2006). https://doi.org/10.1021/la052687c this mechanism is not widely accepted due to the insufficient surface stress for vesicle rupture.4242. G. J. Hardy, R. Nayak, and S. Zauscher, Curr. Opin. Colloid Interface Sci. 18, 448 (2013). https://doi.org/10.1016/j.cocis.2013.06.004 Analogously, for vesicle fusion on the MSNs, most researchers support that the process proceeds by a “two-step” mechanism (Fig. 1). In time-dependent cryo-TEM (cryotransmission electron microscopy), Mornet et al. revealed, in detail that liposomes are first absorbed on silica nanoparticles followed by deforming and rupturing, which is consistent with the “two-step” process.4343. S. P. Mornet, O. Lambert, E. Duguet, and A. Brisson, Nano Lett. 5, 281 (2005). https://doi.org/10.1021/nl048153y For MSNs, Durfee et al. also observed the process of vesicle adsorption and deformation by cryo-TEM, and they deduced that these deformed vesicles would, subsequently, rupture and fuse on the MSNs surface in a similar “two-step” process.4444. P. N. Durfee et al., ACS Nano 10, 8325 (2016). https://doi.org/10.1021/acsnano.6b02819 However, the evidence obtained from cryo-TEM is not enough to explain the mechanisms of vesicle fusion on MSNs, more comprehensive study is needed.Although mechanisms are not clear, many factors have been proved to definitely influence vesicle fusion to form LMSNs (Table I). The single unilamellar vesicles (SUV)/SiO2 surface ratio (SASUV/SASiO2) can affect vesicle fusion on MSNs. The SASUV/SASiO2 = 1 is the critical ratio needed to exactly form a complete SLB on SiO2 nanoparticles. When SASUV/SASiO2 2 nanoparticles were observed by Savarala et al., the surface of the SiO2 was partially covered, easily inducing aggregation/precipitation. Interestingly, this phenomenon was surface coverage dependent: For SASUV/SASiO2 SUV/SASiO2 ≥ 0.4, there was no aggregation/precipitation found during the 4 days; while in the range 0.005 SUV/SASiO2 SUV/SASiO2 ≤ 0.35, the aggregation/precipitation was compositional and time-dependent and also irreversible. They postulated that aggregates were formed by bridging a single lipid bilayer in which each lipid monolayer was bound to two adjacent SiO2 nanoparticles. Decreasing the lipid bilayer patches and increasing the bare SiO2 surface increased the probability that the dimer/multiplet formation through collision would be followed up by the fast rate of aggregation/precipitation.4545. S. Savarala, F. Monson, M. A. Ilies, and S. L. Wunder, Langmuir 27, 5850 (2011). https://doi.org/10.1021/la200636k When SASUV/SASiO2 > 1, the excess SUVs could provide steric/undulatory repulsion to reverse colloidal instability for systems where SASUV/SASiO2 = 1 at I > 55 mM NaCl and keep the suspensions stable. However, the addition and incubation of excess SUVs could not reverse the aggregation/precipitation for systems where SASUV/SASiO2 4545. S. Savarala, F. Monson, M. A. Ilies, and S. L. Wunder, Langmuir 27, 5850 (2011). https://doi.org/10.1021/la200636k Despite excess SUVs, additional multilayered SLBs on the SiO2 nanoparticles were not observed since it was well known that zwitterionic SUVs did not fuse on SLB surfaces.4646. C. A. Keller and B. Kasemo, Biophys. J. 75, 1397 (1998). https://doi.org/10.1016/S0006-3495(98)74057-3 In fact, even double SLBs were very difficult to form by the fusion of SUVs onto substrates.4242. G. J. Hardy, R. Nayak, and S. Zauscher, Curr. Opin. Colloid Interface Sci. 18, 448 (2013). https://doi.org/10.1016/j.cocis.2013.06.004 Durfee et al. found the same rule during the formation of LMSNs by vesicle fusion: when SASUV/SASiO2 SUV/SASiO2 > 1, stable and uniform nanoparticles were observed, indicating complete lipid coating.4444. P. N. Durfee et al., ACS Nano 10, 8325 (2016). https://doi.org/10.1021/acsnano.6b02819 It can be seen that excess lipid facilitates the vesicle fusion process and the formation of monosized LMSNs.

TABLE I. Influence factors for the three lipid coating methods.

MethodsInfluence factorsHow to influenceVesicle fusionSASUV/SASiO2<1, incomplete supported lipid bilayers (SLBs) =1, complete SLBs and unstable LMSNs >1, complete SLBs and stable LMSNsIonic strengthsPure water, unformed SLBs Sufficient, complete SLBs and stable LMSNs Too high, complete SLBs and unstable LMSNsNanoparticles propertiesSpherical MSNs, rodlike MSNs, small pores mMSNs (2.8, 5, 8 nm), complete SLBsSmall nanoparticles (4–6, 8 nm) and large pores MSNs (18 nm), unformed SLBsLipid typeUnsaturated DOPC, complete SLBs and unstable LMSNs Saturated DSPC, complete SLBs and stable LMSNsDrug loadingNegligible effect on the LMSN formationThin-film hydrationMSNs/lipid ratio (w/w)1:0.9, complete SLBs with 5–7 nm thickness 1:1.11, complete SLBs with 13−15 nm thickness 1:1.3, complete SLBs with 30 −33 nm thicknessSolvent exchange methodNanoparticles/lipid ratio (w/w)1:1–1:32, the lipid density increased with the weight ratio and reached a plateau at 1:8Lipid typePhospholipid-PEG (phospholipids with 14–18 carbons, PEG molecular weights 1000–5000 Da), successfully lipid coating Phospholipid-PEG (PEG molecular weights <750 Da), nanoparticles aggregationAnother factor that can affect vesicle fusion on MSNs is ionic strengths and sufficient ionic strengths are necessary for this process. On the one hand, small, low polarizable cations were able to convert the phosphate headgroups from dipoles to positive monopoles.47,4847. S. Garcia-Manyes, G. Oncins, and F. Sanz, Biophys. J. 89, 1812 (2005). https://doi.org/10.1529/biophysj.105.06403048. B. Seantier and B. Kasemo, Langmuir 25, 5767 (2009). https://doi.org/10.1021/la804172f On the other hand, it was suggested that salt could increase the deprotonation of silanol groups, inducing the surface charge density increase of the silica.4949. T. F. Tadros and J. Lyklema, J. Electroanal. Chem. 17, 267 (1968). https://doi.org/10.1016/S0022-0728(68)80206-2 Both the effects strengthened vesicle–silica interaction and deformed the vesicle, which, therefore, facilitate vesicle fusion. Durfee et al. reported that samples prepared in pure water exhibited a comparable zeta potential with MSN (ζ = −41.0 mV), indicating SLBs were not formed since vesicle fusion was severely inhibited. As the ionic strengths of the fusion conditions increased to 40 mM in phosphate buffered saline (PBS), samples prepared with SAsuv:SASiO2 > 1, had a diameter about 30 nm greater than the parent MSN and a zeta potential (ζ = −3.3 mV) comparable to the liposomes but higher than the parent MSN. When transferred to 160 mM PBS, aggregation was not observed, indicating that SLBs successfully fused on the MSNs.4444. P. N. Durfee et al., ACS Nano 10, 8325 (2016). https://doi.org/10.1021/acsnano.6b02819 It is important to note that the ionic strengths needed to form complete SLBs on MSNs with different sizes may differ.4545. S. Savarala, F. Monson, M. A. Ilies, and S. L. Wunder, Langmuir 27, 5850 (2011). https://doi.org/10.1021/la200636k Moreover, if the ionic strengths are too high, precipitation of the SLBs may be induced, not only due to the electrostatic shielding by both lipid and salt but also due to the reduced repulsive forces for lipids on solid surfaces.4545. S. Savarala, F. Monson, M. A. Ilies, and S. L. Wunder, Langmuir 27, 5850 (2011). https://doi.org/10.1021/la200636k Therefore, a low but sufficient ionic strength is needed for vesicle fusion on MSNs to increase the relative rate and help avoid any accompanying aggregation.Except SASUV/SASiO2 and ionic strengths, the physicochemical properties of MSNs (particle size, charge, shape, pore diameters, and morphologies) and lipids (charge and type) are also known to influence vesicle fusion. Savarala et al. studied the fusion process between liposomes [formed by zwitterionic 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)] and silica nanoparticles with diameters ranging from 100 to 4–6 nm. At certain ionic strengths and SASUV/SASiO2, they found that complete SLBs formation of the larger nanobeads was easier and faster than that for the smaller nanobeads (20–100 nm). This might be because the smaller nanobeads possessed less area of contact with the SUVs. Meanwhile, the lipids bilayer on the smaller nanobeads had more free volume, increasing the chain interdigitation of the lipids and hindering lipids packing on the nanobeads.5050. S. Savarala, S. Ahmed, M. A. Ilies, and S. L. Wunder, Langmuir 26, 12081 (2010). https://doi.org/10.1021/la101304v Even with the particle size up to 9 ± 1 μm, the lipid bilayer could still successfully coat the porous silica microparticles.5151. I.-A. Pavel et al., J. Mater. Chem. B 6, 5633 (2018). https://doi.org/10.1039/C8TB01114A However, 4–6 nm silica beads appeared to surround and decorate the exterior of the SUVs instead of forming SLBs, which was confirmed by another report.5252. S. Savarala, S. Ahmed, M. A. Ilies, and S. L. Wunder, ACS Nano 5, 2619 (2011). https://doi.org/10.1021/nn1025884 Liu et al. also confirmed that silica nanoparticles with a diameter of 8 nm could not be coated by liposomes but absorbed on them, whereas larger silica nanoparticles (diameters 30, 50, 80, and 130 nm) were successfully coated by a lipid bilayer.5353. J. Liu, A. Stace-Naughton, and C. J. Brinker, Chem. Commun. 2009, 5100. https://doi.org/10.1039/b911472f Durfee et al. tested the effects of MSNs properties on the lipid fusion process, including shapes (spherical or rodlike), pore diameters (2.8–18 nm), and pore morphologies (aligned cylindrical, isotropic wormlike, and dendritic) (Fig. 2). They found that conformal SLBs were formed on all of the tested particles except spherical MSNs with the largest pores of ∼18 nm. In this case, complete SLB was not formed on the MSNs, although by cryo-TEM, vesicle adsorption and deformation were observed. They proposed that for this highly porous particle, the van der Waals and electrostatic interactions are too weak to cause rupture/fusion to form an SLB, and the deep scratches on their surface might also arrest the spreading of the SLBs.4444. P. N. Durfee et al., ACS Nano 10, 8325 (2016). https://doi.org/10.1021/acsnano.6b02819 They also investigated the effect of the degree of lipid unsaturation on vesicle fusion and found liposomes composed of unsaturated 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) or saturated 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) could both form complete SLBs on MSNs but might influence their long-term stability and the later possessed better stability.4444. P. N. Durfee et al., ACS Nano 10, 8325 (2016). https://doi.org/10.1021/acsnano.6b02819 As described above, since vesicle fusion on MSNs was mainly driven by electrostatic interactions, it was easy to speculate that the charge of the MSNs and the lipids have a direct effect on vesicle absorption and rupture. Unfortunately, there is no systematic investigation, although we can find some evidence from several research studies.29,5429. J. Liu, A. Stace-Naughton, X. Jiang, and C. J. Brinker, J. Am. Chem. Soc. 131, 1354 (2009). https://doi.org/10.1021/ja808018y54. H. Shi et al., ACS Appl. Mater. Interfaces 11, 3645 (2019). https://doi.org/10.1021/acsami.8b15390As drug carriers, a drug may also be an influencing factor. Loading of a drug in MSNs can be expected to fill in, to some extent, the surface pores or absorbed on the surfaces and thereby influence the vesicle fusion by changing the physicochemical properties of the MSNs, such as the surface area, charge, and surface roughness. Unfortunately, such reports are limited. Han et al. speculated that drug loading would not affect the vesicle fusion since the weight loss between LMSNs and drug loading LMSNs, obtained from the thermogravimetric analysis (TGA), was comparative.2626. N. Han, Y. Wang, J. Bai, J. Liu, Y. Wang, Y. Gao, T. Jiang, W. Kang, and S. Wang, Microporous Mesoporous Mater. 219, 209 (2016). https://doi.org/10.1016/j.micromeso.2015.08.006 Meanwhile, the bilayer thickness of loaded and unloaded LMSNs was 6.0 ± 0.94 and 5.4 ± 0.91 nm, respectively, indicating a negligible effect of drug loading on the vesicle fusion.3131. A. E. LaBauve et al., Sci. Rep. 8, 13990 (2018). However, these research studies obviously cannot fully explain the effect of the drug loading, more comprehensive studies are needed. Considering that the influence is mutual, vesicle fusion can also affect drug loading and release. For drug-loaded MSNs, the loading efficiency would slightly decrease during the vesicle fusion process. But for drugs that cannot load into the MSNs pores, vesicle fusion may increase the LMSN's loading efficiency. Liu et al. reported that negatively charged calcein could be loaded in LMSNs when fused with positively charged liposomes, whereas no calcein loading was observed when fused with negatively charged liposomes.2929. J. Liu, A. Stace-Naughton, X. Jiang, and C. J. Brinker, J. Am. Chem. Soc. 131, 1354 (2009). https://doi.org/10.1021/ja808018y It may be because calcein is loaded into the lipid bilayer rather than the MSNs pores. Through vesicle fusion, a single lipid bilayer will be coated on the MSNs, thereby decreasing drug release from the LMSNs. Meanwhile, the drug release can also be controlled by specific lipids, for example, acid-sensitive lipids were applied to trigger drug release under low pH conditions.55,5655. X. Zhang, F. Li, S. Guo, X. Chen, X. Wang, J. Li, and Y. Gan, Biomaterials 35, 3650 (2014). https://doi.org/10.1016/j.biomaterials.2014.01.01356. D. Wang et al., Biomaterials 34, 7662 (2013). https://doi.org/10.1016/j.biomaterials.2013.06.042

In short, vesicle fusion is a reliable, easy, and versatile method to produce high-quality SLBs, and the forming process does not require a sophisticated instrument. Moreover, liposomes with different particle sizes can be prepared to accommodate MSNs coating. In order to achieve complete lipid coating, the liposomes have to undergo two steps, including absorption and deformation, disruption, and fusion. However, this stepwise liposomal procedure consumes time and energy, leading to inefficiency and nonuniform coating. Therefore, more attention should be paid to the increase of efficiency when applying this method.

III. THIN-FILM HYDRATION METHOD

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. VESICLE FUSION METHODIII. THIN-FILM HYDRATION ... <<IV. SOLVENT EXCHANGE METH...V. COMPARISON AND SELECTI...VI. CONCLUSION AND FUTURE...REFERENCESThe thin-film hydration method is proposed by Bangham et al. as a simplified liposome preparation method.5757. A. D. Bangham, M. M. Standish, and J. C. Watkins, J. Mol. Biol. 13, 238 (1965). https://doi.org/10.1016/S0022-2836(65)80093-6 Basically, the lipid material is dispersed in volatile organic solvents, such as hexane, methanol, and chloroform, followed by the evaporation of the solvent. After all the solvent gets removed and the thin lipid film is formed, the hydrophilic cargo solution is then added to hydrate the film and induce liposome formation.58–6058. A. Samad, Y. Sultana, and M. Aqil, Curr. Drug Delivery 4, 297 (2008). https://doi.org/10.2174/15672010778215126959. W.-C. Tsai and S. S. H. Rizvi, Trends Food Sci. Technol. 55, 61 (2016). https://doi.org/10.1016/j.tifs.2016.06.01260. B. William, P. No mie, E. Brigitte, and P. G. Raldine, Chem. Eng. J. 383, 123106 (2020). https://doi.org/10.1016/j.cej.2019.123106 This hydration process usually requires energy input from intensive heating and sonication. Although this method is still the simplest procedure for liposome formation, it is gradually replaced by other methods because of some of its limitations, such as low encapsulation efficiency, homogenization, difficulty in removing organic solvent, and small scale production.6161. L. A. Meure, N. R. Foster, and F. Dehghani, AAPS PharmSciTech 9, 798 (2008). https://doi.org/10.1208/s12249-008-9097-x Despite its decline in liposomal preparation, the application of the thin-film hydration method in preparing LMSNs is just beginning. In 2015, two research groups independently prepared LMSNs by the thin-film hydration method. Meng et al. developed a coated lipid film method in which drug-soaked MSN water solutions were used to hydrate a continuous lipid film obtained by solvent evaporation, resulting in rapid and uniform surface coverage upon controlled probe sonication (Fig. 3). Compared with vesicle fusion, this method can obtain instantaneous and complete LB coating without multiple washing procedures, avoiding the jeopardy of drug loss and providing effective drug loading.6262. H. Meng, M. Wang, H. Liu, X. Liu, A. Situ, B. Wu, Z. Ji, C. H. Chang, and A. E. Nel, ACS Nano 9, 3540 (2015). https://doi.org/10.1021/acsnano.5b00510 Although this lipid film coating procedure was not named in this article, its principle is similar to that of the thin-film hydration method. Han et al. used a modified thin-film hydration method to prepare LMSNs, in which hydrophobic MSNs were directly mixed with lipid solution, followed by evaporation and hydration, allowing a lipid layer coating on the surface of MSNs. It is worth noting that the obtained LMSNs could only carry a drug since the lipid film formed on the hydrophobic MSNs was monolayer.6363. N. Han et al., ACS Appl. Mater. Interfaces 7, 3342 (2015). https://doi.org/10.1021/am5082793 Since then, this one-step thin-film hydration method is widely applied in LMSN preparation.64–6764. L. Wang et al., J. Controlled Release 318, 197 (2019). https://doi.org/10.1016/j.jconrel.2019.10.01765. R. K. Thapa, H. T. Nguyen, M. Gautam, A. Shrestha, E. S. Lee, S. K. Ku, H. Choi, C. S. Yong, and J. O. Kim, Drug Delivery 24, 1690 (2017). https://doi.org/10.1080/10717544.2017.139638266. S. Yang, S. Song, K. Han, X. Wu, L. Chen, Y. Hu, J. Wang, and B. Liu, Microporous Mesoporous Mater. 284, 212 (2019). https://doi.org/10.1016/j.micromeso.2019.04.04367. Y. Feng, N. X. Li, H. L. Yin, T. Y. Chen, Q. Yang, and M. Wu, Mol. Pharm. 16, 422 (2019). https://doi.org/10.1021/acs.molpharmaceut.8b01073Similar to the vesicle fusion method, the lipid coating of the thin-film hydration method is carried out by electrostatic interactions and van der Waals attractive forces. Meng et al. suspected that van der Waals forces played a role in LB forming process and electrostatic charge contributed to LB adhesion to the MSN surface.6262. H. Meng, M. Wang, H. Liu, X. Liu, A. Situ, B. Wu, Z. Ji, C. H. Chang, and A. E. Nel, ACS Nano 9, 3540 (2015). https://doi.org/10.1021/acsnano.5b00510 For the thin-film hydration method, the obtained lipid coating is usually bilayer since the MSN surface is often hydrophilic. Converting the MSN surface into hydrophobic, monolayer lipid coating will be obtained through the thin-film hydration method. Zhou et al. modified the MSNs with hydrophobic organosiloxane precursor and then monolayer lipid was coated on the surface of the MSNs by a thin-film hydration method based on hydrophobic interaction.6868. G. Zhou, L. Li, J. Xing, J. Cai, J. Chen, P. Liu, N. Gu, and M. Ji, J. Sol-Gel Sci. Technol. 82, 490 (2017). https://doi.org/10.1007/s10971-017-4330-2 So it is likely that van der Waals forces play a major role in the thin-film hydration method. Of course, it is difficult to well understand the mechanics and physical processes involved in this method through the above studies, further biophysical studies are needed to look at the contribution of free energy change between the membrane and the particle surface, the thickness of the hydration layer, LB fluidity, stability, diffusivity, etc.6969. E. Sackmann and M. Tanaka, Trends. Biotechnol. 18, 58 (2000). https://doi.org/10.1016/s0167-7799(99)01412-2Unfortunately, the mechanism study of the thin-film hydration method is so few. There were just several research studies simply investigating the influence factors of this method, which usually focused on the weight ratio of the MSN and the lipid (MSNs/lipid, w/w) (Table I).70,7170. X. Liu, M. Li, W. Yuan, Y. Liu, Y. Wang, and Y. Wang, Die Pharm. 73, 447 (2018). https://doi.org/10.1691/ph.2018.806571. Y. Qiu, C. Wu, J. Jiang, Y. Hao, Y. Zhao, J. Xu, T. Yu, and P. Ji, Mater. Sci. Eng., C 71, 835 (2017). https://doi.org/10.1016/j.msec.2016.10.081 Choi et al. optimized MSNs/lipid ratios to obtain suitable nanoparticles for drug delivery and found that 1:1 MSNs/lipid ratio (w/w) resulted in nanoparticles with ideal particle size and zeta potential.7272. J. Y. Choi et al., Acta Biomater. 39, 94 (2016). https://doi.org/10.1016/j.actbio.2016.05.012 Except for the particle size and zeta potential, the thickness of the lipid bilayer is a crucial factor that will affect the character of LMSNs. Theoretically, a lipid bilayer with ideal thickness should uniformly cover the surface of the MSNs and facilitate drug release. Thinner bilayer results in premature drug release, while a thicker bilayer hinders the drug release in vivo, both inducing dissatisfactory therapeutic effects. Lin et al. screened and optimized the bilayer thickness by modifying the MSNs/lipid ratios during the thin-film hydration procedure, providing that the 13–15 nm lipid bilayer obtained at a ratio of MSNs to lipid (w/w) = 1:1.11 showed ideal thickness to avoid the burst effect. It also found that the thickness of the lipid bilayer tended to decrease with increasing MSNs/lipid ratio. The TEM results demonstrated that an ununiform lipid coating with 5–7 nm bilayer thickness was obtained at 1:0.9 MSNs/lipid ratio (w/w), while 30 −33 nm bilayer thickness that might hinder drug loading rate was obtained at 1:1.3 MSNs/lipid ratio(w/w) (Fig. 4). Moreover, LMSNs prepared with this optimal factor setting demonstrated high drug encapsulation efficiency and well stability.7373. J. Lin et al., Int. J. Pharm. 536, 272 (2018). https://doi.org/10.1016/j.ijpharm.2017.10.043 Meanwhile, it was easy to find that the lipid bilayer obtained in this report was significantly thicker than the single lipid bilayer (4–7 nm), presuming multilayers were formed. Another study also obtained 30 nm lipid bilayer coating on the MSNs by the thin-film hydration method. Such phenomenon was different from the vesicle fusion method, in which additional multilayered SLB was not observed even excess SUVs were used.7171. Y. Qiu, C. Wu, J. Jiang, Y. Hao, Y. Zhao, J. Xu, T. Yu, and P. Ji, Mater. Sci. Eng., C 71, 835 (2017). https://doi.org/10.1016/j.msec.2016.10.081 This difference may originate from their different mechanisms. It should be noted that the influence factors of the thin-film hydration method are not limited to the MSNs/lipid ratio, though the procedure of this method is very simple. Other factors, like the characters of the lipids