Executive Summary

Liposomes — self-assembled phospholipid bilayer vesicles — are among the most clinically validated nanocarrier platforms, used to deliver small molecules, proteins, and, most recently and consequentially, mRNA in lipid nanoparticle (LNP) form. Aqueous liposomal suspensions are physically and chemically unstable over long-term storage: vesicles fuse, aggregate, leak their payload, and the phospholipids themselves can hydrolyze or oxidize. Freeze-drying (lyophilization) converts a liposomal suspension into a dry, more chemically stable cake that can, in principle, be stored and shipped at higher temperatures and reconstituted on demand — but the freezing, drying, and rehydration steps each impose distinct physical stresses capable of destroying the very membrane integrity the process is meant to preserve.

This white paper synthesizes several decades of liposome freeze-drying research, from the foundational mechanistic studies of Crowe, Crowe, and Carpenter establishing the water-replacement and vitrification hypotheses, through systematic cryoprotectant comparisons (Stark, Pabst & Prassl 2010; Franzé et al. 2018; Zhang, Ling & Li 2021), to the surge of recent work (2022–2026) applying these principles to lyophilized mRNA-LNP vaccines. The consistent finding is that sugars — above all trehalose, closely followed by sucrose and other disaccharides — remain the dominant and most effective class of liposome lyoprotectant, acting through a combination of direct hydrogen-bonding to phospholipid head groups (water replacement) and formation of a protective amorphous glass around the vesicles (vitrification). Freezing rate, cryoprotectant concentration, lipid composition (including PEGylation and ionizable-lipid chemistry), and residual moisture content are all significant, independently tunable process levers. The paper closes with a review of the most active current application of this science: lyophilized mRNA-LNP vaccines, where formulation and process optimization are being pursued specifically to escape the ultracold (−20 °C to −70 °C) supply chain that has constrained global distribution of these products.

Table of Contents

1. Introduction

2. Why Liposomes Are Vulnerable to Freezing and Drying

   2.1 Freeze-Concentration and Ice-Crystal Damage

   2.2 Dehydration and the Membrane Phase Transition

   2.3 Rehydration

3. Protective Mechanisms of Sugars

   3.1 The Water-Replacement Hypothesis

   3.2 The Vitrification Hypothesis

   3.3 Concentration-Dependent Roles

4. Cryoprotectant Selection: Evidence from the Literature

   4.1 Trehalose and Sucrose

   4.2 Other Sugars and Polyols

   4.3 Cyclodextrins and Combination Formulations

   4.4 Membrane-Bound (Surface-Linked) Cryoprotectants

   4.5 What Doesn't Work Well: Surfactants Alone

5. Process Parameters

   5.1 Freezing Method and Rate

   5.2 Cryoprotectant Concentration

   5.3 Lipid Composition and PEGylation

   5.4 Residual Moisture Content

6. Case Study: Lyophilization of mRNA-Lipid Nanoparticle Vaccines

   6.1 Why Dry mRNA-LNPs

   6.2 Formulation Findings

   6.3 Process Findings

   6.4 Remaining Challenges

7. Practical Guidelines for Liposome Lyophilization

8. Conclusions and Outlook

References

1. Introduction

Liposomes are spherical vesicles formed by one or more concentric phospholipid bilayers enclosing an aqueous core. Since their introduction as drug carriers in the 1970s, they have become one of the most clinically mature nanomedicine platforms, used to deliver chemotherapeutics, antifungals, vaccine antigens, and, since 2020, messenger RNA in the lipid nanoparticle (LNP) formulations that underlie the major COVID-19 vaccines. Their appeal lies in the same feature that makes them fragile: a self-assembled, non-covalent bilayer structure that can encapsulate hydrophilic cargo in the aqueous core and hydrophobic or amphiphilic cargo within the membrane itself, but that is held together only by weak, reversible intermolecular forces.

Aqueous liposomal suspensions are not shelf-stable. Over time, vesicles fuse and aggregate, encapsulated payload leaks out, and the phospholipids themselves undergo hydrolysis (releasing free fatty acids and lysophospholipids) and oxidation, particularly for unsaturated acyl chains. For mRNA-LNPs specifically, the aqueous environment also permits hydrolytic degradation of the RNA payload, which is the principal reason the current generation of licensed mRNA vaccines requires frozen storage and transport. Freeze-drying offers an attractive route to a more stable, ambient- or refrigerator-temperature product: removing the bulk water arrests hydrolytic chemistry and can, if done correctly, leave the liposome or LNP structurally intact in a redispersible dry cake.

The difficulty is that lyophilization is itself an aggressive process. Freezing concentrates solutes and mechanically stresses membranes via ice-crystal contact; drying strips away the hydration shell that keeps the phospholipid bilayer in its native, fluid state; and rehydration reintroduces water abruptly enough to destabilize a membrane that has just adapted to the dry state. Successfully freeze-drying liposomes therefore depends, as with cells and other biologically derived colloids, on selecting an effective cryoprotectant/lyoprotectant system and controlling the process parameters around it. This white paper reviews what is currently known about how to do that.

2. Why Liposomes Are Vulnerable to Freezing and Drying

2.1 Freeze-Concentration and Ice-Crystal Damage

As an aqueous liposomal suspension cools, ice nucleates and grows, excluding both dissolved solutes and the liposomes themselves from the advancing ice front. This freeze-concentration effect compresses the vesicles into an increasingly small pocket of unfrozen liquid, dramatically raising both solute concentration and the frequency of vesicle–vesicle collision. Direct mechanical contact with growing ice crystals can rupture bilayers outright, and comparative freezing studies have shown that formulations containing only a surfactant (with no sugar or polyol) leave liposomes “strongly damaged and fragmented by ice crystals,” while disaccharides and mannitol protect vesicle integrity through this same freezing step — underscoring that freezing alone, independent of the subsequent drying step, is a major and separable source of liposome damage.

2.2 Dehydration and the Membrane Phase Transition

Once frozen, water is removed by sublimation (primary drying) and desorption (secondary drying) under vacuum. As in cell membranes, water molecules ordinarily hydrogen-bond to the polar head groups of membrane phospholipids, keeping them spaced apart and maintaining the bilayer in a fluid, liquid-crystalline state with a phase-transition temperature (Tm) below ambient temperature. Removing this hydration shell allows the head groups to pack more closely, increases van der Waals interaction between adjacent acyl chains, and raises Tm — forcing the dry membrane into a rigid gel phase at room temperature. This transition is a principal driver of fusion, aggregation, and leakage of encapsulated contents during drying, exactly as it is for cell membranes undergoing the same dehydration stress.

2.3 Rehydration

Reintroducing water reverses the process: the phospholipid head groups regain their hydration shell and the membrane reverts from the gel-like to the liquid-crystalline phase. If this reversed transition happens while excess free water is already present and excipients are still dissolving, it is frequently accompanied by transient leakage of encapsulated payload — a well-documented failure mode for both small-molecule and macromolecular (protein, RNA) liposomal cargo. As with cells, the rate, temperature, and osmolality of the rehydration step therefore materially affect final product quality, and are not simply a passive final step of the process.

3. Protective Mechanisms of Sugars

Because sugars, and disaccharides in particular, are by far the most effective and most extensively characterized liposome lyoprotectants, two complementary (not mutually exclusive) mechanistic hypotheses — both originating from the foundational work of Crowe, Crowe, and Carpenter beginning in the 1980s — anchor the field's mechanistic understanding.

3.1 The Water-Replacement Hypothesis

First proposed by Crowe and colleagues, this hypothesis holds that disaccharides such as trehalose can hydrogen-bond directly to the polar head groups of membrane phospholipids, substituting for the hydrogen bonds normally provided by water. By maintaining head-group spacing and reducing van der Waals interaction among the acyl chains, sugars depress Tm in the dry state, keeping the bilayer in its native liquid-crystalline configuration through drying and rehydration rather than allowing the damaging gel-phase transition described in Section 2.2.

3.2 The Vitrification Hypothesis

The complementary vitrification model holds that as the sugar solution freeze-concentrates and then dries, it forms a rigid, high-viscosity amorphous glass in which the liposomes become physically trapped. This glassy matrix drastically restricts molecular mobility, mechanically inhibiting the vesicle–vesicle contact needed for fusion and aggregation, independent of any direct chemical interaction between sugar and phospholipid head group. Contemporary reviews generally treat water replacement and vitrification as operating together rather than as competing explanations: hydrogen bonding protects the bilayer at the molecular level, while the surrounding glass provides bulk mechanical protection against fusion and aggregation.

3.3 Concentration-Dependent Roles

Detailed dose–response studies on soybean-phosphatidylcholine large unilamellar vesicles have shown that the functional role of the sugar shifts with its concentration: at low concentrations, the dominant effect is head-group spacing that prevents fusion; at intermediate concentrations, membrane stabilization against leakage becomes the more prominent effect; and at higher concentrations, the bulk glassy sugar matrix becomes the main driver of reduced aggregation. This concentration-dependence explains why liposome lyophilization studies frequently report a threshold sugar-to-lipid ratio below which protection is markedly weaker — the mechanism operating below that threshold may simply be different (and weaker) than the mechanism operating above it.

Table 1. Damage mechanisms and corresponding sugar-protection mechanisms

Synthesized from Crowe & Crowe (1988, 1996), Koster et al. (1994), Sun, Leopold, Crowe & Crowe (1996), and subsequent reviews.

4. Cryoprotectant Selection: Evidence from the Literature

4.1 Trehalose and Sucrose

Trehalose is consistently identified as the single most effective liposome lyoprotectant across independent studies. In a classic comparison of glucose, sucrose, mannitol, and trehalose using differential scanning calorimetry, trehalose-containing liposomal suspensions showed the highest glass-transition temperature (Tg) of the four, and trehalose produced both the smallest change in vesicle size during freeze-drying and the highest retention of encapsulated payload (a model chemotherapeutic and a lipid-soluble vitamin), with less leakage than glucose-protected formulations. Sucrose performs nearly as well in many systems and, notably, is the cryoprotectant used in both major licensed mRNA-LNP COVID-19 vaccines (in their frozen-liquid, not lyophilized, formulations), reflecting decades of prior validation in biopharmaceutical freeze-drying more broadly.

4.2 Other Sugars and Polyols

Beyond the two dominant disaccharides, cellobiose and lactose have shown encouraging results maintaining liposome morpho-functional parameters through complete freeze-drying cycles in direct comparative studies, while glucose (a monosaccharide) and mannitol (a polyol) generally underperform the disaccharides. Mannitol illustrates a specific failure mode also seen in other freeze-drying contexts (see the companion white paper on cell preservation): it protects effectively during the freezing step alone but is prone to crystallizing out of the amorphous phase, and the resulting crystals can physically damage membranes, making mannitol a better cryoprotectant than lyoprotectant for liposomes. Oligosaccharides such as fructooligosaccharide and inulin have also shown protective activity, with inulin proposed to work partly by increasing phospholipid bilayer fluidity and micropolarity, a somewhat distinct mechanism from the classical disaccharide model.

4.3 Cyclodextrins and Combination Formulations

Cyclodextrins — cyclic oligosaccharides capable of forming inclusion complexes — have shown particular promise in combination formulations. Work on protein and, more recently, LNP lyophilization has found that combining a cyclodextrin with sucrose (CD/Suc), or a hydroxypropyl-cyclodextrin with PVP and sucrose (HPβCD/PVP/Suc), outperformed sucrose alone for extended storage stability, and separately that a CD/Suc formulation could shorten the lyophilization cycle time by roughly half relative to a sucrose-only formulation — a practically significant finding for manufacturing throughput, in addition to the stability benefit.

4.4 Membrane-Bound (Surface-Linked) Cryoprotectants

A distinctive strand of liposome-specific research, without a direct cellular analogue, has examined cryoprotectants covalently or physically anchored to the vesicle surface itself. Incorporating hexadecyl-β-D-galactopyranoside — a sugar head group tethered to a lipid anchor — directly into the bilayer of soybean-phosphatidylcholine liposomes showed that fixing the sugar head at the membrane surface specifically prevented vesicle fusion, even though the tethered galactoside alone (without free sugar in solution) did not provide general membrane stabilization. Combined with free carbohydrate in solution, the membrane-bound sugar improved protection in a hyperadditive (greater-than-additive) manner, attributed to hydrogen bonding between the surface-tethered and free sugar molecules — an early demonstration that surface engineering and bulk additive strategies can be combined synergistically, a theme echoed in the more recent hyaluronic-acid-conjugated cryoprotectant work discussed in Section 4.3 of the companion nanoparticle literature.

4.5 What Doesn't Work Well: Surfactants Alone

A recurring negative finding across the comparative literature is that non-ionic surfactants, used without an accompanying sugar or polyol, do not adequately protect liposomes through freezing: one systematic freezing-methodology comparison found that in the presence of a surfactant alone, liposomes were “strongly damaged and fragmented by the ice crystals,” in clear contrast to the disaccharides and mannitol tested in the same study, which successfully protected vesicle integrity. This mirrors the analogous finding for gold nanoparticles (see the companion white paper), where surfactant-only protection was likewise adequate for freeze–thaw but inadequate once the more demanding stresses of full lyophilization were introduced — reinforcing, across quite different colloidal systems, that interfacial/steric stabilization by a surfactant is a comparatively weak substitute for the hydrogen-bonding and vitrification mechanisms provided by sugars.

Table 2. Comparative performance of liposome cryoprotectants

Compiled from Zhang, Ling & Li (2003), Franzé et al. (2018), Trenkenschuh & Friess (2021), and related sources; see References.

5. Process Parameters

5.1 Freezing Method and Rate

As with cell freeze-drying, the freezing step is not a passive precursor to drying but an independently controllable and consequential variable. Comparative studies isolating the freezing step from the rest of the lyophilization cycle have evaluated both rapid quenching in liquid nitrogen and slower, shelf-ramped freezing, finding that the optimal choice depends on the specific lipid bilayer composition being frozen — there is no single freezing protocol that is universally optimal across liposome formulations, echoing the same conclusion reached for cell freeze-drying (see the companion white paper).

5.2 Cryoprotectant Concentration

Because the protective mechanism itself shifts with concentration (Section 3.3), cryoprotectant dose must be optimized empirically for each formulation rather than assumed from a single reference concentration. Studies on PLGA nanoparticles and liposomes together have found that only the highest tested percentages of cryoprotectant permitted resuspension of stable nanocarriers after freeze-drying, indicating that under-dosing the cryoprotectant is a common and consequential formulation error, particularly for more structurally demanding lipid nanocarrier systems.

5.3 Lipid Composition and PEGylation

Liposome susceptibility to freeze-drying damage depends strongly on bilayer composition. Comparative work on DOPC-DOTAP liposomes with and without a PEGylated lipid (DSPE-PEG) component, matched for the same phase-transition temperature, found that PEGylation measurably altered freeze-drying behavior relative to the non-PEGylated counterpart, consistent with PEG's steric bulk providing partial, composition-dependent protection independent of the added sugar cryoprotectant — a parallel to the PEG surface-passivation strategies described for gold and silica nanoparticles in the companion white paper. For mRNA-LNPs specifically, the identity of the ionizable lipid component has been shown to strongly affect post-lyophilization mRNA payload retention and bioactivity even when colloidal (particle-size) stability is comparable across different ionizable lipids — indicating that lipid chemistry, not just cryoprotectant choice, is an independent formulation variable that must be optimized for lyophilized lipid nanoparticle products.

5.4 Residual Moisture Content

As with cells and proteins, residual moisture content after drying is a critical, independently controlled variable for liposomal products. Recent mRNA-LNP lyophilization work has shown that formulations stored at moderately elevated temperature (20 °C, versus refrigerated 4 °C) degrade faster in part because residual moisture and increased molecular mobility within the lyophilized glassy matrix accelerate lipid oxidation and hydrolysis, and because the protective glassy matrix itself becomes less effective at higher temperature due to increased diffusion and possible excipient crystallization — directly linking drying endpoint (residual moisture) and storage temperature as jointly determining formulation shelf life.

6. Case Study: Lyophilization of mRNA-Lipid Nanoparticle Vaccines

6.1 Why Dry mRNA-LNPs

The nucleoside-modified mRNA-LNP vaccines that became central to the global COVID-19 response demonstrated the power of the platform but also exposed its principal logistical weakness: limited thermostability requiring storage and distribution at −20 °C to −70 °C, which severely constrains distribution in low-resource and warm-climate settings. Because encapsulated mRNA remains subject to hydrolytic degradation even inside the LNP, and because colloidal destabilization of the LNP itself is an additional failure mode, lyophilization has been pursued by multiple independent groups since roughly 2022 as a way to substantially improve thermostability while, ideally, using existing cold-chain infrastructure only for shorter-duration, less extreme-temperature storage or distribution.

6.2 Formulation Findings

Sucrose has emerged as the leading single-component cryoprotectant for mRNA-LNP lyophilization, mirroring its established role in classical liposome freeze-drying: a 20% w/v sucrose formulation enabled successful lyophilization of mRNA-LNPs (using a reduction-sensitive ionizable lipid) via both conventional batch freeze-drying and an innovative continuous spin-lyophilization process, with colloidal stability maintained after reconstitution regardless of the specific ionizable lipid's chemical structure. More recent formulation-screening work has expanded the excipient set under evaluation to include maltose, PEG-1500, PVP-K12, and lysine in addition to sucrose, and has found that formulations combining a cyclodextrin with sucrose, or a hydroxypropyl-cyclodextrin/PVP/sucrose mixture, can outperform sucrose alone for extended (multi-month) storage stability at elevated temperature. Separately, Lyophilized SARS-CoV-2 and monkeypox mRNA-LNP vaccine candidates using optimized cryoprotectant and buffer systems have demonstrated no measurable change in physicochemical properties or bioactivity after storage at 25 °C for six months, and have elicited robust humoral and cellular immune responses in mice, rabbits, and non-human primates — direct evidence that lyophilized mRNA-LNPs can, with the right formulation, match the performance of their frozen-liquid counterparts.

6.3 Process Findings

Beyond formulation, mRNA-LNP-specific process research has examined freezing method, temperature, cooling rate, and primary/secondary drying conditions as independently optimizable variables, achieving freeze-dried products with uniform appearance and residual moisture content below 1% in optimized protocols. A notable process innovation is continuous (as opposed to batch) spin lyophilization, which has been shown to enable successful drying while allowing storage at higher temperatures than conventional shelf freeze-drying achieves for comparable formulations, potentially offering manufacturing throughput advantages alongside the stability benefit. Lipid composition also interacts with the drying process itself: susceptibility of RNA-LNPs to thin-film freezing and drying has been shown to depend on the specific lipid composition of the formulation, indicating that process and formulation optimization cannot be fully decoupled for this class of product.

6.4 Remaining Challenges

Despite substantial progress since 2022, mRNA-LNP lyophilization is not yet a fully solved problem. Maintaining optimized freeze-dried mRNA-LNP formulations at intermediate temperatures (e.g., 20 °C) remains more difficult than at refrigerated temperature (4 °C): residual moisture and increased molecular mobility within the glassy matrix are understood to accelerate lipid oxidation and hydrolysis at the higher temperature, reducing encapsulation efficiency and promoting nanoparticle aggregation or fusion over time. Researchers in this space have generally been careful to note that these degradation mechanisms are inferred from observed stability trends and established literature precedent rather than always directly measured in a given study, indicating that the detailed molecular-level failure pathways for lyophilized mRNA-LNPs at intermediate storage temperatures remain an active area of investigation rather than a fully closed question.

7. Practical Guidelines for Liposome Lyophilization

Drawing on the mechanistic and formulation literature reviewed above, the following practical guidance can be offered for laboratories or manufacturers freeze-drying liposomal or lipid-nanoparticle products:

• Default to a disaccharide, and specifically trehalose where cost and formulation constraints allow, as the primary lyoprotectant; sucrose is a well-validated, lower-cost alternative with only modestly reduced performance in most systems.
• Do not rely on a surfactant alone. Surfactant-only formulations can fail during freezing itself, well before the drying step is reached; a sugar or polyol must be present to protect through the freeze-concentration stage.
• Treat cryoprotectant concentration as a variable requiring empirical optimization, not a fixed reference value — the protective mechanism itself shifts with concentration, and under-dosing is a common cause of poor redispersibility.
• Consider combination formulations (sugar + cyclodextrin, sugar + PVP, or a membrane-anchored sugar plus free sugar in solution) where single-component protection is marginal or where cycle-time reduction is a priority.
• Optimize freezing rate and method for the specific lipid composition in use; do not assume a freezing protocol validated for one liposome formulation transfers directly to a different bilayer composition or a lipid nanoparticle system.
• For LNP/mRNA systems specifically, treat the ionizable lipid identity as an independent formulation variable — it can materially affect payload retention and bioactivity after lyophilization even when particle size and colloidal stability appear unaffected.
• Target low residual moisture and account for storage temperature jointly with drying endpoint; a formulation validated for refrigerated storage should not be assumed stable at room temperature without separate validation, since matrix mobility and excipient crystallization both become more significant at higher temperature.
• Characterize the reconstituted product directly — vesicle size, polydispersity, zeta potential, encapsulated-payload retention, and (for RNA cargo) payload integrity — rather than inferring performance from the pre-freezing suspension or from cake appearance alone.

8. Conclusions and Outlook

Freeze-drying liposomal particles is a well-studied, mechanistically well-understood problem with a strong track record of practical success, built on foundational work establishing the water-replacement and vitrification hypotheses and refined through decades of comparative cryoprotectant and process studies. Disaccharides — trehalose foremost, sucrose close behind — remain the dominant and most reliable class of lyoprotectant, acting through a combination of direct hydrogen bonding to phospholipid head groups and formation of a protective glassy matrix, with their relative contribution shifting according to concentration. Polyols such as mannitol and simple surfactants are comparatively weak substitutes, particularly through the drying step itself, and combination formulations (sugar plus cyclodextrin or polymer, or membrane-anchored sugar plus free sugar) frequently outperform any single component.

The field's center of gravity has shifted markedly since 2022 toward lyophilized mRNA-lipid nanoparticle vaccines, where the same underlying sugar-based lyoprotection principles are being extended, refined, and industrially scaled to solve a problem of substantial public-health consequence: eliminating the ultracold cold chain that has constrained global vaccine distribution. Early results — lyophilized mRNA-LNP vaccines stable for six months at 25 °C with preserved immunogenicity in multiple animal models — are genuinely encouraging, though intermediate-temperature (room-temperature-adjacent) stability remains harder to achieve than refrigerated stability, and the detailed degradation chemistry at these temperatures is still being worked out. Continued progress is likely to come from combination cryoprotectant formulations, tighter control of residual moisture and glass-transition temperature relative to intended storage conditions, and continued attention to how ionizable-lipid and PEG-lipid chemistry interacts with the drying process — extending, rather than replacing, the classical liposome lyoprotection framework established over the preceding four decades.

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