Structural characterization of lipidic systems under nonequilibrium conditions

Rational formulation and industrial manufacturing of lipid-based complex injectables: Landmarks and trends

Lipid-based complex injectables are renowned for their effectiveness in delivering drugs, with many approved products. While significant strides have been made in formulating nanosystems for small molecular weight drugs, a pivotal breakthrough emerged with the recognition of lipid nanoparticles as a promising platform for delivering nucleic acids. This finding has paved the way for tackling long-standing challenges in molecular and delivery aspects (e.g., mRNA stability, intracellular delivery) that have impeded the clinical translation of gene therapy, especially in the realm of immunotherapy. Nonetheless, developing and implementing new lipid-based delivery systems pose significant challenges, as industrial manufacturing of these formulations often involves complex, multi-batch processes, giving rise to issues related to scalability, stability, sterility, and regulatory compliance. To overcome these obstacles, embracing the principles of quality-by-design (QbD) is imperative. Furthermore, adopting cutting-edge manufacturing and process analytical tools (PAT) that facilitate the transition from batch to continuous production is essential. Herein, the key milestones and insights derived from the development of currently approved lipid- nanosystems will be explored. Additionally, a comprehensive and critical overview of the latest technologies and regulatory guidelines that underpin the creation of more efficient, scalable, and flexible manufacturing processes for complex lipid-based nanoformulations will be provided.

A novel droplet-based approach to study phase transformations in lyotropic liquid crystalline systems

HYPOTHESIS

Lyotropic liquid crystals (LLC) and their phase transformations in response to stimuli have gathered much interest for controlled and 'on-demand' drug applications. Bulk methods of preparation impose limitations on studying the transformations, especially induced by compositional changes, such as enzymatic changes to lipid structure. Here we hypothesise that controlled microfluidic production and coalescence of dissimilar aqueous and lipid droplets emulsified in a third mutually immiscible liquid will provide a new approach to the spatio-temporal study of structure formation in lyotropic liquid crystalline materials.

EXPERIMENTS

Separate lipid and aqueous droplets, dispersed in a fluorocarbon oil were generated using a microfluidic format. The chip, prepared as a hybrid polydimethylsiloxane (PDMS) and glass microfluidic device, was constructed to enable in-situ acquisition of time-resolved synchrotron small angle X-ray scattering (SAXS) and crossed polarised light microscopy of the coalesced droplets to determine the structures present during aging.

FINDINGS

Janus-like droplets formed upon coalesce, with distinct lipid and aqueous portions with a gradient between the two sides of the merged droplet. SAXS and polarised light microscopy revealed a progression of mesophases as the lipid portion was hydrated by the aqueous portion via the diffusion limited interface which separated the portions. Thus demonstrating, on a droplet scale, a new approach for studying the phase transformation kinetics and identification of non-equilibrium phase in droplet-based lyotropic liquid systems.

Microfluidic Nanomaterial Synthesis and In Situ SAXS, WAXS, or SANS Characterization: Manipulation of Size Characteristics and Online Elucidation of Dynamic Structural Transitions

With the ability to cross biological barriers, encapsulate and efficiently deliver drugs and nucleic acid therapeutics, and protect the loaded cargos from degradation, different soft polymer and lipid nanoparticles (including liposomes, cubosomes, and hexosomes) have received considerable interest in the last three decades as versatile platforms for drug delivery applications and for the design of vaccines. Hard nanocrystals (including gold nanoparticles and quantum dots) are also attractive for use in various biomedical applications. Here, microfluidics provides unique opportunities for the continuous synthesis of these hard and soft nanomaterials with controllable shapes and sizes, and their in situ characterization through manipulation of the flow conditions and coupling to synchrotron small-angle X-ray (SAXS), wide-angle scattering (WAXS), or neutron (SANS) scattering techniques, respectively. Two-dimensional (2D) and three-dimensional (3D) microfluidic devices are attractive not only for the continuous production of monodispersed nanomaterials, but also for improving our understanding of the involved nucleation and growth mechanisms during the formation of hard nanocrystals under confined geometry conditions. They allow further gaining insight into the involved dynamic structural transitions, mechanisms, and kinetics during the generation of self-assembled nanostructures (including drug nanocarriers) at different reaction times (ranging from fractions of seconds to minutes). This review provides an overview of recently developed 2D and 3D microfluidic platforms for the continuous production of nanomaterials, and their simultaneous use in in situ characterization investigations through coupling to nanostructural characterization techniques (e.g., SAXS, WAXS, and SANS).

Investigation of diclofenac release and dynamic structural behavior of non-lamellar liquid crystal formulations during in situ formation by UV-Vis imaging and SAXS

In situ formation of high viscous inverse lyotropic non-lamellar liquid crystalline phases is a promising approach for sustained drug delivery in the joint. The in situ forming process on exposure of two diclofenac-loaded preformulations to aqueous media was characterized with respect to depot size and shape, initial release and structural transitions using UV-Vis imaging and spatially and time-resolved synchrotron small-angle X-ray scattering (SAXS). The preformulations consisted of 10 % (w/w) ethanol, 10 % (w/w) water and a binary lipid mixture of glycerol monooleate (GMO):1,2-dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) (DOPG) or GMO:medium chain triglycerides (MCT). Upon injection of preformulations into an employed injection-cell containing excess of bio-relevant medium, rapid generation of liquid crystalline depots was observed. UV-Vis images and constructed 2D SAXS maps of the injection-cell showed depots with different shapes and sizes, and features with high nanostructural heterogeneity. More extensive swelling of the GMO:DOPG-based preformulation was observed compared to the GMO:MCT-based preformulation. The UV image analysis found that a higher amount of diclofenac was released in the image area after 20 h from the GMO:MCT-depot compared to the GMO:DOPG-depot. The injection-cell setup employing UV-Vis imaging and synchrotron SAXS constitutes an attractive approach for evaluating the in situ forming processes of liquid crystalline depots.

Spatially and time-resolved SAXS for monitoring dynamic structural transitions during in situ generation of non-lamellar liquid crystalline phases in biologically relevant media

Formation of high viscous inverse lyotropic liquid crystalline phases in situ upon exposure of low viscous drug-loaded lipid preformulations to synovial fluid provides a promising approach for design of depot formulations for intra-articular drug delivery. Rational formulation design relies on a fundamental understanding of the synovial fluid-mediated dynamic structural transitions occurring at the administration site. At conditions mimicking the in vivo situation, we investigated in real-time such transitions at multiple positions by synchrotron small-angle X-ray scattering (SAXS) combined with an injection-cell. An injectable diclofenac-loaded quaternary preformulation consisting of 72/8/10/10% (w/w) glycerol monooleate/1,2-dioleoyl-glycero-3-phospho-rac-(1-glycerol)/ethanol/water was injected into hyaluronic acid solution or synovial fluid. A fast generation of a coherent drug depot of inverse bicontinuous Im3m and Pn3m cubic phases was observed. Through construction of 2D spatial maps from measurements performed 60 min after injection of the preformulation, it was possible to differentiate liquid crystalline rich- and excess hyaluronic acid solution- or synovial fluid-rich regimes. Synchrotron SAXS findings confirmed that the exposure of the preformulation to the media leads to alterations in structural features in position- and time-dependent manners. Effects of biologically relevant medium composition on the structural features, and implications for development of formulations with sustained drug release properties are highlighted.

Recent advances in drug delivery applications of cubosomes, hexosomes, and solid lipid nanoparticles

The use of lipid nanocarriers for drug delivery applications is an active research area, and a great interest has particularly been shown in the past two decades. Among different lipid nanocarriers, ISAsomes (Internally self-assembled somes or particles), including cubosomes and hexosomes, and solid lipid nanoparticles (SLNs) have unique structural features, making them attractive as nanocarriers for drug delivery. In this contribution, we focus exclusively on recent advances in formation and characterization of ISAsomes, mainly cubosomes and hexosomes, and their use as versatile nanocarriers for different drug delivery applications. Additionally, the advantages of SLNs and their application in oral and pulmonary drug delivery are discussed with focus on the biological fates of these lipid nanocarriers in vivo. Despite the demonstrated advantages in in vitro and in vivo evaluations including preclinical studies, further investigations on improved understanding of the interactions of these nanoparticles with biological fluids and tissues of the target sites is necessary for efficient designing of drug nanocarriers and exploring potential clinical applications.

Advances in microfluidic synthesis and coupling with synchrotron SAXS for continuous production and real-time structural characterization of nano-self-assemblies

Microfluidic platforms have become highly attractive tools for synthesis of nanoparticles, including lipid nano-self-assemblies, owing to unique features and at least three important aspects inherent to miniaturized micro-devices. Firstly, the fluids flow under controlled conditions in the microchannels, providing well-defined flow profiles and shorter diffusion lengths that play important roles in enhancing the continuous production of lipid and polymer nanoparticles with relatively narrow size distributions. Secondly, various geometries adapted to microfluidic device designs can be utilized for enhancing the colloidal stability of nanoparticles and improving their drug loading. Thirdly, microfluidic devices are usually compatible with in situ characterization methods for real-time monitoring of processes occurring inside the microchannels. This is unlike conventional nanoparticle synthesis methods, where a final solution or withdrawn aliquots are separately analysed. These features inherent to microfluidic devices provide a tool-set allowing not only precise nanoparticle size control, but also real-time analyses for process optimization. In this review, we focus on recent advances and developments in the use of microfluidic devices for synthesis of lipid nanoparticles. We present different designs based on hydrodynamic flow focusing, droplet-based methods and controlled microvortices, and discuss integration of microfluidic platforms with synchrotron small-angle X ray scattering (SAXS) for in situ structural characterization of lipid nano-self-assemblies under continuous flow conditions, along with major challenges and future directions in this research area.

In situ monitoring of the formation of lipidic non-lamellar liquid crystalline depot formulations in synovial fluid

Administration of parenteral liquid crystalline phases, forming in-vivo with tunable nanostructural features and sustained release properties, offers an attractive approach for treatment of infections and local drug delivery. It has also a potential use for postoperative pain management after arthroscopic knee surgery. However, the optimal use of this drug delivery principle requires an improved understanding of the involved dynamic structural transitions after administration of low-viscous stimulus-responsive lipid precursors and their fate after direct contact with the biological environment. These precursors (preformulations) are typically based on a single biologically relevant lipid (or a lipid combination) with non-lamellar liquid crystalline phase forming propensity. In relation to liquid crystalline depot design for intra-articular drug delivery, it was our interest in the present study to shed light on such dynamic structural transitions by combining synchrotron SAXS with a remote controlled addition of synovial fluid (or buffer containing 2% (w/v) albumin). This combination allowed for monitoring in real-time the hydration-triggered dynamic structural events on exposure of the lipid precursor (organic stock solution consisting of the binary lipid mixture of monoolein and castor oil) to excess synovial fluid (or excess buffer). The synchrotron SAXS findings indicate a fast generation of inverse bicontinuous cubic phases within few seconds. The effects of (i) the organic solvent N-methyl-2-pyrolidone (NMP), (ii) the lipid composition, and (iii) the albumin content on modulating the structures of the self-assembled lipid aggregates and the implications of the experimental findings in the design of liquid crystalline depots for intra-articular drug delivery are discussed.

Refining internal bilayer structure of bicelles resolved by extended-q small angle X-ray scattering

The internal profile across the bilayer reveals important structural information regarding the crystallinity of acyl chains or the positions of encapsulated species. Here, we demonstrate that a simple five-layer-core-shell discoidal model can be employed to best fit the extended-q small angle X-ray scattering (SAXS) data and resolve the bilayer internal structure (with sub-nanometer resolution) of a nanoscale discoidal system comprised of a mixture of long- and short- chain lipids (known as "bicelles"). In contrast to the traditional core-shell discoidal model, the detailed structure in the hydrophobic core such as the methylene and methyl groups can be distinguished via this model. The refined model is validated by the SAXS data of bicelles whose electron scattering length density of the hydrophobic core is adjusted by the addition of a long-chain lipid with a fluorine-end group. The higher resolution of the bilayer internal structure can be employed to advance our understanding of the interaction and conformation of the membrane and associated molecules, such as membrane-associated proteins and locations of entrapped species in the lipid nanoparticles.

Synchrotron Scattering Methods for Nanomaterials and Soft Matter Research

This article aims to provide an overview of broad range of applications of synchrotron scattering methods in the investigation of nanoscale materials. These scattering techniques allow the elucidation of the structure and dynamics of nanomaterials from sub-nm to micron size scales and down to sub-millisecond time ranges both in bulk and at interfaces. A major advantage of scattering methods is that they provide the ensemble averaged information under in situ and operando conditions. As a result, they are complementary to various imaging techniques which reveal more local information. Scattering methods are particularly suitable for probing buried structures that are difficult to image. Although, many qualitative features can be directly extracted from scattering data, derivation of detailed structural and dynamical information requires quantitative modeling. The fourth-generation synchrotron sources open new possibilities for investigating these complex systems by exploiting the enhanced brightness and coherence properties of X-rays.

Direct monitoring of lipid transfer on exposure of citrem nanoparticles to an ethanol solution containing soybean phospholipids by combining synchrotron SAXS with microfluidics

Lipid exchange among citrem nanoparticles and an ethanol micellar solution containing soy phosphatidylcholine was investigated in situ by coupling small angle X-ray scattering with a microfluidic device. The produced soy phosphatidylcholine/citrem nanoparticles have great potential in the development of hemocompatible nanocarriers for drug delivery.

A sub-ms pressure jump setup for time-resolved X-ray scattering

We present a new experimental setup for time-resolved solution small-angle X-ray scattering (SAXS) studies of kinetic processes induced by sub-ms hydrostatic pressure jumps. It is based on a high-force piezo-stack actuator, with which the volume of the sample can be dynamically compressed. The presented setup has been designed and optimized for SAXS experiments with absolute pressures of up to 1000 bars, using transparent diamond windows and an easy-to-change sample capillary. The pressure in the cell can be changed in less than 1 ms, which is about an order of magnitude faster jump than previously obtained by dynamic pressure setups for SAXS. An additional temperature control offers the possibility for automated mapping of p-T phase diagrams. Here we present the technical specifications and first experimental data taken together with a preview of new research opportunities enabled by this setup.

Direct monitoring of calcium-triggered phase transitions in cubosomes using small-angle X-ray scattering combined with microfluidics

This article introduces a simple microfluidic device that can be combined with synchrotron small-angle X-ray scattering (SAXS) for monitoring dynamic structural transitions. The microfluidic device is a thiol–ene-based system equipped with 125 µm-thick polystyrene windows, which are suitable for X-ray experiments. The device was prepared by soft lithography using elastomeric molds followed by a simple UV-initiated curing step to polymerize the chip material and simultaneously seal the device with the polystyrene windows. The microfluidic device was successfully used to explore the dynamics of the structural transitions of phytantriol/dioleoylphosphatidylglycerol-based cubosomes on exposure to a buffer containing calcium ions. The resulting SAXS data were resolved in the time frame between 0.5 and 5.5 s, and a calcium-triggered structural transition from an internal inverted-type cubic phase of symmetryIm3mto an internal inverted-type cubic phase of symmetryPn3mwas detected. The combination of microfluidics with X-ray techniques opens the door to the investigation of early dynamic structural transitions, which is not possible with conventional techniques such as glass flow cells. The combination of microfluidics with X-ray techniques can be used for investigating protein unfolding, for monitoring the formation of nanoparticles in real time, and for other biomedical and pharmaceutical investigations.

Small Angle X-ray and Neutron Scattering: Powerful Tools for Studying the Structure of Drug-Loaded Liposomes

Nanovectors, such as liposomes, micelles and lipid nanoparticles, are recognized as efficient platforms for delivering therapeutic agents, especially those with low solubility in water. Besides being safe and non-toxic, drug carriers with improved performance should meet the requirements of (i) appropriate size and shape and (ii) cargo upload/release with unmodified properties. Structural issues are of primary importance to control the mechanism of action of loaded vectors. Overall properties, such as mean diameter and surface charge, can be obtained using bench instruments (Dynamic Light Scattering and Zeta potential). However, techniques with higher space and time resolution are needed for in-depth structural characterization. Small-angle X-ray (SAXS) and neutron (SANS) scattering techniques provide information at the nanoscale and have therefore been largely used to investigate nanovectors loaded with drugs or other biologically relevant molecules. Here we revise recent applications of these complementary scattering techniques in the field of drug delivery in pharmaceutics and medicine with a focus to liposomal carriers. In particular, we highlight those aspects that can be more commonly accessed by the interested users.

Cubosomes and hexosomes as versatile platforms for drug delivery

Nonlamellar liquid crystalline phases are attractive platforms for drug solubilization and targeted delivery. The attractiveness of this formulation principle is linked to the nanostructural versatility, compatiblity, digestiblity and bioadhesive properties of their lipid constituents, and the capability of solubilizing and sustaining the release of amphiphilic, hydrophobic and hydrophilic drugs. Nonlamellar liquid crystalline phases offer two distinct promising strategies in the development of drug delivery systems. These comprise formation of ISAsomes (internally self-assembled ‘somes’ or particles) such as cubosomes and hexosomes, and in situ formation of parenteral dosage forms with tunable nanostructures at the site of administration. This review outlines the unique features of cubosomes and hexosomes and their potential utilization as promising platforms for drug delivery.

Design of Light-Triggered Lyotropic Liquid Crystal Mesophases and Their Application as Molecular Switches in "On Demand" Release

Here, we present the design and assembly of a new light-responsive functional lyotropic liquid crystal system using host-guest lipidic mesophases (LMPs). Light as an external stimulus has many advantages in comparison to other stimuli: it is milder than acids or bases, and variation of intensity and duration can provide a high level of pharmacological control. The LMPs are composed of monoolein (MO) and oleic acid (OA) as host lipids and a small amount of a judiciously synthesized lipid bearing an azobenzene photoactive unit as a guest. While preserving the structure and stability of the host lipidic aggregates, the guest lipids render them specific functionalities. Single-step and sequential light-triggered release and retention of the embedded dye molecules are demonstrated, thereby achieving exquisite temporal, spatial, and dosage control of the release, opening up the possibility of using such lipidic biomaterials as effective matrices in therapy, when a continuous release of active drugs might be toxic.

Stability of the Metastable α-Polymorph in Solid Triglyceride Drug-Carrier Nanoparticles

Colloidal dispersions of crystalline nonpolar lipids are under intensive investigation as carrier systems in pharmaceutics and nutrition. In this context, the controlled preparation of particles in a metastable polymorphic state is of some interest for the delivery of active substances. In the present study, tristearin particles stabilized with three α-polymorph-preserving emulsifier regimes ((I) sodium glycocholate/saturated long-chain phospholipids, (II) sodium glycocholate, and (III) poly(vinyl alcohol) (PVA)) were investigated concerning the stability of the metastable α-polymorph after controlled crystallization of the particles from the melt. Upon long-term storage, the α-polymorph was preserved best in PVA-stabilized dispersions, followed by those stabilized with the glycocholate/phospholipid mixture and finally those stabilized solely with the bile salt. In particular for rapidly crystallized nanoparticles, the formation of an α-polymorph with highly reduced lamellarity was observed. According to time-/temperature-resolved synchrotron X-ray diffraction analysis with simultaneous DSC (differential scanning calorimetry) studies, this less-ordered α-polymorph transformed into the common, lamellar α-form upon heating. Although the presence of the less-ordered form is probably related to the extraordinarily high stability of the metastable α-polymorph observed in some of the dispersions, it could not completely prevent the transition into the stable β-polymorph. The higher the transition temperature of the less-ordered α-form to the ordered one, the slower was the polymorphic transition to the stable β-polymorph. To estimate the polymorphic stability of the differently stabilized particles upon isothermal long-term storage, standard DSC measurements on samples stored at 23 °C for 4 weeks seem to be of predictive value.

Multicompartment lipid cubic nanoparticles with high protein upload: millisecond dynamics of formation

Membrane shapes, produced by dynamically assembled lipid/protein architectures, are crucial for both physiological functions and the design of therapeutic nanotechnologies. Here we investigate the dynamics of lipid membrane-neurotrophic BDNF protein complexes formation and ordering in nanoparticles, with the purpose of innovation in nanostructure-based neuroprotection and biomimetic nanoarchitectonics. The kinetic pathway of membrane states associated with rapidly occurring nonequilibrium self-assembled lipid/protein nanoarchitectures was determined by millisecond time-resolved small-angle X-ray scattering (SAXS) at high resolution. The neurotrophin binding and millisecond trafficking along the flexible membranes induced an unusual overlay of channel-network architectures including two coexisting cubic lattices epitaxially connected to lamellar membrane stacks. These time-resolved membrane processes, involving intercalation of discrete stiff proteins in continuous soft membranes, evidence stepwise curvature control mechanisms. The obtained three-phase liquid-crystalline nanoparticles of neurotrophic composition put forward important advancements in multicompartment soft-matter nanostructure design.

Influence of nanoparticle-membrane electrostatic interactions on membrane fluidity and bending elasticity

The aim of this work is to investigate the effect of electrostatic interactions between the nanoparticles and the membrane lipids on altering the physical properties of the liposomal membrane such as fluidity and bending elasticity. For this purpose, we have used nanoparticles and lipids with different surface charges. Positively charged iron oxide (γ-Fe2O3) nanoparticles, neutral and negatively charged cobalt ferrite (CoFe2O4) nanoparticles were encapsulated in neutral lipid 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine and negatively charged 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine lipid mixture. Membrane fluidity was assessed through the anisotropy measurements using the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene. Though the interaction of both the types of nanoparticles reduced the membrane fluidity, the results were more pronounced in the negatively charged liposomes encapsulated with positively charged iron oxide nanoparticles due to strong electrostatic attractions. X-ray photoelectron spectroscopy results also confirmed the presence of significant quantity of positively charged iron oxide nanoparticles in negatively charged liposomes. Through thermally induced shape fluctuation measurements of the giant liposomes, a considerable reduction in the bending elasticity modulus was observed for cobalt ferrite nanoparticles. The experimental results were supported by the simulation studies using modified Langevin-Poisson-Boltzmann model.

Interaction between dipolar lipid headgroups and charged nanoparticles mediated by water dipoles and ions

In this work, a theoretical model describing the interaction between a positively or negatively charged nanoparticle and neutral zwitterionic lipid bilayers is presented. It is shown that in the close vicinity of the positively charged nanoparticle, the zwitterionic lipid headgroups are less extended in the direction perpendicular to the membrane surface, while in the vicinity of the negatively charged nanoparticle, the headgroups are more extended. This result coincides with the calculated increase in the osmotic pressure between the zwitterionic lipid surface and positively charged nanoparticle and the decrease of osmotic pressure between the zwitterionic lipid surface and the negatively charged nanoparticle. Our theoretical predictions agree well with the experimentally determined fluidity of a lipid bilayer membrane in contact with positively or negatively charged nanoparticles. The prospective significance of the present work is mainly to contribute to better understanding of the interactions of charged nanoparticles with a zwitterionic lipid bilayer, which may be important in the efficient design of the lipid/nanoparticle nanostructures (like liposomes with encapsulated nanoparticles), which have diverse biomedical applications, including targeted therapy (drug delivery) and imaging of cancer cells.

DNA/Fusogenic Lipid Nanocarrier Assembly: Millisecond Structural Dynamics

Structural changes occurring on a millisecond time scale during uptake of DNA by cationic lipid nanocarriers are monitored by time-resolved small-angle X-ray scattering (SAXS) coupled to a rapid-mixing stopped-flow technique. Nanoparticles (NPs) of nanochannel organization are formed by PEGylation, hydration, and dispersion of a lipid film of the fusogenic lipid monoolein in a mixture with positively charged (DOMA) and PEGylated (DOPE-PEG2000) amphiphiles and are characterized by the inner cubic structure of very large nanochannels favorable for DNA upload. Ultrafast structural dynamics of complexation and assembly of these cubosome particles with neurotrophic plasmid DNA (pDNA) is revealed thanks to the high brightness of the employed synchrotron X-ray beam. The rate constant of the pDNA/lipid NP complexation is estimated from dynamic roentgenograms recorded at 4 ms time resolution. pDNA upload into the vastly hydrated channels of the cubosome carriers leads to a fast nanoparticle-nanoparticle structural transition and lipoplex formation involving tightly packed pDNA.

Monovalent ions and water dipoles in contact with dipolar zwitterionic lipid headgroups-theory and MD simulations

The lipid bilayer is a basic building block of biological membranes and can be pictured as a barrier separating two compartments filled with electrolyte solution. Artificial planar lipid bilayers are therefore commonly used as model systems to study the physical and electrical properties of the cell membranes in contact with electrolyte solution. Among them the glycerol-based polar phospholipids which have dipolar, but electrically neutral head groups, are most frequently used in formation of artificial lipid bilayers. In this work the electrical properties of the lipid layer composed of zwitterionic lipids with non-zero dipole moments are studied theoretically. In the model, the zwitterionic lipid bilayer is assumed to be in contact with aqueous solution of monovalent salt ions. The orientational ordering of water, resulting in spatial variation of permittivity, is explicitly taken into account. It is shown that due to saturation effect in orientational ordering of water dipoles the relative permittivity in the zwitterionic headgroup region is decreased, while the corresponding electric potential becomes strongly negative. Some of the predictions of the presented mean-field theoretical consideration are critically evaluated using the results of molecular dynamics (MD) simulation.