Introduction

Nanotechnology provides advanced tools and economic value to health-care systems1, where multiple products based on nanoparticles (NPs) as drug carriers have been approved for the treatment, prevention and diagnosis of diseases2. The most recent example comprises the COVID-19 vaccines from Pfizer/BioNTech, Moderna and Novavax.

NPs approved for medical use are mostly isotropic, with no variations in geometry or composition noted when observed from different angles. By contrast, anisotropy implies a directionality dependency3 (Box 1). In nature, anisotropy is dominant and assumed to result from adaptation to surrounding environmental cues. Anisotropy often provides advanced functionality; for instance, various bacteria, such as bacilli or spirochete, feature non-spherical shapes and/or specifically oriented surface structures, such as flagella or pili, that improve mobility in hard-to-tackle media4,5. The most recognizable anisotropic cells are erythrocytes, which have a biconcave disc shape that is linked to optimal flow in blood vessels6. Anisotropy also plays a role at the intracellular level to enhance transport. For example, biomolecular motors, such as myosin, dynein and kinesin, are responsible for the spatial organization of cellular organelles and vesicles, contributing to their directional and rapid displacement, particularly if clustered locally rather than randomly distributed7,8. Additionally, some viruses self-assemble into ordered prolate capsids that provide improved cargo protection and reduced free surface energy9. The highly pathogenic Ebola and Marburg viruses also rely heavily on their filamentous morphology to remain infective by featuring oriented cell entry and budding10,11.

The peculiar behaviour of anisotropic structures has inspired the development of highly functional materials. Anisotropic constructs hold unique characteristics in terms of shape, charge, hydrophilicity, and optical, electrical or magnetic properties, allowing applications involving micromotors12,13 and nanomotors14, tissue targeting15, responsive sensors16, stabilizers17 and other material building blocks18. Most anisotropic particles have dimensions in the micrometre range, facilitating the large-scale production of systems with improved control over shape and composition. However, such relatively large systems often do not meet size requirements for suitable biomedical applications, particularly for drug delivery, imaging, cell tagging and labelling, yet downsizing to the nanoscale poses substantial challenges in creating spatially distinct functionality, producing consistent size and shape, and yielding adequate response to stimuli19.

Nonetheless, interest in anisotropic nanomaterials for medical use is rising. For example, the non-spherical geometry of NPs can improve biodistribution and tune cell interactions20. Moreover, compositional anisotropy in spherical NPs can offer advantages over isotropic systems, including enabling tunable biodegradation and drug release21,22 or allowing specific bioactivity, such as the spatial presentation of ligands on pathogens owing to partial surface functionalization with targeting moieties23,24. Control over transport directionality of remotely guided or active colloids is also explored in this field25. Altogether, applications of anisotropic nanosystems include vaccine design26, production of platforms for cancer and photodynamic drug therapies27,28, and fighting antimicrobial resistance29.

Herein, we provide a critical overview of how anisotropic NPs can contribute to the design of advanced diagnostics and therapeutics and how these cues impact interactions with biological structures. We start by addressing the most relevant manufacturing techniques of anisotropic NPs (having at least a single dimension in the nanoscale range)30, which helps to better understand anisotropy and guides the scaling up of production processes. We then focus on how anisotropic NPs can tackle biological hurdles to perform their intended functions.

Production methods of anisotropic systems

Anisotropic NPs can be generated by various techniques that offer control over shape, composition or both (Fig. 1).

Fig. 1: Techniques commonly used for the fabrication of different types of anisotropic NPs for biomedical applications.
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To produce shape anisotropy: a, Seed-mediated method; for example, gold stars35 and rods210 with improved optical properties are typically produced by seed-mediated methods. b, Lithography; for example, poly(lactic-co-glycolic acid) (PLGA) nanocylinders can be generated by nanoimprint lithography40. c, Self-assembly; for example, worms of controlled size can result from self-assembled spherical nanoparticles (NPs) made of thymine-containing polymers by adding adenine-containing polymer at specific adenine-to-thymine molar ratios48, and bowl-shaped stomatocytes result from shape transformation of self-assembled polymersomes, induced by osmotic shock109. d, Film stretching; for example, spherical polystyrene NPs can be elongated into discs by film-stretching methods59. To produce composition anisotropy: e, Template-assisted 2D method; for example, Janus-like mesoporous silica and iron oxide NPs can be partially coated with gold by depositing them in glass slides followed by toposelective sputtering60. f, Template-assisted 3D method; for example, Pickering emulsions are used to hide part of silica NPs74 for asymmetric surface functionalization generating Janus particles. g, Electric field application; for example, compartmental PLGA NPs80 featuring two distinct domains can be attained using electrohydrodynamic co-jetting. Part b adapted with permission from ref. 40, ACS. Part c (worm) adapted from ref. 48, Springer Nature Limited. Part g adapted with permission from ref. 80, Wiley.

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Modulation of shape

Various shapes can be envisioned at the nanoscale. Non-spherical NPs can be obtained by bottom-up approaches such as the seeded growth method, where small seed particles act as nucleation sites that accumulate precursor molecules from a growth solution, growing into larger particles of controlled shape and size3. The technique can produce metallic stars and rods (Fig. 1a) with tunable surface plasmon resonance in the near-infrared (NIR) region. Owing to the ability of infrared light to penetrate tissue, the particles can be applied for cancer cell imaging31,32,33 and combinational photodynamic and photothermal therapy34,35.

Lithography-based techniques can generate NPs of precise shape and controlled dimensions by depositing a liquid precursor into a well-defined patterned mask, without the need for extreme temperatures and etching procedures. A common example is particle replication in non-wetting templates (PRINT), which enables scalable production of monodisperse trapezoidal and conical NPs of less than 100 nm in diameter. The precursor (for example, poly(ethylene glycol diacrylate), tri-acrylate resin, poly(lactic acid) or poly(pyrrole)) is cast onto a substrate and brought into contact with a patterned mould, generating NPs of the precursor material by mild heating, UV light crosslinking or solvent evaporation. Photocurable non-wetting and non-swelling moulds made of perfluoropolyether reduce the affinity to the liquid precursor, resulting in individualized particles36. PRINT is employed for the scalable production of polymeric NPs carrying proteins, such as avidin36 or small interfering RNA37, without affecting their bioactivity. For example, influenza vaccines based on poly(lactic-co-glycolic acid) (PLGA) and cholesterol cylindrical NPs (80 × 320 nm) have been developed by Liquidia Technologies38 using PRINT and tested in a phase I–II clinical trial (NCT01224262). Another interesting lithography technique is flash nanoimprinting, in which a release layer included in the substrate is dissolved at the end of the procedure, thus facilitating NP recovery from the moulds by avoiding the need for scraping. This technique is applied for the fabrication of stimuli-responsive, easily harvestable carriers of antibodies, nucleic acids39 and anticancer drugs40 (Fig. 1b). In combination with lithography techniques, template-assisted electrodeposition can be used to prepare elongated NPs or nanowires made of metals or semiconductor materials. In this technique, an electrolyte solution is reduced onto a conductive template substrate, growing according to its nanopattern. Finally, the template is dissolved or chemically etched41,42.

Anisotropic nanoconstructs, such as cylinders, worms, lamellae and rods, can also be generated spontaneously from amphiphilic block copolymers by self-assembly as promoted by energetic repulsion amongst different segments of copolymers, which leads to partitioning43,44. Autonomous spatial organization and the resulting shape depend on the monomers and the lengths of the polymer blocks. Other factors, such as temperature, pH, ionic strength or solvent, also play a role in the final features of constructs45,46. For instance, multicompartment micelles of ~100 nm in diameter can be prepared by hierarchical co-assembly of polycaprolactone-b-poly(ethylene glycol) (PCL-b-PEG) and PCL-b-poly(4-vinylpyridine). Shell micelles are first spontaneously formed owing to hydrophobic interactions among PCL chains, followed by pH adjustment to form phase-segregated patches. The multicompartment architecture of the obtained systems seems suitable for delivering incompatible compounds simultaneously47. Another example comprises the growth of anisotropic worms from spherical NPs made of amphiphilic block copolymers containing thymine by establishing supramolecular bonds with a second copolymer containing adenine at specific molar ratios (adenine-to-thymine ratio of between 0.2 and 1). The structure can be modified by incorporating bioactive or functional ligands for active drug delivery and targeting48 (Fig. 1c).

Alternative techniques, such as polymerization-induced self-assembly, are employed to generate drug-loaded NPs49. In this case, the polymeric block grows by adding monomers, becoming insoluble and leading to in situ assembly. For example, worm-like and rod-like micelles are prepared from poly(oligo(ethylene glycol)methacrylate)-b-(poly(styrene)-co-poly(vinyl benzaldehyde)) copolymers and further conjugated with doxorubicin50. Crystallization-driven self-assembly is also employed to prepare NPs of various morphologies (for example, diamond-like, platelet-like or cylindrical-shaped NPs) from poly(l-lactide)-based block copolymers, showing potential as platforms for drug delivery and immunotherapy owing to their shape-dependent macrophage activation51,52. Moreover, self-assembled anisotropic nanostructures can be made of shape memory polymers that can alter their conformation in response to specific stimuli. For example, jellyfish-like amphipathic NPs with diameters as small as 80 nm are prepared using a sucrose-PCL copolymer. The hydrophilic hydroxyl groups of sucrose protect the hydrophobic PCL tentacles in the centre, which recoil when abruptly exposed to negative pressures53. This strategy can be further adapted to generate NPs with shape memory that feature propulsive transport owing to dynamic shape-shifting upon external stimuli54.

Erythrocyte-shaped NPs (or stomatocytes) have been devised for active transport by entrapping metallic or biological catalysts that convert substrates into propulsive gases. Stomatocytes result from shape transformation of self-assembled spherical polymeric polymersomes that are susceptible to osmotic shock promoted by dialysis or electrolyte addition55 (Fig. 1c). These anisotropic particles have a bowl-like shape with a neck-like structure that shields the cargo against the surrounding environment55. Processing pre-prepared polymersomes through extrusion in a PEG solution triggers instantaneous osmotic shock and reduces the diameter to values below 200 nm, thus broadening their applicability56.

Film stretching is another approach to manipulate NP shape. In this technique, spherical polymeric particles are embedded in a continuous polymeric film and subjected to mechanical stress above their glass transition temperature or melting point such that they acquire the shape of the surrounding pocket holes after cooling57 (Fig. 1d). For example, various complex-shaped systems, such as prolate and oblate ellipsoidal NPs (inspired by erythrocytes), can be prepared by embedding spherical PLGA NPs into polyvinyl alcohol–glycerol films, followed by controlled stretching above the polyester melting point and cooling58. Despite its simplicity, one of the main drawbacks of the film-stretching technique concerns the complete removal of the materials used as embedding matrices, which might influence particle stability and size59.

Composition anisotropy

Focal changes to the surface composition of NPs can be achieved by topographically selective (toposelective) modification methods by which pre-prepared NPs are immobilized in 2D or 3D substrates for asymmetric functionalization. Such techniques rely on partial adhesion or embedment of NPs onto a template, allowing chemical modification of only the remaining exposed surface area. Deposition of NPs into glass or silicon slides followed by sputtering or electron-beam evaporation with gold is a common example60,61 (Fig. 1e). Other immobilization options could encompass particle embedment in poly-l-lysine–hyaluronic acid films for adsorption and/or functionalization of unmasked surfaces62,63, or attachment to UV-sensitive silicon nanosheets through hydrosilylation as employed for anisotropic functionalization of quantum dots64. Particles can then be released by ultrasonication, controlled desorption through sodium hydroxide exposure, or UV-induced template destruction.

Alternatively, polymeric microspheres can be used as 3D templates. For example, silica NPs down to 100 nm in diameter are partially labelled with gold NPs using polystyrene microsphere templates treated with supercritical carbon dioxide65. The glass transition temperature of polystyrene decreases upon exposure to carbon dioxide, hence allowing the control of NP embedment in the microsphere template by adjusting temperature and pressure. Subsequently, the partially exposed surface of the silica particles is modified with gold NPs. The same principle is employed to prepare strawberry-like structures, where iron oxide NPs, polystyrene and poly(acrylic acid) are used as coatings of silica NPs during functionalization66. Despite the popularity of polystyrene, polycarbonate microparticles are also described as templates67.

An alternative to polystyrene or polycarbonate microsphere templates is a fluid wax-in-water Pickering emulsion, a type of emulsion in which NPs act as stabilizers and accumulate at the oil–water interface to form colloidosomes. Paraffin is often chosen as the oil phase, although hexadecane68, dodecane69 and octanol17 can also be utilized. Stabilization of the dual phase system depends on the interfacial accumulation of NPs, and hardening of the inner oil phase can be further promoted by cooling, thus immobilizing the NPs70. The side of the NPs facing the aqueous phase of the emulsion can be functionalized, and anisotropic NPs can be collected by wax melting or solvent extraction, followed by filtration or centrifugation. In the case of magnetic NPs, the separation of the particles can be aided using a magnet. Silica is the most popular bulk material for NPs produced by such a technique but others of interest for drug delivery, such as zein71, PLGA72 and iron oxide73, are also considered. Pickering emulsion-based methods can produce large amounts of anisotropic NPs70. The technique can be applied to the production of pH-responsive NPs73 or systems with improved cell attachment and/or uptake74 (Fig. 1f).

Variations in composition can further extend to the bulk of NPs, particularly for compartmental systems. Electrospraying is used to produce dual compartmental spherical microparticles by applying a strong electric field to jetting liquids, leading to breakage of the stream into droplets75. These can be used to incorporate different molecules or supramolecular constructs of interest in opposing hemispheres of a polymeric core76,77,78. The production of such systems at the nanoscale, although challenging, is possible. Fine-tuning of the viscosity, conductivity and surface tension of jetting liquids as well as of the applied voltage and flow rates of electrospraying systems can result in particles of diameters as low as 100–200 nm without compromising the formation of individual compartments79. For example, the technique is applied to produce pH-responsive bicompartmental PLGA–acetal-modified dextran NPs (diameter of ~240 nm) for the co-delivery of anticancer drugs such as lapatinib and paclitaxel80 (Fig. 1g).

Combination systems

Systems of anisotropic shape and composition can be produced using a combination of different techniques. For example, dumbbell NPs are attained by selectively depositing cerium oxide or palladium at the tips of pre-prepared metallic nanorods, which spatially separates the structures involved in plasmonic photocatalysis and leads to a red-shift in the longitudinal localized surface plasmon resonance81,82. These systems have been proposed for photothermal therapies in the NIR region for Alzheimer disease and NIR photoacoustic imaging, respectively81,82. Similarly, Janus bimetallic nanorods made of gold and platinum have been prepared for deep tissue imaging exploiting the photoacoustic imaging capacity in the second NIR window, and chemotherapy induced by the cytotoxic effects of oxidized platinum (II) ions83. Hybrid metal–polymer NPs with a snowman-like configuration have also been produced, for example, by loading antibacterial silver cations into polymeric micelles followed by UV light exposure84, or by mixing platinum NPs with poly(divinylbenzene) to act as carriers of miconazole nitrate85, with both systems designed as antimicrobial therapies.

Seeded-emulsion polymerization techniques have also been used to prepare anisotropic NPs with distinctive compositions, for example, dumbbell-like and patchy particles for dual drug delivery86,87, and doxorubicin-loaded core-shell, snowman-like or dumbbell-like NPs (down to 400 nm) from 2-(dimethylamino)ethyl methacrylate and poly(2-hydroxyethyl methacrylate) seeds88. The technique is a two-step process in which seed particles are initially synthesized before swelling by a reactive monomer, resulting in the formation of bulges that are further polymerized89. Phase separation occurs owing to the incompatibility between seed particles and monomers and results in the formation of anisotropic NPs. The final shape is determined by interfacial tension forces so that the total free energy is minimized90. Another possible approach involves microfluidics fabrication. Despite being relatively inefficient in generating anisotropic particles at the nanoscale91,92, some successful attempts to produce nanosystems have been achieved93,94. Microfluidics may be specifically appealing when using polymers of distinct degradation rates and hydrophilicity for time-programmed dual drug delivery applications or to improve cargo release95,96. For example, ellipsoidal doxorubicin-loaded mesoporous silica NPs (average long axis of ~150 nm) with ordered parallel tunnels along their shorter axis have been prepared on spiral-shaped channels of a microfluidic reactor97.

Improving transport in fluids

The behaviour of NPs in biological fluids governs their ability to distribute and reach regions of interest, either at the site of administration or systemically. Transport of NPs in Newtonian biological fluids, such as urine or cerebrospinal fluid, depends largely on the viscosity of the medium rather than on the negligible inertial forces stemming from particulates. However, most biological fluids have non-Newtonian properties, such as blood, mucus and saliva, having rheological and mechanical features where time-reversible particle shape changes can generate propulsion98. As particle size decreases, environmental thermal fluctuations introduce variations to their movement, resulting in randomization of propulsion and uncertainty in transport characterization99. This can be partially addressed by introducing chemical modifications in particles (such as adding propulsive moieties to their composition) or media (for example, by generating gradients) to improve particle transport.

Particle engineering for transport modulation has led to the concept of (nano)motors, which can be defined as entities capable of transforming energy into motion99. A spherical particle experiences translational and rotational diffusion in liquid media, whose coefficients can be described by the Stokes–Einstein equation99. Such particles and the non-spherical ones can display improved transport beyond Brownian motion and be classified as motors, which is determined by their properties and/or propulsive forces. Nanomotor designs can be bioinspired. For example, biomolecular motors, such as kinesin, myosin, dynein and adenosine triphosphate synthase (ATPase), can provide efficient particle transport in aqueous environments through hydrolysis of ATP units100,101. These biosystems have inspired the design of synthetic nanomotors with propulsive and/or directional motion102,103 by converting external sources of energy into mechanical transport. Nanomotors are classified into two main classes: chemically fuelled (Fig. 2a), based on metal or enzymatic catalysis as a source of energy and inspired by microorganisms that harness chemical energy from the surrounding environment; and physically actuated (Fig. 2b), in which energy is provided by ultrasound, a magnetic field or light. Asymmetric reactivity resulting from geometric or surface anisotropy allows for directional transport owing to the imbalance in propulsive agent distribution across the particle. Nonetheless, this concept may be biased when considering nanosystems at a single particle level. The orientation of individualized NPs when introduced into biorelevant media is uncontrolled, leading to random directionality. Some strategies have been devised to tackle this issue, for example, by inducing chemotactic responses to local chemical gradients, which can be particularly relevant when targeting diseased areas that overexpress specific markers (for example, reactive oxygen species (ROS)104 or hydrogen peroxide105). Conferring magnetic properties to nanosystems is another approach that enables ensemble orientation or steering of nanomotors106,107 or, at least, can be used to minimize rotational diffusion of NPs108. Still, fluctuations in velocity and poor directional control of anisotropic NPs require further research, especially when considering transport in complex biological media109.

Fig. 2: Anisotropic NPs designed for improved transport in biorelevant media.
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a, Chemically fuelled anisotropic systems. Improvement in the diffusive behaviour of nanoparticles (NPs) might stem from asymmetric chemical reactions occurring at the surface of particles, such as in the case of snowman-like (top left) or Janus-like motors (top right), or within a confined space such as in stomatocytes (bottom left and bottom right). Such reactions can be potentiated by inorganic or enzymatic catalysts. b, Physically actuated anisotropic systems. Remote control over NP dynamics can be achieved in physically actuated systems using, for example, ultrasound (left), a magnetic field (middle) or light (right). The arrows represent the direction of movement. All micrographs are from transmission electron microscopy, except for part b (left and middle) (scanning electron microscopy). Part a (top left) is adapted from ref. 112, CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Part a (bottom left) is adapted from ref. 117, CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Part a (top right) is adapted with permission from ref. 61, Elsevier. Part a (bottom right) is adapted from ref. 122, Springer Nature Limited. Part b (left) is adapted with permission from ref. 139, American Chemical Society. Part b (middle) is adapted with permission from ref. 142, American Chemical Society. Part b (right) is adapted with permission from ref. 154, Wiley.

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Different approaches may help to improve the mobility of NPs for therapeutics and biomedical applications (Table 1). Irrespective of the strategy used to engineer anisotropic NPs with improved transport properties, a few drawbacks have limited applicability and further development of such nanosystems. These include the lack of precise control of propulsion in complex biological matrices, the need for dedicated equipment to generate power, and the use of toxic and/or scarcely available fuels.

Table 1 Examples of anisotropic nanomotors for biomedical applications
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Chemically fuelled anisotropic systems

Inorganically driven motors

NPs can be driven by chemophoretic mechanisms owing to catalytic reactions occurring in distinct regions populated with one or more propulsive agents. Inorganic materials, such as platinum and gold, can decompose hydrogen peroxide into molecular oxygen110,111,112 (Fig. 2a, top left), whereas magnesium, zinc or aluminium use water as fuel to generate molecular hydrogen113,114. For example, stomatocytes can be designed to entrap metallic catalysts that generate gaseous fluxes and thrust NPs unidirectionally for a limited period55 (Fig. 2a, bottom left).

Stomatocytes loaded with platinum NPs and functionalized with transactivating transcriptional activator cell-penetrating peptides have been tested for specific uptake by HeLa cancer cells115, which produce higher levels of hydrogen peroxide than non-cancerous cells. Similar stomatocytes loaded with doxorubicin showed higher directional mobility in the presence of hydrogen peroxide, with an estimated speed of up to 39 µm s–1 in aqueous media116. However, the inefficient encapsulation of platinum NPs and the harsh processing conditions, which may not be compatible with the incorporation of labile active molecules, could hamper the use of these stomatocytes. Alternatively, the inclusion of manganese dioxide NPs (25 nm in diameter) into PEG-b-poly(d,l-lactide)–polystyrene stomatocytes improves tumour penetration in HeLa cell-based spheroids. The inorganic NPs can generate oxygen by decomposing hydrogen peroxide, leading to variable autonomous motion related to fuel concentration, with particle velocities ranging from 15 to 20 µm s–1 for 5–50 mM of hydrogen peroxide117. In general, PEG is often included as particle shielding increases blood circulation time, controls drug release and lowers adhesive interactions with extracellular matrices118.

Enzyme-powered motors

Enzyme-based motors have a selective capability of substrate conversion. For example, catalase, urease and glucose oxidase are powerful propulsive units for thrusting NPs119, as seen for enzyme-fuelled hollow mesoporous silica NPs (<400 nm in diameter)120. These particles are half-capped with silicon dioxide and modified with either catalase, urease or glucose oxidase on the uncoated side. The NPs show an increasing diffusion coefficient (up to 1.30, 1.10 and 0.99 µm2 s–1, respectively) in aqueous solutions with increasing concentration of fuels (1.5 wt.% hydrogen peroxide, 25 mM urea or 500 mM glucose, respectively). Nonetheless, the motors require higher than physiological levels of urea and glucose to display noticeable propulsion. Still, urease-based and glucose oxidase-based NPs were biocompatible with HeLa cells120. In another study, streptavidin-coated magnetic NPs of 200 nm have been immobilized onto a magnet and asymmetrically coated with glucose oxidase. The remaining surface is functionalized with biotinylated peptides that target cell integrins. NPs show an increase of roughly 23% in the apparent diffusion coefficient with glucose concentration rising from 0 to 60 mM, which reduces binding between NPs and Chinese hamster ovary (CHO) cells121. Urease is a promising candidate to power particle motion. For example, a system composed of silica NPs sputter-coated with gold and functionalized with hyaluronic acid (for tumour targeting) on one side and urease on the other was developed for the delivery of camptothecin61 (Fig. 2a, top right). These Janus NPs feature an increase of 42% in diffusion coefficient (from 4.7 to 6.2 µm2 s–1) in phosphate-buffered saline upon increasing urea concentration from 5 to 10 mM (ref. 61).

A combination of enzymes can partially self-generate substrates that feed nanomotor activity122 (Fig. 2a, bottom right). For example, catalase and glucose oxidase have been encapsulated in nanosized stomatocytes109, where catalase breaks down hydrogen peroxide into water and oxygen, and glucose oxidase acts on glucose to generate gluconic acid and hydrogen peroxide. Thus, hydrogen peroxide can fuel the catalase reaction, making glucose the limiting fuel of the system. The nanomotor achieves a velocity of 6 µm s–1 with a glucose concentration as low as 5 mM without the need for exogenous hydrogen peroxide109.

Additional enzymes described as motors, including lipase123, histamine-metabolizing enzyme124 and cholesterol oxidase125, may also be promising for designing anisotropic nanosystems. Interestingly, above a certain threshold of enzyme concentration, these biocatalysts distribute at the surface of spherical particles in an unbalanced fashion, which is sufficient to cause the symmetry breakage necessary for active transport126. For example, urease-powered spherical nanosystems (that would otherwise be classified as isotropic) have been employed for intracellular drug delivery127, antibacterial activity128 and treating bladder cancer given the natural presence of urea129. Of note, an important aspect is the use of high-purity enzymes to achieve enhanced propulsive behaviour130.

Combining biological and inorganic catalysts can improve particle motion in fluids. For example, sub-micrometre systems have been developed by associating platinum NPs, glucose oxidase and a peptide-fuelled trypsin motor arranged in a Janus-like configuration. Further incorporation of manganese ferrite NPs enables control over directionality under the influence of a magnetic field by limiting particle rotational diffusion. Transport beyond Brownian motion is observed in the presence of 400 mM of glucose and 100 µM of (benzyloxycarbonyl-Ile-Pro-Arg)2-R110 (model peptide fuel)108, which are above physiological levels, highlighting the problem of most enzyme-based motors. Ideally, feasible nanomotors should be powered by readily available energy sources at biologically relevant levels131,132. Moreover, products stemming from enzymatic degradation, such as hydrogen peroxide or ammonia, may elicit toxicity133. Alternative fuels based on enzymatic reactions with biocompatible substrates are being explored and involve the use of external synthetic amino acids (for example, N-methacryl-l-arginine and N-methacryloyl-l-cysteine) that are converted to nitric oxide in the presence of its synthase or ROS. In this case, generated nitric oxide is not only useful for the propulsion of NPs but also acts as a bioactive agent owing to its vasodilating properties, which can potentiate cardiac targeting and angiogenesis applications104,134,135.

Physically actuated anisotropic systems

Toxicity associated with chemically fuelled systems and the variable or scarce availability of natural substrates at sites of interest have substantiated the development of fuel-free nanomotors that are responsive to external physical stimuli136.

Ultrasound

Low-power ultrasound, typically in the single-digit megahertz frequency range, can improve movement of anisotropic NPs136. Acoustically powered gold–nickel–gold nanowires can move through saliva and serum when submitted to ultrasound (10 V, 2.51 MHz), reaching speeds of approximately 10–50 µm s–1 (ref. 137). Furthermore, doxorubicin-loaded porous gold nanowires of similar dimensions have been designed for NIR light-triggered drug release. Movement results from the asymmetric pressure gradient at the concave extremity of the wire formed upon exposure to ultrasound, and the inherent porosity of the wires allows eightfold higher doxorubicin loading capacity. The nanocarriers can be directed towards adherent HeLa cells under ultrasound stimulus, showing potential for targeted drug delivery138. In another example, gold nanowires have been wrapped with a rolling circle amplification DNA strand and used as carriers for small interfering RNA delivery (Fig. 2b, left). The system can disrupt the cell membrane and circulate intra-cellularly upon ultrasound exposure, leading to 70–94% mRNA silencing (compared with less than 15% under static conditions) upon 5 min of treatment at 6 V and 2.66 MHz using green fluorescent protein-expressing HEK293 and MCF7 cell lines. The acoustic effect on membrane permeability remains unclear but potential cell damage has been dismissed as time and ultrasound intensity are deemed low139.

Magnetic field

Magnetism has been widely explored to guide NPs to sites of interest (for example, tumours) but it can also be used as an energy source to trigger the propulsion of anisotropic NPs140. This responsiveness of the particles initiates from the presence of magnetic materials in their composition. For instance, nanowires composed of a gold head and a nickel tail joined together by a silver bridge display magnetically driven propulsion under an external rotating 5 G magnetic field, arising from mechanical deformation and causing system symmetry breaking, thus inducing movement. These systems show high mobility in urine and hyperosmotic fluids, with speed increasing from 1.5 to 6 µm s–1 with varying magnetic frequency within the 5–15 Hz range141.

Magnetically driven helical silica–nickel propellers of nanoscaled size (100 × 400 nm) feature better directionality and mobility in synovial fluid surrogates than in a glycerol–water mixture, with velocity values increasing from 0.36 to 1.48 µm s–1 at 80 Hz (ref. 142) (Fig. 2b, middle). The small size of helical nanomotors makes magnetic field use impractical when aiming for directed synchronous motion in Newtonian fluids143. Additionally, nanomotors tend to aggregate, which limits their stability, particularly in hyperosmotic media141. This issue can be addressed by, for example, the addition of zinc ferrite coatings to silica–iron nanomotors, which increases particle stability for up to 6 months without impairing performance144. Such coatings can even improve haemocompatibility145 and qualify the helical nanomotors as cytocompatible146.

Magnetically activated nanomotors have also been applied to solid tumours. For example, the bioaccumulation of zinc-doped iron oxide nanocubes (60 nm) modified with the tripeptide arginine-glycine-aspartic acid (RGD) following intratumoural injection to U87 subcutaneous xenograft model on BALB/c mice was improved by the application of a rotating magnetic field (5 Hz) compared with the absence of a magnetic field or under a lower field strength (1 Hz). The nanocubes self-organize into elongated swarms capable of synchronous rotation under the magnetic field, thus promoting in situ residence and tumour growth suppression at 14 days post-injection147. However, the inefficiency of energy transfer across biological samples in deeper regions of the body (contrasting with the superficial tumour of the considered animal model) may impair magnetic activation147.

Light exposure

Light can power the motion of Janus particles featuring a metal-coated hemisphere owing to an asymmetric optothermal response that generates a propelling temperature gradient148,149,150. NIR light is typically selected owing to high tissue penetration depth and reduced photodamage, although UV radiation can also be useful for superficial applications151. Light-powered systems have mostly been developed at the microscale149 but nanomotors have also been explored. Nanoconstructs partially sputter-coated with gold have been designed for thermal ablation of cancer cells and photothermal therapy152,153. Janus gadolinium-doped, gold-coated mesoporous silica NPs have been developed for magnetic resonance imaging (Fig. 2b, right). The system shows solid tumour penetration by thermomechanical perforation of cell membranes when NPs are intravenously administered to BALB/c mice bearing subcutaneous 4T1 cell-derived tumours, and the diseased area is irradiated with NIR light154. Examples of other light-induced nanomotors include FoF1-ATPases asymmetrically distributed across the surface of polyelectrolyte-coated silica particles. Once exposed to NIR light, a generated proton gradient moves through the FoF1-ATPase channels, inducing their rotation and producing ATP. However, this approach has only been demonstrated for large particles (average diameter of 3 µm)155. Alternative strategies involve the development of hybrid nanomotors combining light with ultrasound156, a magnetic field157 or catalytic activation158. Combined physical stimuli offer control over particle aggregation and separation or the possibility of shifting between energy sources for better control of directionality158.

Tackling biological barriers

The fate of a nanosystem in the body depends on established interactions with the surrounding environment. Different anisotropic nanosystems have been shown to be effective for transport in biological fluids, cell internalization, overall biodistribution and immune evasion.

Transport across biological fluids

Different natural fluids can hinder the mobility of molecules and supramolecular structures, and various strategies have been devised to address such effects. Shape anisotropy can be beneficial for promoting permeation across mucus. For example, mesoporous silica nanorods (80 × 240 nm) show higher diffusivity and distribution in rat intestinal mucus than their 200-nm spherical counterparts composed of the same material and with similar surface charge and composition (Fig. 3a). Such behaviour is attributed to the rotational diffusion of nanorods promoted by shear flows and structural cues of the mucin network159. Similar results were observed for rod-shaped lovastatin nanocrystals with a hydrodynamic diameter of 400 nm. The nanostructures are produced by sonoprecipitation combined with high-pressure homogenization. Improved transport in rat mucus as well as cell and tissue uptake led to a higher oral bioavailability for the rod-shaped formulation160. Short self-assembled amphiphilic α-lactalbumin nanotubes (length of 100–200 nm) also demonstrate higher permeability in rat intestinal mucus than nanospheres or longer nanotubes. The nanotubes reach epithelial cells more effectively based on ex vivo mucus penetration studies and permeability assays in co-cultured monolayers of Caco-2 enterocyte-type and mucus-producing (as induced by methotrexate exposure) HT29-MTX cells. They also reveal the highest cell uptake and drug bioavailability when used as carriers for curcumin in Sprague–Dawley rats161. Once again, rotational diffusion — alongside size control that allows nanotubes to fit the tight aqueous channels created by the mucin mesh-like structure of mucus162 — appears to play an important role in improving transport.

Fig. 3: Selected strategies based on shape anisotropy for tackling biological barriers.
figure 3

a, Rods can permeate fresh rat intestinal mucus more efficiently than spheres of similar chemical composition and zeta potential (–5 mV). Both ensemble-average mean squared displacement (MSD) over time and distribution of the logarithms of single particle effective diffusivity (Deff) at a time scale of 1 s indicate improved transport of nanorods in mucus. Ex vivo particle distribution studies in rat intestinal mucus (in green) confirm that rods (in red, lower panel) exhibit improved penetration159. b, Tripeptide arginine-glycine-aspartic acid bearing gold nanoparticles (NPs) of varying aspect ratio (AR) are incubated with human mesenchymal stem cells (hMSCs) to assess anisotropic ligand presentation at the nanoscale. NPs with higher AR values potentiate efficient attachment of nanopodia (pointed out by white arrows) and result in improved cell adhesion. Following incubation of NPs with stem cells in NOD/SCID mice, cell adhesion and spreading are more pronounced for the groups of higher AR (AR4 and AR7) than in groups with lower AR (AR1 and AR2). Higher AR also translates into higher YES-associated protein (YAP) nuclear translocation, which is a mechanosensitive transcriptional factor for the regulation of cell fate towards specific cell lineages such as osteoblasts. Here, F-actin and NPs are coloured in red and yellow, respectively175. SD, Sprague–Dawley. Part a is adapted with permission from ref. 159, American Chemical Society. Part b is adapted with permission from ref. 175, Wiley.

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The use of nanomotors to improve transport in mucus is also promising. For example, cisplatin-loaded self-propelled hollow mesoporous copper sulfide NPs (diameter <180 nm) half-capped with titanium have been proposed for thermophoretic mobility upon exposure to NIR light. These structures are further coated with cell membrane extracts from Staphylococcus aureus. Bacterial protein A can bind IgG, which is overexpressed in colorectal cancer, thus conferring tumour-targeting properties. Moreover, NPs show increased diffusivity in mucus surrogates in vitro with increasing laser power. These nanomotors inhibit tumour growth in orthotopic tumour-bearing mice following oral administration and NIR laser exposure163.

Anisotropy may also influence the way nanosystems behave when in blood circulation. Self-assembled cylinder-shaped filomicelles (20–60 nm × 8 µm) display blood circulation of up to 1 week, which is 10 times longer than that for spheres of similar composition. Particles with a length greater than 2.5 µm show the maximum half-life values of 5.2 days following intravenous injection to Sprague–Dawley rats or C57 mice. This is attributed to the structural similarities between filomicelles and natural filoviruses, as well as to controlled biodegradation (decrease in length) over time, until clearance. Moreover, paclitaxel-loaded filomicelles achieved tumour size regression in a lung cancer murine model164. In another study, elongated oleic acid-coated iron oxide nano-assemblies with superparamagnetic properties loaded with vincristine exhibited prolonged blood circulation residence over isotropic spherical counterparts, with an area under the plasma concentration–time curve and mean residence time values 1.2-fold and 1.7-fold higher, respectively, and a decrease of nearly 50% in clearance. Extended blood circulation of the drug carrier following intravenous administration contributes to the lowering of leukocyte count and improved expression of anti-leukaemia markers in a NOD/SCID leukaemia mouse model165. Similarly, prolonged blood circulation is observed for nanorods of PEG-b-dendritic polylysine–camptothecin (<500 nm in length) in comparison to spherical counterparts, with a half-life time of 5.8 h and 1.6 h, respectively, following intravenous administration to BALB/c mice166.

Nanoparticle–cell interactions

Unlike composition anisotropy, the impact of size, shape and aspect ratio of non-spherical NPs on cell attachment and uptake have been explored, although often leading to conflicting results167. As the NP shape can affect blood circulation time, disc-shaped mesoporous silicon particles (400 × 1,000 nm) show higher endothelial cell adhesion to diseased vasculature under in vitro microvascular flow conditions than rods or spherical counterparts. Complementary computational analysis suggests that the area of adhesion decreases from discs to rods to spheres as opposed to the dislodging hydrodynamic forces15. Similarly, platelet-like nanoporous silicon particles (400 × 1,000 nm) show higher accumulation in the tumour vasculature of melanoma-bearing mice168. The effect of shape on cell uptake has also been explored under non-flow dynamics such as in mesenchymal stem cells or HeLa cancer cells. For example, quasi-ellipsoidal NPs are less prone to undergo cell uptake than spherical counterparts, where the effect is related to the increasing aspect ratio59. Interestingly, studying the uptake of various anisotropic NPs under static or dynamic conditions yields different outcomes. For example, larger cuboid-shaped (800 × 100 × 100 nm) and disc-shaped (325 nm diameter × 100 nm height) PEG-diacrylate NPs exhibit higher uptake by endothelial cells than smaller particles (400 × 100 × 100 nm or 220 nm diameter × 100 nm height, respectively) under physiological-like shear flow conditions, although the contrary is observed for static conditions169. Moreover, in the presence of red blood cells and shear flow, oblate-shaped and rod-shaped biotin-functionalized polystyrene particles align and adhere more effectively to avidin-coated vessel walls of a microfluidic chamber than equivalent spherical particles. However, this effect is only evident for sizes exceeding 500 nm (ref. 170). Molecular simulations also suggest that penetration across cell membrane models is more effective for rods (1 nm × 3 nm), followed by discs (3 nm × 1 nm) and spheres (3 nm) of similar surface chemistry (composed of either polar, non-polar, charged or non-charged beads)171. Regarding antibacterial effects, short cylindrical micelles (length ≤4 µm) are more efficient in rupturing bacterial membranes and killing bacteria, as shown by their lower minimum inhibition concentration values against S. aureus, than their longer (length >10 µm) or spherical (200 nm) counterparts172.

Geometric anisotropy plays a definitive role in cell targeting of nanosystems by optimizing the presentation of targeting moieties to specific receptors173. For example, rod-shaped polystyrene NPs (approximately 130 × 370 nm) coated with the anti-receptor tyrosine-protein kinase ERBB2 antibody trastuzumab demonstrate higher specific uptake and surface binding when using different ERBB2+ breast cancer cell lines than discs or nanospheres174. In another example, an increase in the aspect ratio of gold nanorods coated with the cell adhesion motif RGD leads to augmented cell spreading, alignment of the basal cytoskeletal structure and maturation of cell focal adhesions owing to higher recruitment of β1 and β3 integrins when compared with spherical NPs (Fig. 3b). This also translates into higher expression of the osteogenic transcription factors Runt-related transcription factor 2 (RUNX2) and YES-associated protein, which promote stem cell differentiation into osteogenic lineage, showing the capacity of particle shape for tuning stem cell commitment175. In an inflammation cell culture model under dynamic conditions, polystyrene nanorods coated with antibodies to vascular adhesion molecule 1 (VCAM1), an endothelial inflammation marker, exhibit increased cell binding compared with their spherical counterparts and non-specific IgG-coated rod-like NPs. However, apart from the lungs, where VCAM1 rods show higher accumulation, no differences in biodistribution across other tissues (brain, kidney, liver and spleen) are observed for the different particle shapes following their intravenous administration in induced systemic inflammation mice models. These observations were attributed to the small differences in aspect ratio between the tested shapes176.

Overall, the definitive processes involved in anisotropic NP uptake by different cell types are still poorly understood. However, the combined analysis of factors, including particle shape, size, rigidity and surface chemistry, and cell properties (for example, preferential pathway for particle uptake, membrane flexibility and presence of receptors) can explain differences in uptake dynamics177,178. Hence, general guidance seems premature at this point without further extensive research.

Tissue penetration

Extracellular matrices and cell clusters constitute challenging barriers and anisotropy can provide a promising approach for NPs to bypass them. However, the systematization of evidence is challenging owing to the scarce data available on the topic and the plethora of particle design possibilities that are introduced by anisotropy. Some examples, however, provide compelling evidence of the effects that anisotropy can have on the biodistribution of NPs and support the use of such nanosystems for targeted delivery. For example, spherical, rod-shaped and star-shaped gold NPs administered intravenously to mice show different organ accumulation patterns179. Rods present lower biodistribution to tissues (including the liver) and rapid clearance, whereas spherical NPs accumulate mostly in the lungs but none of these NPs passes the blood–brain barrier (BBB)179. The findings may be attributed to the variable protein corona formed during blood circulation for differently shaped gold NPs180. Conversely, asymmetric snowman-like self-assembled vesicles (50 nm in diameter) made of poly(ethylene oxide)-poly(butylene oxide) with either poly((2-methacryloyl)ethyl phosphorylcholine)-poly(2-(diisopropylamino)ethyl methacrylate) or poly(oligo(ethylene glycol)methyl methacrylate)-poly(2-(diisopropylamino)ethyl methacrylate) and encapsulating glucose oxidase (with or without catalase) overcome the BBB. The system responds to external glucose gradients such as the positive gradient observed from the blood to the vessel wall and into the nervous system, thus allowing directional transport from the blood towards the brain181. The encapsulated enzymes react with their substrates in a confined space and produce a flow of hydrogen peroxide and gaseous oxygen that is preferentially expelled through the small and permeable bulge formed by poly(ethylene oxide)-poly(butylene oxide) of the anisotropic vesicle, thus generating propulsion. Favourable transport across the BBB is shown using an in situ perfusion rat model181. Even though composition and size are the most critical factors for overcoming the BBB182, anisotropic shapes may further help to access the nervous system by improving presentation183. For example, nanorods associate less extensively with the endothelium of a 3D BBB model than spherical NPs, but this only results in mild improvement in transport across the BBB184,185.

Aside from systems featuring geometric anisotropy, Janus-like NPs can be used as promising multifunctional platforms for theranostic applications. For example, silica NPs coated with gold in one hemisphere and further functionalized with a probe to sense hydrogen peroxide (3-mercaptophenylboronic acid), a photosensitizer (5,10,15,20-tetrakis(4-aminophenyl)porphyrin) and catalase have been developed (Fig. 4a). The system is responsive to the increased concentrations of hydrogen peroxide observed in the tumour microenvironment. Catalase decomposes hydrogen peroxide into oxygen, which improves transport and feeds the photosensitizer to produce ROS. Moreover, upon exposure to NIR light, the gold coating heats and induces a photothermal effect, promoting self-thermophoretic mobility. Additionally, fluorescent and photoacoustic imaging are possible owing to the presence of the gold nanoshell and the strong signal from the photosensitizer, respectively. Altogether, these NPs show synergistic photothermal and photodynamic effects, reducing tumour weight and size following intratumoural injection in a subcutaneous 4T1 breast tumour mouse model153. The addition of targeting moieties, such as affibodies, is also an interesting approach to promoting specific particle accumulation and, thus, improving the visualization and treatment of tumours186.

Fig. 4: Examples of anisotropic nanomotors for combinational therapies.
figure 4

a, Dual-powered mesoporous silica nanoparticles (NPs) devised for theranostics. Half of the particle is coated with gold and functionalized with Raman reporter 3-mercaptophenylboronic acid (3-MPBA), which allows for surface-enhanced Raman scattering sensing (by detecting hydrogen peroxide (H2O2), used as a tumour marker) and enables photoacoustic imaging and photothermal therapy upon exposure to near-infrared (NIR) light. Irradiation also leads to excitation of the pre-loaded photosensitizer 5,10,15,20-tetrakis (4-aminophenyl) porphyrin (TAPP), thus generating reactive oxygen species. The addition of catalase to act as a photodynamic therapy (PDT) enhancer promotes the diffusion and deeper tumour penetration of the nanomotor through the release of oxygen (O2). The nanomotor is responsive in terms of diffusivity to both increasing concentrations of H2O2 and NIR light. Once injected intratumourally in 4T1 tumour-bearing mice, the full combinational therapy is the most promising regarding tumour volume and weight reductions compared with the use of NIR alone, combined with NPs without TAPP/catalase, or the full system without NIR153. b, Magnetic nanobullets are used for combined PDT and magnetic hyperthermia in an anti-metastatic immunotherapy approach. The mesoporous organosilica framework grows asymmetrically on iron oxide NPs and carries the photosensitizer chlorine e6. These structures are further cloaked with cancer cell membranes for homologous tumour-targeted accumulation and immune escape. Once exposed to a glutathione solution, which mimics the acidic and reductive tumour environment, nanobullets degrade and enable the release of chlorine e6. In vivo studies using an orthotropic 4T1 BALB/c mouse tumour model show that intravenous administration of the nanobullets and irradiation with NIR light in combination with an alternating current magnetic field (ACMF) and intravenous anti-CTLA4 antibody (an immune-checkpoint inhibitor) induce anticancer activity, reduce metastases in lungs and result in a sequence of immunogenic cell death25. MSD, mean squared displacement. Part a is adapted with permission from ref. 153, AAAS. Part b is adapted from ref. 25, CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).

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Anisotropic nanomotors can help penetration into pathological tissues and other tissue-like structures that are difficult to access. For example, peptide-modified polydopamine nano-bowls loaded with the nitric oxide donor N,N′-di-sec-butyl-N,N′-dinitroso-1,4-phenylenediamine have been developed for the treatment of thrombus. These nanomotors show propulsive properties upon NIR light stimulus as polydopamine absorbs and transfers energy into the nitric oxide donor and leads to the release of nitric oxide bubbles, resulting in deep thrombus infiltration and improved site-specific mechanical thrombolysis. The therapeutic effect in a tail thrombus mouse model outperformed clinical standard of care with recombinant tissue plasminogen activator, whilst avoiding recurrence owing to the anti-platelet adhesion and vasodilation effects of nitric oxide187. Considering the wound-healing properties of nitric oxide, calcium nanomotors have been partially capped with polydopamine grafted with cysteine-nitric oxide for S. aureus biofilm infiltration in diabetic foot ulcers of mice by decomposing calcium dioxide into molecular oxygen. The subsequent release of low levels of nitric oxide from polydopamine-g-cysteine-nitric oxide is also triggered by glutathione, which is naturally present in biofilms. Combined deep biofilm penetration of nanomotors and the intrinsic antibacterial activity of nitric oxide generated in situ results in accelerated wound closure188.

Immune response

The immune system can be seen as a scattered barrier to the biodistribution of particles, in which shape, size and their interrelation play a dominant role in determining immune cell uptake and influencing immunological outcomes189,190. For example, spheres and short filomicelles (diameter of ~60 nm and length of up to 2 μm) are more internalized by activated human-derived macrophages in vitro than long filomicelles (up to 3 μm) under flow conditions164. Similarly, the effect of aspect ratio on the interactions of stealth (PEG-coated) and targeted (RGD-coated) anisotropic nanorods with immune cells has been investigated in a colon cancer mouse model. The nanorods are produced by biotemplating through RNA-guided self-assembly of tobacco mosaic virus coat protein. The accumulation and penetration of PEG-coated nanorods in tumours is dependent on aspect ratio, with lower values outperforming higher ones in terms of transport from the blood vessel to the extravascular space. Tested aspect ratio values are 3.5, 7 and 16.5, corresponding to lengths varying between 60, 130 and 300 nm. Conversely, similar RGD-coated nanorods are quickly recognized by the immune system and eliminated. However, mid-sized (130 nm) RGD-coated rods feature the best size and ligand density combination, resulting in the highest specific tumour co-localization191.

Anisotropy not only influences immune cell uptake but can also trigger different immune responses. For example, 1D cylindrical glyco-NPs induce in vitro macrophage-mediated inflammation more pronouncedly than spherical counterparts despite being less internalized192. Moreover, 2D diamond-shaped glyco-platelets are poorly taken up by macrophages but lead to improved release of various pro-inflammatory cytokines (even more than for cylinder-shaped NPs), showing their potential for immune therapies51. These observations suggest that immune triggering may be induced by interactions at the cell membrane level, which are potentiated by the higher surface area-to-volume ratio of non-spherical constructs, without the need for uptake and intracellular processing. Similarly, ellipsoidal PLGA nanoconstructs produced by film stretching and used as antigen-presenting cell mimetics improve macrophage escape and antigen-specific induction of cytotoxic T lymphocytes. A strong immune cell response is further observed in irradiated THY1.2+ C57Bl/6 mice as an adoptive immunotherapy animal model after intravenous administration, contrasting with a mild response to spherical NPs193.

Overall, the distinct ability of anisotropic NPs to interact with immune cells opens new possibilities for immune engineering, with emphasis on the recent development of vaccines for cancer immunotherapy and infectious disease prevention. For example, antigen-presenting disc-shaped NPs made of apolipoprotein A1-mimetic peptide and phospholipids (diameter of ~10 nm) and co-loaded with aldehyde dehydrogenase (ALDH) antigen peptides and adjuvant molecules induce specific T cell responses and reduce cancer stem cells with a high expression of ALDH. The particles show antitumour efficacy in both D5 melanoma and 4T1 breast cancer murine models, particularly in combination with immune-checkpoint blockade with anti-PD-L1 (ref. 194). Similar systems incorporating human papillomavirus antigen improve the treatment of TC-1 mucosal tumours in C57Bl/6 mice as compared with the use of the soluble form of the antigen. Additionally, subcutaneous administration generates superior T cell responses compared with the same vaccine used by the intranasal route, which is only effective for the treatment of tumours that are in the vicinity of the vaccination site (namely for lung metastasis and sublingual inner lip tumours but not for intravaginal tumours)195. Treatment of gliomas has also been discussed by studies combining synthetic high-density lipoprotein nanodisc vaccines and chemotherapeutic agents196, or immune-checkpoint blockade with anti-PD-L1, particularly for personalized medicine approaches using endogenous antigens197. For infection prevention, vaccines based on gold nanorods have been designed as platforms to improve the efficacy of a synthetic RNA adjuvant following intranasal administration in mice. These vaccines can elicit stronger mucosal immune response against influenza virus infection as compared with similar spherical systems, even at lower antigen doses198.

Geometric anisotropic NPs can also be combined with magnetic responsiveness to induce hyperthermia in immunotherapy applications. An example is silica-based magnetic nanobullets carrying a photosensitizer intended for photodynamic therapy (Fig. 4b). After intravenous administration to 4T1 tumour-bearing mice, this system induces immunogenic cell death and leads to the elimination of deep metastatic tumours upon laser irradiation and application of a magnetic field in combination with immune-checkpoint inhibitors such as antibodies to CTLA4 (ref. 25).

Outlook

Introducing the concept of anisotropy into the engineering of nanomaterials opens a wide range of design possibilities for various biomedical applications. From creating smart multi-drug delivery systems for targeted therapies to developing tools for diagnostics, anisotropy allows modulation of the interactions of NPs with the surrounding environment and influences transport and distribution in biological settings. Once administered to complex organisms, engineered anisotropic nanosystems can reach target organs, tissues or even cell types, thus maximizing the chances for successful medical interventions. Overcoming well-known barriers, particularly by using nanomotors sensitive to remote guiding signals or featuring self-propulsion, represents a main direction of research in the field. Although external power sources offer some advantages (for example, real-time guidance control), the use of naturally occurring biomolecules to boost the movement of NPs seems the most promising approach for achieving active and directional transport. However, evidence for this derives from studies performed using surrogate media that fall short in representing complex biological environments142. Improving transport and control over the directionality of nanomotors is often close to the limits of experimental reproducibility and statistical significance, making it hard to distinguish between experimental bias and actual highly diffusive and oriented phenomena99. Furthermore, NP fabrication typically involves multistep and complex processes, which leads to low yields and poor reproducibility. Thus, these and other issues should be considered to allow the translation of anisotropic NPs into the clinic (Box 2). Overall, the introduction of anisotropic principles in the design process of NPs holds potential to endow nanomedicines with a whole new set of functionalities that may contribute to improved therapeutics, prophylaxis and diagnosis.