Polymeric and Polymer-Functionalized Drug Delivery Vectors: From Molecular Architecture and Elasticity to Cellular Uptake

Abstract

Polymers and polymer composites offer versatile possibilities for engineering the physico-chemical properties of materials on micro- and macroscopic scales. This review provides an overview of polymeric and polymer-decorated particles that can serve as drug-delivery vectors: linear polymers, star polymers, diblock-copolymer micelles, polymer-grafted nanoparticles, polymersomes, stealth liposomes, microgels, and biomolecular condensates. The physico-chemical interactions between the delivery vectors and biological cells range from chemical interactions on the molecular scale to deformation energies on the particle scale. The focus of this review is on the structure and elastic properties of these particles, as well as their circulation in blood and cellular uptake. Furthermore, the effects of polymer decoration in vivo (e.g., of glycosylated plasma membranes, cortical cytoskeletal networks, and naturally occurring condensates) on drug delivery are discussed.

Keywords: biomolecular condensates; cellular uptake; circulation times; drug delivery; hairy nanoparticles; linear chains; microgels; nanogels; passive endocytosis; polymer-grafted nanoparticles; star polymers; stealth liposomes; translocation.

Figure 2

Linear polymers. (a) Typical configurations of a linear chain with $N=102$ monomers in the coil state and a liquid-like globular state. Reprinted with permission from Ref. [119]. Copyright 2017 by the American Physical Society. (b) Chemical structure of polyethylene glycol (PEG). (c) Shapes and Flory parameters $\nu$ of polymer chains in different solvent qualities. The three dotted lines denote the theoretical values of $\nu$ for different solvent qualities. Reproduced from Ref. [121] with permission from Springer Nature. (d) Reduced mean square radius of gyration $S^2={(R_{\mathrm{g}}/{\ell }_{\mathrm{K}})}^2$ (plus symbols in the left scale) and mean-squared deviation of the total energy from its average $\tilde{C}$ (proportional to the specific heat; circles in the right scale) as functions of the reduced temperature $k_{\mathrm{B}}T/ϵ$, determined from the Monte Carlo simulations of Gaussian chains for $N=102$. Reprinted with permission from Ref. [119]. Copyright 2017 by the American Physical Society.

Figure 3

Star polymers. (a) Form factors and corresponding fits (solid lines) of the 8-arm (squares) and 18-arm (circles) polyisoprene stars (8-arm data are multiplied by a factor of 0.3 for visibility). Figure reproduced from Ref. [124]. Published under licence by IOP Publishing Ltd. (b) Liquid-drop model. Reduced central lateral extension $\zeta$ of a diametrically compressed spherical polymer brush with $f=60$ arms containing $N_{\mathrm{c}}=30$ and 50 monomers plotted against slit width L (black datapoints/curve). The datapoints are obtained using molecular dynamics simulations, and the curve is the liquid drop model fit for the reduced Egelstaff–Widom length $\Psi =0.6$ (see Equation (8)). The deformation energies for these two cases, together with the fits (red datapoints/curves), are also shown. The inset illustrates the diametral-compression geometry, with arrows representing hydrostatic pressure, and contains the definition of $\zeta$. Figure reproduced from Ref. [122]. CC BY-NC 3.0. (c) A representation of the Daoud–Cotton model: every branch is made of a succession of blobs with a size $\xi$ increasing from the centre of the star to the outside. From outside to inside, $r_1$ indicates the transition between the swollen and the unswollen region, and $r_2$ the transition between the unswollen region and the core. The terms swollen, unswollen, and core refer to regions with increasing monomer concentration. Figure reprinted from Ref. [125].

Figure 4

Polymer-grafted nanoparticles. (a) Cryo-electron micrograph of mixed poly(acrylic acid)/polystyrene brush-grafted silica nanoparticles in water. The scale bar corresponds to $100\mathrm{nm}$. Reprinted with permission from Ref. [22]. Copyright 2015 American Chemical Society. (b,c) Simulation snapshot and schematic representation of the two-layer model. Indicated are the nanoparticle radius R, the total radius of the polymer-grafted nanoparticle $R_{\mathrm{tot}}$, and the thicknesses of the dry and interpenetration layers, $h_{\mathrm{dry}}$ and $h_{\mathrm{inter}}$, respectively. Reprinted with permission from Ref. [10]. Copyright 2020 American Chemical Society.

Figure 5

Polymer-decorated membranes. 3D reconstruction of the pericellular hyaluronan coat by particle exclusion assay. (a,b) Fluorescence micrographs of rhodamine-labeled chondrocytes immersed in a medium containing fluorescein isothiocyanate (FITC)-labeled silica beads. Cells were allowed to adhere to glass coverslips for $25\min$ before fixation and labeled with tetramethyl rhodamine isothiocyanate (red). They were then incubated with FITC-labeled $0.4\mathsf{\mu }\mathrm{m}$ silica beads (green). Micrographs were taken with a digital microscope (DeltaVision) able to generate 3D images by image reconstruction from a series of z-sections at $0.5\mathsf{\mu }\mathrm{m}$ resolution. The excluded volume is dark. Untreated cells have a 5 to $6\mathsf{\mu }\mathrm{m}$ wide excluded zone around them (a), whereas beads reach up to the surface of hyaluronidase-treated cells (b). The scale bars correspond to $5\mathsf{\mu }\mathrm{m}$. Reprinted from Ref. [126], with permission from the Biophysical Society. (c) Pegylated liposomal doxorubicin. Reprinted from Ref. [127], Copyright 2004, with permission from Elsevier.

Figure 11

Biomolecular condensates: colocalization, coalescence, and interface decoration by protein clusters. (a) Cytoplasmic enhanced green fluorescent protein (EGFP)–yes-associated protein (YAP) selectively enriches different proteins. Live-cell image showing the colocalization of EGFP–YAP (green) condensates with mCherry–Dcp1a (red) condensates in the cytoplasm of HEK293T cells after hyperosmotic stress that induced condensate formation, $20\mathrm{s}$ after sorbitol treatment. The dotted line indicates the nucleus, green fluorescence EGFP-YAP condensates, red mCherry-Dcp1a condensates, and yellow colocalization. The scale bars correspond to $5\mathsf{\mu }\mathrm{m}$ (whole-cell image) and $1\mathsf{\mu }\mathrm{m}$ (magnified view of the boxed region). Reproduced from Ref. [214] with permission from Springer Nature. (b) Time-lapse of nucleolar coalescence after the nucleolar signal at $t=0\mathrm{s}$ in the nucleus of a live HeLa cell. The frames show the progress of the nucleolar fusion. The scale bar corresponds to $2\mathsf{\mu }\mathrm{m}$. Reprinted with permission from Ref. [209]. Copyright 2018 by the American Physical Society. (c) MEG-3 forms low-dynamic clusters that adsorb to the surface of PGL-3 condensates. Photomicrographs of a P granule reconstituted in vitro with purified PGL-3 and MEG-3 trace-labeled with Dylight 488 and Alexa 647, respectively. The scale bar corresponds to $3\mathrm{mm}$ and applies to all images in the set. The top panels are a maximum projection of a z-stack through the granule. The middle panels are a single x-y plane through the middle of the same granule. The lower panels are a single z-x plane through the middle of the same granule. Reproduced from Ref. [20], AAAS.

Figure 12

Circulation times. (a) Circulation half-life times of gold nanoparticles (GNP) with various diameters and PEG coatings of various molecular weights (MW). The color of the data points corresponds to the MW of the PEG coating. Adapted from Ref. [82]. CC BY 3.0. (b) Clearance profiles of PEG particles with diameters between 2 and $3\mathsf{\mu }\mathrm{m}$ and elastic moduli of 50, 850, and 5000 $\mathrm{kPa}$ after intravenously injecting them into mice. Reproduced from Ref. [218] with permission from Springer Nature.

Figure 1

Simulation snapshots of polymeric particles. (a) Star polymer with $f=35$ arms and $N_{\mathrm{s}}=50$ monomers (dark, blue beads) per chain and a linear chain with $N_c=40$ monomers (bright, green beads) at a center-to-center distance of ten times the bead radius. Reprinted with permission from Ref. [8]. Copyright 2007 American Chemical Society. (b) Diblock-copolymer micelle. Reprinted with permission from Ref. [9]. Copyright 2006 American Chemical Society. (c) Polymer-grafted nanoparticle. Reprinted with permission from Ref. [10]. Copyright 2020 American Chemical Society. (d) Swollen microgel particle with a uniform crosslink distribution. Reprinted with permission from Ref. [11].

Figure 6

Microgel collapse: in silico modeling and scattering experiments. (a) Snapshots of a microgel particle, consisting of crosslinked linear chains with beads that interact using the Weeks–Chandler–Andersen (WCA) potential for ’bead size’ $\sigma$, exhibiting the typical volume phase transition from swollen to compact. (b) Numerical form factors, averaged over four different realizations, for microgels with $N\approx$ 41,000 monomers and $c=3.2\%$ of crosslinkers generated in a sphere of radius $Z=30\sigma$ (symbols) for various solvent qualities (corresponding to temperatures). Solid lines are fits of the curves using the fuzzy sphere model of Equation (18). (c) Averaged density profiles obtained from molecular dynamics (MD) simulations of a microgel with $N\approx$ 41,000, $c=3.2\%$, and $Z=30\sigma$ (symbols) and from the fit of the form factors using the fuzzy sphere model (solid lines). Reprinted with permission from Ref. [140].

Figure 7

The elastic moduli of deposited ${\mathrm{PNIPAM}}_{\mathrm{BIS}}$ microgels that are crosslinked using N,N’-methylenebis(acrylamide) (BIS) and of ${\mathrm{PNIPAM}}_{\mathrm{SCL}}$ self-crosslinked microgels as a function of the radial position. (a) Typical topography and elastic modulus images of a single PNIPAM microgel (top: BIS, bottom: SCL). Scale bars: 200 nm. (b) Plot of the elastic modulus normalized by the elastic modulus at the centre, $Y_{\mathrm{c}}=340\pm 10\mathrm{kPa}$ for ${\mathrm{PNIPAM}}_{\mathrm{BIS}}$ and $Y_{\mathrm{c}}=13\pm 3\mathrm{kPa}$ for ${\mathrm{PNIPAM}}_{\mathrm{SCL}}$, vs. the radial position calculated from at least five microgels using high-resolution elastic modulus mapping and the height trace (dashed lines) reconstructed from vertical tip position during force map acquisition. Reprinted from Ref. [142]. CC BY 3.0.

Figure 8

Representative snapshots of in silico microgels down to ultra-low crosslinked for microgels with $N\approx$ 336,000 monomers interacting via the WCA potential for ‘bead size’ $\sigma$ in good solvent conditions, mimicking $T=20$ °C in experiments. To improve the visualization, all the plots represent a slice of the microgels of width $30\sigma$. Monomers are reported in green, while crosslinkers are in red. For ultra-low crosslinked (ULC) microgels, we use $c_{\mathrm{cross}}=0.1\%$, which agrees well with the experimental data in terms of swelling ratio and form factors. Reprinted with permission from Ref. [151]. Copyright 2023 American Chemical Society.

Figure 9

Biomolecular condensates for delivering small molecules. (a) Uniform manifold approximation and projection (UMAP) representation of 1700 small molecules used in the analysis [187], which is based on physical features generated in QikProp [188]. (b) Partition coefficients (PC) determined using mass spectrometry vary over nearly six orders of magnitude. Bar chart of PC values, ordered from smallest to largest, for the partitioning of 1037 compounds into the SUMOSIM condensate (composed of polySUMO and polySUMO-interaction motif (SIM) proteins [189]). Red and green dashed lines indicate $log\mathrm{PC}=0$ and $log{\mathrm{PC}}_{\mathrm{SUMOSIM}}=1.77$, respectively. The numbers of compounds with $log\mathrm{PC}<0$ and $log\mathrm{PC}>log{\mathrm{PC}}_{\mathrm{SUMOSIM}}$ are indicated. The grey-coloured areas represent the bar plots for the mean values of the data, and green and purple dots represent metabolites and drug compounds, respectively. Reprinted from Ref. [186]. CC BY 4.0.

Figure 10

Biomolecular condensates containing proteins. (a) Intrinsically disordered proteins (IDPs) mediate phase separation that underlies the formation of a biomolecular condensate. Reprinted with permission from Ref. [194]. Copyright 2024 American Chemical Society. (b) Two chemically active protein condensates, pinned to a planar glass substrate functionalized with polyethylene glycol diacrylate with $M_{\mathrm{n}}=700$ (PEGDA 700) at a distance compared to the protein diameter, show internal flows induced by the presence of the other condensate. Time projection over $1\min$ of fluorescent particles inside two adjacent chemically active protein droplets catalyzing the urea-urease reaction (overall enzyme concentration $c_{\mathrm{e}}=0.6\mathsf{\mu }\mathrm{M}$, substrate concentration $c_{\mathrm{s}=100\mathrm{mM}}$). Arrows indicate the internal flow direction. The scale bar corresponds to $10\mathsf{\mu }\mathrm{m}$. Reprinted from Ref. [199]. CC BY 4.0. (c,d) Permeability of giant unilamellar vesicles (GUV) and lipid-coated protein condensates (RL48 protocells). A dye compound, calcein ($20\mathsf{\mu }\mathrm{M}$), 21 base pair DNA ($0.5\mathsf{\mu }$M), and green fluorescent protein (GFP) are treated to (c) GUV and (d) RL48 protocells for $10\min$ before confocal analysis. Lipid coatings (red) and dye-labeled external materials (green) are imaged. The scale bars correspond to $5\mathsf{\mu }\mathrm{m}$. Reprinted from Ref. [116]. CC BY-NC 3.0.

Figure 13

Cellular uptake. (a–c) Uptake kinetics for NIPAM-MAA-5S microgels by HEK293T cells. (a) Fluorescence confocal images 2, 4, 6, and 10 min after microgel application. A donut-shaped region-of-interest (ROI) demarks the cytosolic area of a representative cell. (b) Increase in the fluorescence in the ROI marked in (a). Normalized intensity changes ($\mathsf{\Delta }F/f_0$) as a function of time (black trace) and as a monoexponential fit (red), used to calculate the cell-specific time constant $\tau$. The gray background marks the experimental period represented in (a). (c) Violin plot for the distribution of time constants, starting at $\tau \approx 0\mathrm{s}$. Solid horizontal lines indicate the mean (black) and median (red) values, and the upper and lower quartiles (dashed lines). Adapted with permission from Ref. [238]. Copyright 2020 American Chemical Society. (d) Uptake of liposomes (blue) and extracellular vesicles (green) with and without protein corona (PC), determined using flow cytometry to measure the mean fluorescent intensities characterizing the uptake of the particles into THP1 and moDC cells after $16\mathrm{h}$. Mean values and standard deviations of median fluorescence intensities are shown ($n=3$). Reprinted from Ref. [53]. CC BY 4.0. (e) Quantification of nanodiamonds taken up by macrophages after $12\mathrm{h}$ incubation time, determined using extinction spectroscopy. The data are represented as the mean value ± standard deviation of three independent replicates. Reprinted with permission from Ref. [54]. Copyright 2020 American Chemical Society.

Figure 14

Simulation snapshots of a linear polymer chain with intermediate hydrophobicity translocating through a lipid-bilayer membrane. Used with permission of the Royal Society of Chemistry, from Ref. [63]; permission conveyed through Copyright Clearance Center, Inc.

Figure 15

Wetting, endocytosis, and lipid-bilayer translocation of condensates at liposomes. (a) Translocation of condensates composed or oligoarginine (${\mathrm{R}}_{10}$, $2.7\mathrm{kDa}$) and torula yeast RNA ($\mathrm{tyRNA}$) (red, labelled with DNA ${\mathrm{polyA}}_{15}$-$Cy5$ oligonucleotides, marked with arrows) at liposomes consisting of ${\mathrm{POPC}}_{0.4}/{\mathrm{cholesterol}}_{0.1}/{\mathrm{POPG}}_{0.5}$ (green), labelled with DOPE-AttO 488. The scale bar corresponds to $10\mathsf{\mu }\mathrm{m}$. (b) Morphological state diagram of the interplay between complex condensates and liposomes as a function of their absolute $\zeta$-potential difference $\mathsf{\Delta }\zeta$ and the condensate’s lipid partition coefficient $K_{\mathrm{p}}$. * Denotes a special case of ${\mathrm{R}}_{40}/\mathrm{polyA}$ condensates that were significantly smaller (average diameter $<1\mathsf{\mu }\mathrm{m}$) than most other condensate samples and were found to penetrate the liposome membrane despite a strong surface attraction. Reprinted from Ref. [113]. CC BY 4.0.

Figure 16

Images of direct stochastic optical reconstruction microscopy measurements of adsorbed PNIPMAM microgels at surfaces of different functionalization. The microgel conformation changes from a fried egg shape at hydrophobic surfaces coated with trichloro(1H,1H,2H,2H-perfluorooctyl)silane (FOCTS) or n-octadecyltrimethoxysilane (ODS) to a spherical shape at hydrophilic surfaces coated with PEG. The solid–liquid interfaces are placed at $z=0$. Reprinted with permission from Ref. [245]. Copyright 2019 American Chemical Society.

Figure 17

Snapshots of three systems of a PGN interacting with attractive substrates having: (a) zero (flat), (b) positive (hole/concave), and (c) negative (bump/convex) curvatures. Reprinted from Ref. [248]. CC BY 4.0.

Figure 18

Wrapping of non-spherical particles. (a) Shallow- and deep-wrapped states of a cube at a triangulated membrane. Reprinted with permission from Ref. [252]. Copyright 2014 American Chemical Society. (b) Shallow- and deep-wrapped states of prolate vesicles: attached (green) and free (red) vesicle membrane area. Reprinted from Ref. [256]. CC BY 4.0.

Figure 19

Wrapping diagrams for particles at initially planar membranes. (a) Diagram for a hard spherical particle in the plane of reduced adhesion strength $\tilde{w}$ and reduced membrane tension $\tilde{\sigma }$ close to the triple point $\mathrm{T}$. The dashed line “$\mathrm{W}$” marks the continuous transition at which partial wrapping sets in, the bold solid line “$\mathrm{E}$” indicates the discontinuous transition between partially wrapped and fully enveloped, and the short dashed lines “${\mathrm{S}}_1$” and “${\mathrm{S}}_2$” are the spinodals belonging to “$\mathrm{E}$”. The fine dotted line $\tilde{w}=4+2\tilde{\sigma }$ close to $\mathrm{E}$ indicates where the fully wrapped state has zero energy. Figure adapted with permission from Ref. [272]. Copyrighted by the American Physical Society. (b) Diagram for a prolate elastic particle modeled by a vesicle with reduced volume $v=0.8$ at a membrane with reduced tension $\tilde{\sigma }=0.5$ in the ${\kappa }_{\mathrm{v}}/{\kappa }_{\mathrm{p}}$-$\tilde{w}$-plane. Here, ${\kappa }_{\mathrm{v}}$ is the bending rigidity of the vesicle membrane and ${\kappa }_{\mathrm{p}}$ the bending rigidity of the initially planar membrane. The binding transition ${\mathrm{W}}_1$ separates the non-wrapped (NW) from the shallow-wrapped (SW) regime, the transition ${\mathrm{W}}_2$ the SW from the deep-wrapped (DW) regime, and the envelopment transition ${\mathrm{W}}_2$ the DW from the complete-wrapped (CW) regime. The red-dashed lines indicate the wrapping transitions for a hard particle with reduced volume $v=0.8$. Reprinted from Ref. [256]. CC BY 4.0.

Figure 20

The endocytosis of the oncologic H-1 parovirus (H-1PV) is clathrin-dependent. HeLa cells were infected with H-1PV for $1\mathrm{h}$ at 4 °C to allow H-1PV cell surface attachment but not entry. Cells were then shifted to 37 °C to allow H-1PV cell internalization. The cells were collected every $5\min$ for a total of 30 min and processed for EM analysis. (a) At 4 °C, H-1PV particles are found attached to electro-dense (clathrin-rich) regions on the plasma membrane. (b) In the first 5 min after release at 37 °C, H-1PV particles are detected in early-forming clathrin-coated pits. (c) From 10 to 30 min, H-1PV particles moved into the cells within deeply invaginated clathrin-coated pits that were still connected to the plasma membrane, forming an hourglass-like membrane neck. (d) Later in the infection (10–30 min at 37 °C), H-1PV particles are seen being trafficked within the cell inside clathrin-coated vesicles. Reprinted from Ref. [278]. CC BY 4.0.

Figure 21

Cytoskeleton-driven active uptake processes. (a,b) Uptake of short histidine-rich, pH-responsive beak peptide (HBpep) coacervates by HeLa cells. (a) Filopodial capture visualized using scanning electron microscopy and (b) almost entirely engulfed HBpep-SP coacervates with an additional lysine residue, visualized via ultrathin sections of fixed and resin-embedded cells $15\min$ after incubation. The yellow arrow indicates the membrane around the condensate, and N the nucleus. Reprinted from Ref. [112]. CC BY 4.0. (c) Macropinocytic cups of dictyostelium obtained from a 3D-rendered fluorescent image. Reprinted from Ref. [286]. CC BY 4.0.