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. 2019 Jun 5;5(6):960–969. doi: 10.1021/acscentsci.8b00749

Functionalized Spiky Particles for Intracellular Biomolecular Delivery

Hui-Jiuan Chen †, Tian Hang †, Chengduan Yang †, Di Liu §, Chen Su †, Shuai Xiao †, Chenglin Liu †, Di-an Lin †, Tao Zhang †,∥, Quanchang Jin †, Jun Tao †, Mei X Wu ‡, Ji Wang †,‡,*, Xi Xie †,*
PMCID: PMC6598163  PMID: 31263755

Abstract

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The intracellular delivery of biomolecules is of significant importance yet challenging. In addition to the conventional delivery of nanomaterials that rely on biochemical pathways, vertical nanowires have been recently proposed to physically penetrate the cell membrane, thus enabling the direct release of biomolecules into the cytoplasm circumventing endosomal routes. However, due to the inherent attachment of the nanowires to a planar 2D substrate, nanowire cell penetrations are restricted to in vitro applications, and they are incapable of providing solution-based delivery. To overcome this structural limitation, we created polyethylenimine-functionalized microparticles covered with nanospikes, namely, “spiky particles”, to deliver biomolecules by utilizing the nanospikes to penetrate the cell membrane. The nanospikes might penetrate the cell membrane during particle engulfment, and this enables the bound biomolecules to be released directly into the cytosol. TiO2 spiky particles were fabricated through hydrothermal routes, and they were demonstrated to be biocompatible with HeLa cells, macrophage-like RAW cells, and fibroblast-like 3T3-L1 cells. The polyethylenimine-functionalized spiky particles provided direct delivery of fluorescent siRNA into cell cytosol and functional siRNA for gene knockdown as well as successful DNA plasmid transfection which were difficult to achieve by using microparticles without nanospikes. The spiky particles presented a unique direct cell membrane penetrant vehicle to introduce biomolecules into cell cytosol, where the biomolecules might bypass conventional endocytic degradation routes.

Short abstract

The spiky particles presented a unique direct cell membrane penetrant vehicle to introduce biomolecules into cell cytosol bypassing conventional endocytic degradation routes.

Introduction

The intracellular delivery of various biological effectors (e.g., DNAs, RNAs, proteins, and peptides) is of significant importance for therapeutics,1−3 yet it is challenging because the cell membrane prevents various biomolecules from accessing the cytosol.4−7 Recent advances have spawned novel nanomaterials (e.g., nanoparticles, nanotubes, and suspended nanowires) for intracellular drug delivery.8−15 Conventional methods of the intracellular delivery of biomolecules utilized endocytosis or phagocytosis. However, the biomolecule-bound nanomaterials were frequently trapped within the endosome or phagosome without achieving cytosolic release (Figure 1).16 The biomolecules can only escape into the cytosol and perform their respective functions following the rupture of the endosome or phagosome. However, due to the poorly controlled endosomal disruption process and the enzymatic degradation of biomolecules within the endosome,7,9,17−19 biomolecules delivered through endocytosis or phagocytosis have limited access to cytosol, and transfection efficiency tends to be low.20−24

Figure 1.

Figure 1

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Illustration of biomolecular delivery via PEI-functionalized microparticles. (a) Plain particles deliver biomolecules through endocytosis, where the biomolecule-bound nanomaterials are often trapped within the endosome. Spiky particles deliver biomolecules by NW penetration, where biomolecules can be directly released into the cytosol. (b) Schematic of the fabrication process of spiky particles. (c) SEM image of the produced TiO2 nanostructural bundles. (d) Schematic image and SEM images, (e) TEM image, and (f) optical microscopy image of spiky particles. (g) Schematic and SEM images of plain particles. (h) Schematic and SEM images of rough particles.

Recently, vertical nanowires (NWs) have been developed as a physical platform to penetrate the cell membrane and to provide the direct delivery of biomolecules into the cytoplasm.20−30 Direct cell penetration bypassed endocytotic or phagocytotic pathways, and it avoided endosomal degradation of biomolecules.20,22 Despite early successes of NW penetration in vitro,20,21,25 the attachment of NWs to a planar 2D substrate hinders the usage of this technique in solution-based delivery and in vivo applications. Freely diffusing NWs with a higher aspect-ratio have been shown to suffer from ineffective cellular internalization compared to lower-aspect-ratio nano-objects.24,27,28 Even though the free diffusing NWs (similar to nanoparticles) are taken up by cells through endocytosis, the load is trapped within the endosome, and it lacks direct access to the cytosol.29−35

Thus, we proposed a heterogeneous nanostructure of suspended nanoparticles covered with sharp nanospikes, namely, “spiky particles”, as a unique vehicle to achieve gene delivery by utilizing the sharp nanospikes’ cell membrane penetration capability. The natural particle phagocytosis process encourages spiky particle engulfment and generates force on the nanospikes,15,29,31 which may induce cell membrane penetration and allow biomolecules bound on the particle surface to be directly released into the cytosol (Figure 1). To demonstrate this design, sub-cell-size spiky particles were fabricated through hydrothermal routes. The particles were biocompatible with HeLa cells, macrophage-like RAW cells, and fibroblast-like 3T3-L1 cells. The cells actively engulfed or internalized the particles as revealed by particles–cell interface studies. The polyethylenimine (PEI)-functionalized spiky particles were demonstrated to mediate the direct delivery of fluorescent siRNA into the cytosol, to knock down genes efficiently through the delivery of functional siRNA, and to successfully transfect DNA plasmids. The previously mentioned functions were difficult to achieve by using microparticles without nanospikes. This work demonstrated the successful extension of biomolecular delivery through substrate-based nanowires using a suspended microparticle platform and presented a unique strategy to directly introduce biomolecules into the cytosol, where the biomolecules might bypass conventional endocytic degradation routes (Supporting Information, Figure S1-1).

The spiky particles were fabricated by the assembly of one-dimensional nanostructural bundles via the hydrothermal method (Figure 1b). Briefly, TiO2 powders were treated in 5 M NaOH aqueous solution to form nanostructural bundles at 120 °C for 24 h in a Teflon-lined stainless-steel autoclave (Figure 1c). The nanostructural bundles were treated with 1 M NaOH and 30% H2O2 at 150 °C for 6 h to produce spiky particles. The spiky particles were then agitated with 0.05 M HNO3 and calcinated, and well-dispersed particles were selected for further applications. The spiky particles (namely “spiky”) were observed with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and optical microscopy. As shown in the SEM and TEM images (Figure 1d,e), nanospikes (with diameter ∼20 nm and length ∼200–400 nm) radially protruded from the particles’ surface. The average particle size was 1.5 ± 0.3 μm, with 536 ± 61 spikes/particle. The optical microscopy image revealed that the spiky particles dispersed homogeneously in water (Figure 1f). Another two types of microparticles, plain microspheres (namely, “plain”, with diameter of 1.6 ± 0.1 μm) and particles with rough surface topography (namely, “rough”, with diameter of ∼1.3 ± 0.3 μm), were employed as control samples for a comparison with the spiky particles. The rough particles were produced by vigorously sonicating spiky particles overnight to remove the nanospikes.

HeLa cells were used as the model cell type to study the interactions between the cells and the particles and the potential applications of intracellular biomolecular delivery. The cytotoxicities of spiky particles, plain particles, and rough particles at different doses were investigated via a live/dead cell staining assay, where live cells were stained with calcein AM (green), dead cells with ethidium bromide (red), and cell nuclei with Hoechst (blue). Previously studied spiky particles had limited biological applications due to the cytotoxicity of the ZnO particles.36 However, TiO2 has been generally accepted as a biosafe material, and it is widely applied in the fields of cosmetics, food, and pharmaceutical industries.37−41 In this study, HeLa cells were incubated with particles at different dosages ranging from 0.02 to 0.16 particles/μm2 for 48 h (Figure 2a). Then, HeLa cells were treated with TiO2 nanoparticles ranging from 25 to 200 pg/μm2. ZnO nanoparticles with a dose of 50 pg/μm2 were used as the negative control. Fluorescence microscopy images of live/dead cell staining assays are shown in Figure 2a and section S2 in the Supporting Information; viability results are shown in Figure 2b. Cell viability was higher than 90% for spiky particles, plain particles, and rough particles at every tested dosage. This indicates that TiO2 microparticles did not induce adverse cytotoxicity to HeLa cells compared to negative controls. Additionally, cells incubated with TiO2 nanoparticles at every tested dosage exhibited high cell viability (>95%), suggesting that TiO2 was biosafe for HeLa cells. These results significantly differed from the viability of cells treated with ZnO nanoparticles, in which case the cell viability was reduced to 55.1 ± 3.6%. In addition, HeLa cells cultured with spiky particles of 0.16 particles/μm2 for 96 h displayed a viability of 90.6 ± 3.4%, which is comparable to the control with a viability of 93.5 ± 2.5% after 96 h of incubation.

Figure 2.

Figure 2

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HeLa cell viability and the study of cell–particle interface via SEM. (a) Fluorescence and optical images showing the live/dead assays of HeLa cells upon culturing with spiky particles, plain particles, or rough particles or without particles (control) for 48 h. The dose of the microparticles was 0.16 particles/μm2. In the other two examples, cells were cultured with TiO2 nanoparticles of 200 pg/μm2 for 48 h and with spiky particles of 0.16 particles/μm2 for 96 h. Live cells were stained with calcein AM (green), dead cells with ethidium bromide (red), and cell nuclei were stained with Hoechst (blue). Scale bar: 400 μm. (b) Statistical analysis of HeLa cell viability when incubated with different particles at different doses for 48 h. * indicates cell viability is significantly lower than the control group. p < 0.05; n = 4/group. (c) Proliferation profiles of GFP-expressing HeLa cells upon incubation with spiky particles of 0.16 particles/μm2 were monitored for 72 h with fluorescence microscopy. Scale bar: 400 μm. (d) Quantitative analysis of HeLa cell proliferation rates upon incubation with different microparticles for 72 h. n = 4/group. SEM images showing HeLa cells interfaced with (e) spiky particles, (f) plain particles, and (g) rough particles. The cells were observed to engulf or fully uptake the spiky, plain, and rough particles. Data were presented as mean ± SD. Significance was calculated by one-way ANOVA.

Furthermore, HeLa cells incubated with microparticles spread and proliferated with similar profiles as the control cells. The cell proliferation profiles of GFP-expressing HeLa cells (green fluorescent protein, GFP, for cell tracking) were monitored for 72 h with fluorescence microscopy (Figure 2c and Figures S2-6 and S2-7 in the Supporting Information), and the proliferation rates were quantified. The cells treated with spiky particles, plain particles, and rough particles displayed higher proliferation rates compared to control cells. The above results suggested that the TiO2 nanoparticles were biocompatible with HeLa cells, and they did not disrupt the cells’ spreading and proliferation behaviors. To reveal the cell–particle interaction profiles, HeLa cells interfaced with microparticles, including spiky, plain, and rough particles, were examined with SEM. Cells were incubated with microparticles of 0.08 particles/μm2 for 24 h and then fixed and prepared with critical point drying for SEM imaging. The cells were observed to engulf or fully uptake the spiky, plain, and rough particles. As shown in Figure 2e–g, the cell membrane wrapped around the microparticles, and a few particles appeared to be fully internalized, likely via endocytosis or phagocytosis. During the cell–particle interactions, the sharp nanospikes might induce localized stress leading to membrane penetration and potential intracellular cargo delivery.

The interactions between cells and microparticles were further investigated with confocal fluorescence microscopy (section S3 in the Supporting Information). To visualize the microparticles, the microparticles were first conjugated with amine groups and then covalently coupled with red fluorescent dye (Alexa Fluor 660 fluorescent dye). HeLa cells were incubated with spiky, plain, or rough particles for 3, 6, or 24 h to track particle uptake (Figure 3a and section S3 in the Supporting Information). Cell cytosol was labeled with green fluorescent dye calcein AM. The confocal fluorescence images were reconstructed with both orthographic view and 3D view to visualize the cell–particle interface. The microparticles were observed to be either internalized within the cell, engulfed, or adhered to the cell surface. The particle internalization profiles at different time points (3, 6, and 24 h) were quantified and are shown in Figure 3b. The total volume of the particles within a cell was compared to the total volume of the cell, and this ratio demonstrated the particle uptake as the cell spread at different time points. The results showed that the particle uptake generally increased with time in a near-linear manner. The particle uptake for spiky particles was comparable with rough particles at the three examined time points, suggesting that the presence of long nanospikes did not significantly interfere with the uptake of microparticles. The uptake of the plain particles was almost twice as high as those of the spiky particles and the rough particles; this is likely due to the smooth surface and better dispersity of the fabricated plain particles which facilitated particle internalization.

Figure 3.

Figure 3

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Interface between HeLa cells and microparticles revealed via confocal fluorescence microscopy. (a) HeLa cells (cytosol labeled with calcein AM) were incubated with spiky, plain, or rough particles for 3, 6, or 24 h to monitor the internalization of the microparticles. The confocal fluorescence images were reconstructed with orthographic view (left) and 3D view (right). Green fluorescence, cell cytosol; red fluorescence, microparticles. (b) Quantification of the particles’ internalization profiles at different time points (3, 6, and 24 h). n = 8/group. (c) Interface between HeLa cell membrane (green fluorescence) and spiky particles (red fluorescence) for 24 h of incubation with particles. (d) Interface between the HeLa cells’ actin network (green fluorescence) and spiky, plain, or rough particles (red fluorescence) for 24 h of incubation with particles. (e) Interface between HeLa cells’ endosome (green fluorescence) and spiky, plain, or rough particles (red fluorescence) for 24 h of incubation with particles. Data were presented as mean ± SD. Significance was calculated by one-way ANOVA.

To visualize cell engulfment of the microparticles, the cell membranes of the HeLa cells were specifically labeled with green fluorescent protein (GFP) by transfecting the cells with BacMam 2.0. Cells were incubated with spiky particles for 24 h and then imaged (Figure 3c). We observed that some spiky particles were wrapped by the cell membrane, and this is consistent with the interface results revealed by SEM. On the other hand, particle phagocytosis and internalization by cells are generally mediated by the driving force generated by the local assembly of actin filaments.42 In this study, HeLa cells were fixed and stained with phalloidin conjugated with Alexa Fluor 488 (green fluorescent dye) on their actin network to examine their interfacial profiles with spiky, plain, and rough particles. Many microparticles were found to stick on top of the actin network, while some of them were observed to be internalized (Figure 3d). Following internalization, particles trapped within the endosomes were tracked, and the endosomes were labeled with GFP by using BacMam 2.0 that consisted of a fusion construct of Rab5a and emGFP used to transfect the cells (Figure 3e). Some internalized particles located within the endosomes; this indicated that the particles underwent phagocytosis or endocytosis (Figure S3-7 in the Supporting Information). These cell–particle interface characterization results suggested that HeLa cells actively engulfed spiky, plain, and rough particles. The nanospiky structure did not significantly prevent particle uptake, and the nanospikes interfaced closely with the cell membrane and its actin network.

To demonstrate the potential of spiky particles as gene delivery vehicles, the particles were evaluated in three types of assays: delivering fluorescent dye-conjugated siRNA, siRNA for GFP knockdown, and GFP plasmids into HeLa cells. Microparticles were coupled with 3-glycidoxypropyl trimethoxysilane (GPS) and then conjugated with PEI (MW ∼ 800 Da) to present amine groups for biomolecular binding (Figures S4-1 and S4-2 in the Supporting Information). The terminal −NH2 groups could provide nonspecific electrostatic or van der Waals binding to biomolecules and allow subsequent release of the bound biomolecules.43 In addition, as a cationic polymer, the PEI surface coating might reduce the energy barrier for cell membrane rupture and thus facilitate cell membrane penetration. In the first assay, 20 μM siRNA conjugated with red fluorescent dye, cyanine 5, was incubated overnight with PEI-coated microparticles including spiky, plain, and rough particles for coupling. The microparticles were supplemented to HeLa cell cultures for 6 h. The fluorescent dye distribution within the cells were examined with confocal fluorescence microscopy (Figure 4a and Figure S4-3 in the Supporting Information). The cell cytosol was stained with calcein AM (green fluorescence) to identify the cell border, and the cell border was indicated with dashed lines in the red fluorescence images. Many red fluorescent particles appeared to be engulfed or internalized by the cells, visualized as individual puncta within the cell. For the spiky particles, some cells displayed a uniform cytosolic distribution of red fluorescent dye (these cells were indicated with yellow dashed lines), implying that the fluorescent siRNA was likely directly delivered into the cytosol. This result was consistent with a previous report that delivery through cell penetration by a vertical nanowire array would resulted in the uniform cytosolic distribution of fluorescent dye (see ref (22)). This result is in contrast to the plain and rough particles groups, where red fluorescence was only visualized as individual puncta without a uniform cytosolic distribution. The fluorescent siRNA along with the plain or rough particles was likely trapped within the phagosome or endosome and failed to escape to the cytosol. As another control group, fluorescent siRNA was directly added to the cell culture. After 6 h of incubation, the siRNA presented punctate fluorescence with a profile that had been widely observed on dye delivery through endocytosis. In addition, HeLa cells were first cultured with nonfluorescent spiky particles for 6 h, followed by the supplementation of fluorescent siRNA and incubation for another 6 h. The cells exhibited a punctate fluorescence profile as well, suggesting that the siRNA was mainly entering cells through endocytosis,22 rather than through the interstitial space between the cell membrane and the nanospikes. These results supported the hypothesis that PEI-functionalized spiky particles might induce cell membrane penetration, and the some of the bound biomolecules could be directly released into the cytosol bypassing the conventional endosome or phagosome entrapments. In our hypothesis, the engulfment and uptake of particles relied on the active phagocytosis or endocytosis mechanism, while the delivery of biomolecules into cytosol was more likely a mixed process of direct penetration and endosomal escape. During the particle engulfment, the nanospikes on the PEI-functionalized particle surface might induce some permeation on the cell membrane. Some biomolecules bound on the particles’ surface may thus release into the cell cytosol. After the initial particle engulfment, the particles were gradually internalized and finally entrapped in the endosome, but some biomolecules bound on these particles might be eventually released into cytosol through conventional endosomal escape (section S1 in the Supporting Information).

Figure 4.

Figure 4

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Delivery of genetic materials into HeLa cells via PEI-functionalized microparticles. (a) HeLa cells were incubated with fluorescent siRNA (red)-complexed microparticles (spiky, plain, and rough particles), purely siRNA dye, or a combination of siRNA dye and spiky particles. Confocal fluorescence microscopy images showing the fluorescent dye distribution within the cells. Cell cytosol was stained with calcein AM (green fluorescence) to identify the cell border. Cell borders were indicated with dashed lines in the red fluorescence images. For spiky microparticles, yellow dashed lines indicated that cells displayed a uniform cytosolic distribution of red fluorescent dye. Scale bar: 10 μm. (b) GFP-expressing HeLa cells were incubated with microparticles complexed with GFP-targeting siRNA for 48 h. Fluorescence microscopy imaging showing the GFP knockdown results using different particle treatments. Scale bar: 400 μm. (c) The GFP knockdown results were quantitatively analyzed with a GFP ELISA assay. * indicates that the GFP expressing level was significantly lower than the other groups. p < 0.05; n = 2/group. (d) Quantification of DNA binding to PEI-functionalized microparticles. (e) HeLa cells were treated with DNA plasmid-complexed microparticles for GFP transfection. Fluorescence images showed the GFP transfection results. Cells were stained with calcein red to determine the cell number. Scale bar: 400 μm. (f) Quantification of DNA transfection efficiency utilizing different particle treatments. * indicates that the GFP expressing level was significantly higher than the other groups. p < 0.05; n = 4/group. Data were presented as mean ± SD. Significance was calculated by one-way ANOVA.

As the second assay, PEI-functionalized microparticles including spiky, plain, and rough particles were bound with GFP-targeting siRNA and released to silence GFP expression in HeLa cells (Figure S4-4 in the Supporting Information). As comparisons, blank microparticles without coupling siRNA, microparticles coupled with negative siRNA whose sequence was scrambled, and microparticles coupled with positive GFP-targeting siRNA were prepared and added to GFP-expressing HeLa cells. For the siRNA transfection assay, PEI-functionalized spiky particles were applied with the dose of 0.16 particles/μm2. The cells were incubated with the microparticles–siRNA complex for 48 h, and the GFP knockdown results were examined with fluorescence microscopy (Figure 4b and Figure S4-5 in the Supporting Information). The spiky particles coupled with GFP-targeting siRNA induced an obvious GFP knockdown compared to spiky particles with negative siRNA or without siRNA. By contrast, the plain or rough particles–siRNA complex did not knock down GFP expression compared to blank microparticles groups or the control cells without any treatment of the microparticles–siRNA complex. The GFP knockdown results were quantitatively analyzed with a GFP ELISA assay (Figure 4c). The results showed that the spiky particles coupled with GFP-targeting siRNA induced ∼64.3% knock down of GFP compared to the control groups, while the other microparticle groups showed <15% reduction of GFP expression. As a positive control, commercial Lipofectamine mediated a GFP knockdown efficiency of 85.8 ± 6.1% (Figure S4-6 in the Supporting Information). These results demonstrated that spiky particles effectively delivered GFP silencing siRNA into HeLa cells, which was challenging for conventional microparticles.

Additionally, PEI-functionalized microparticles were coupled with DNA plasmid and used to transfect HeLa cells to express GFP. The microparticles were incubated with 500 ng/μL DNA plasmid overnight, and the DNA binding was determined to be (12.2 ± 1.7) × 10–8 μg per spiky particle, (10.6 ± 1.0) × 10–8 μg per plain particle, and (7.7 ± 1.6) × 10–8 μg per rough particle, as measured and analyzed using a Nanodrop instrument (Figure 4d and Figure S4-7 in the Supporting Information). Microparticle–DNA constructs were added to HeLa cells. Cells were incubated with microparticles for 48 h, and the results were imaged with fluorescence microscopy. The HeLa cells were stained with calcein red to determine total cell number and to quantify transfection efficiency (Figure 4e and Figures S4-8 and S4-9 in the Supporting Information). Commercial Lipofectamine-mediated DNA transfection was also conducted as a positive control. Results showed that the spiky particles induced GFP expression in 12.7 ± 4.0% cells, while the plain particles and rough particles only provided 1.7 ± 1.2% and 2.0 ± 1.7% GFP transfection efficiency, respectively (Figure 4f). These results indicated that spiky particles mediated DNA transfection in HeLa cells, which was difficult to achieve with both plain particles and rough particles (Figure S4-10 in the Supporting Information).

Furthermore, cell–particle interfacing and the application of siRNA delivery and DNA plasmid transfection were demonstrated on two more cell types, RAW 264.7 cells, a macrophage-like cell line with active phagocytic ability, and 3T3-L1 cells, a fibroblast-like cell line. RAW cells were cultured with spiky particles or plain particles of 0.08 particles/μm2 for 24 h, and the cell–particle interface was characterized with SEM imaging. Many spiky particles or plain particles were found to be fully internalized by the RAW cells, indicating aggressive phagocytosis of the microparticles by the cells (Figure 5a). To test cytotoxicity, RAW cells were cultured with different types of microparticles ranging from 0.01 to 0.08 particles/μm2 for 48 h. The viability of RAW cells following incubation with spiky, plain, and rough particles at different doses was evaluated with live/dead cell staining assay, similar to the HeLa cells. The fluorescence microscopy images of RAW cell viabilities are shown in Figure 5b and section S5 in the Supporting Information, and the results are quantified in Figure 5c. The RAW cells exhibited a viability higher than 93% for most microparticles of various dosages, except that the viability was 87.5 ± 2.7% for rough particles at 0.08 particles/μm2. This result suggested that the microparticles did not significantly impair the cell viability of RAW cells. The interfaces between RAW cells and the spiky particles were further examined with confocal fluorescence microscopy. The spiky particles were labeled with red fluorescent dye, and the RAW cells’ cytosol was stained with green fluorescent dye calcein AM (Figure 5d and section S6 in the Supporting Information). RAW cells were incubated with spiky particles of 0.08 particles/μm2 for 24 h. The cell bodies were observed to be fully embedded with spiky particles, indicating that the particles were engulfed or internalized by the cells. To demonstrate the applications of spiky particle-mediated drug delivery into RAW cells, the PEI-coated microparticles were complexed with fluorescent siRNA and treated with RAW cells for 6 h (Figure 5e and Figure S7-1 in the Supporting Information). For confocal fluorescence microscopy, the cytosol was stained green with fluorescent dye to identify the cell border (also indicated with dashed lines). Similar to the siRNA dye delivery experiment using HeLa cells, in addition to the individual puncta associated with fluorescent particles within the cell, the cells (indicated with yellow dashed lines) treated with spiky particles exhibited a uniform cytosolic distribution of red fluorescent dye. On the other hand, cells treated with plain or rough particles did not display a uniform cytosolic distribution, indicating a failure to deliver fluorescent siRNA into the cytosol. This result was consistent with the results demonstrated by the HeLa cells that the nanospike offered direct delivery of fluorescent siRNA into the cytosol; this was difficult to achieve using conventional microparticles. To demonstrate functional gene delivery, microparticles complexed with GFP-encoded DNA plasmid were applied to transfect RAW cells. RAW cells were incubated with microparticle–DNA constructs for 48 h. Cell nuclei were stained with Hoechst stain to count the cell number (Figure 5f and Figure S7-2 in the Supporting Information). The RAW cells were transfected with GFP by the spiky particles with a transfection efficiency of 2.6 ± 0.6%, while plain particles and rough particles were incapable of providing DNA transfection to RAW cells (Figure 5g and Figure S7-3 in the Supporting Information). These results indicated that the spiky particles were able to interact with macrophage-like RAW cells and mediated the direct delivery of genetic materials into the cellular cytosol.

Figure 5.

Figure 5

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(a–g) Particles–cell interface and application of gene delivery on RAW cells. (a) SEM images showing RAW cells interfaced with spiky particles or plain particles. (b) Fluorescence and optical images showing the live/dead cell assays of RAW cells after culturing with microparticles for 48 h. Particle doses were 0.08 particles/μm2. Green, live cells; red, dead cells; blue, cell nuclei. Scale bar: 400 μm. (c) Statistical analysis of RAW cell viabilities. n = 4/group. (d) Confocal fluorescence microscopy images showing the interface between RAW cells and spiky particles. Red, particles; green, cytosol. (e) Microparticles-mediated delivery of fluorescent siRNA (red) into RAW cells. Confocal fluorescence microscopy images showing the fluorescent dye distribution within the cells. Green, cytosol; white dashed lines, cell border; yellow dashed lines, cells displaying uniform cytosolic distribution of red fluorescent dye. Scale bar: 10 μm. (f) Microparticles-mediated delivery of DNA plasmid into RAW cells. Fluorescence images showed the GFP transfection results. Blue, cell nuclei. Scale bar: 400 μm. (g) Quantification of DNA transfection efficiency using different particle treatments. * indicates that the GFP expression level was significantly higher than the other groups. p < 0.05; n = 4/group. (h–l) Particles–cell interface and application of gene delivery on 3T3-L1 cells. (h) Fluorescence and optical images showing the live/dead cell assays of RAW cells upon culturing with microparticles for 48 h. Particle doses were 0.08 particles/μm2. Green, live cells; red, dead cells; blue, cell nuclei. Scale bar: 400 μm. (i) Statistical analysis of 3T3-L1 cell viabilities. n = 4/group. (j) Confocal fluorescence microscopy images showing the interface between 3T3-L1 cells and particles. Red, particles; green, cytosol. (k) Microparticle-mediated delivery of DNA plasmid into 3T3-L1 cells. Fluorescence images showed the GFP transfection results. Red, cell cytosol stained with calcein red. Scale bar: 400 μm. (l) Quantification of DNA transfection efficiency using different particle treatments. * indicates that the GFP expression level was significantly higher than the other groups. p < 0.05; n = 4/group. Data were presented as mean ± SD. Significance was calculated by one-way ANOVA.

Similarly, cell–particle interfacing and the gene transfection were investigated on fibroblast-like 3T3-L1 cells. The 3T3-L1 cells were cultured with microparticles of 0.04 and 0.08 particles/μm2 for 48 h (Figure 5h and Figure S8 in the Supporting Information). The live/dead cell assay revealed that the cell viabilities for most of the particle groups were between 70% and 80%, indicating that the viabilities were slightly affected but not significantly compromised by the addition of the microparticles (Figure 5i). The interface between 3T3-L1 cells and various microparticles after 24 h of culture was observed with confocal fluorescence microscopy by labeling microparticles with red fluorescent dye and staining the 3T3-L1 cell cytosols with green fluorescent dye (Figure 5j and Figure S9 in the Supporting Information). The spiky, plain, and rough particles were all observed to be engulfed or internalized by the 3T3-L1 cells. Lastly, PEI-coated microparticles were complexed with GFP-encoded DNA plasmid and then applied to the 3T3-L1 cells for 48 h. The cells were stained with calcein red to facilitate cell number counting (Figure 5k and Figure S9 in the Supporting Information). Spiky particles induced GFP transfection on 5.4 ± 2.1% of the cells, while plain particles and rough particles were incapable of providing DNA transfection (Figure 5l and Figure S10 in the Supporting Information). These results were consistent with the results on HeLa cells and RAW cells, and the nanospikes selectively provided efficient transfection of genetic materials into biological cells.

Conclusion

In summary, spiky particles were fabricated and demonstrated as a unique drug delivery vehicle by combining the advantages of nanowire penetration and the suspension nature of microparticles. This presented a direct cell membrane penetrant vehicle in contrast to conventional nanoparticle delivery. The cell cytotoxicity upon incubation with spiky particles and the cell–particle interfaces were systematically evaluated on model cell types including HeLa cells, macrophage-like RAW cells, and fibroblast-like 3T3-L1 cells. Cells engulfed and internalized the spiky particles, indicating active interactions between the cells and the particles. The PEI-functionalized spiky particles successfully provided the direct release of fluorescent siRNA into the cytoplasm, functional siRNA for gene knockdown, and DNA plasmid transfection. The previously mentioned capabilities were difficult to achieve using microparticles without nanospikes. This work successfully extended delivery through direct cell penetration by substrate-based NWs to suspending microparticles, which presented a unique strategy to directly introduce biomolecules into the cell cytosol. Future works of fabricating spiky particles with other biodegradable materials and using new strategies to enhance NW cell penetrations would enable the spiky particle to become a universal in vivo delivery vehicle.

Acknowledgments

This work is supported in part by the National Natural Science Foundation of China (Grant 61771498, 51705543 and 31530023) to X.X., T.H., and J.T. and National Institutes of Health Grants AI089779 and AI 113458 and department funds to M.X.W. The authors wish to thank the Youth 1000 Talents Program of China and 100 Talents Program of Sun Yat-Sen University (76120-18821104) to X.X., 100 Talents Program of Sun Yat-Sen University to J.W., Youth Teacher Training Program of Sun Yat-Sen University (18lgpy18, 18lgpy21), and Wellman Center Photopathology Core for their help in the histology analysis during this project. The authors also would like to thank Prof. Nick Melosh at Stanford University for discussing the ideas of spiky particles and providing helpful suggestions.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscentsci.8b00749.

  • Experimental method and additional figures including a process illustration, fluorescence and optical images, TGA results, quantification of DNA binding, and SEM images (PDF)

Author Contributions

⊥ H.-J.C. and T.H. contributed equally to this work.

Author Contributions

H.-J.C., T.H., J.W., and X.X. designed experiments, analyzed data, and wrote the manuscript. H.-J.C., T.H., C.D.Y., D.L., C.S., S.X., and C.L. performed experiments. D.-A.L., T.Z., Q.C.J., J.T., H.-J.C., and T.H. performed statistical analyses of data sets and aided in the preparation of displays communicating data sets. H.-J.C., T.H., J.W., and X.X. provided conceptual advice. J.W. and X.X. supervised the study. All authors discussed the results and assisted in the preparation of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

oc8b00749_si_001.pdf (4.2MB, pdf)

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