Biocompatible two-dimensional platelets with tunable sizes from polycarbonate-based block copolymers

Abstract

Two-dimensional (2D) nanoparticles have received considerable attention due to their versatile applications ranging from catalysis, optoelectronics to nanomedicine. However, it remains challenging to access size tunable flat nanostructures with spatially tailored chemistries. The seeded-growth method, “living” crystallization-driven self-assembly (CDSA) has emerged as a promising approach for preparing well-defined 1D and 2D core–shell micellar assemblies from crystallizable block copolymers (BCPs). Nevertheless, the development of biocompatible aliphatic polycarbonates, such as poly(trimethylene carbonate) (PTMC), as core-forming blocks for CDSA is considerably less explored and represents a key challenge due to their low crystallinity. Herein, we report the development of poly(dimethyltrimethylene carbonate) (PDTC) as a crystallizable core-forming block through the introduction of side chains to PTMC. The BCPs containing crystallizable PDTC were shown to undergo living CDSA to prepare uniform and size-controlled 2D platelets. In addition, uniform segmented platelets with spatially localized coronal chemistries were successfully constructed. The colloidal stability of the platelets in aqueous solution allowed for an assessment of their toxicity toward healthy WI-38 and cancerous U-87 MG cells. These studies reveal that PDTC nanostructures exhibit no discernible cytotoxicity and excellent biocompatibility, indicating great potential for biomedical applications.

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Introduction

Two-dimensional (2D) nanostructures have been extensively studied in the fields of catalysis,^1,2 sensing,^3,4 optoelectronics,^5–8 and biomedical materials.^9,10 Recently, considerable attention has been devoted to the use of 2D nanostructures in biomedical applications due to their prolonged circulation times in the bloodstream, reduced accumulation in the liver, and reduced immune recognition.^11–15 However, the controlled synthesis of biocompatible 2D nanostructures with predictable morphologies, sizes, and compositions is challenging.^16,17

Crystallization-driven self-assembly (CDSA) enables amphiphilic block copolymers (BCPs) with a crystalline core-forming block to drive the solution self-assembly of low curvature morphologies, such as one-dimensional (1D) nanofibers and 2D platelets.¹⁷ Using this approach, 2D platelets are preferentially formed from crystalline–coil BCPs with block ratios close to 1 : 1 or by tuning the solvent ratio to facilitate crystallization.^4,18–26 The size of the 2D nanoparticles synthesized by CDSA can be controlled using a seeded-growth approach, termed ‘living’ CDSA.^21,26–33 This versatile and robust method has been used to prepare a wide range of well-defined and complex 2D platelets with tailored coronal chemistries and modular multifunctionality, including tracking capabilities and cargo encapsulation.^{11,13,14,33,34} To date, a range of biocompatible and biodegradable crystalline core-forming blocks have been shown to form 2D platelets by living CDSA. These include poly(L-lactide) (PLLA),^8,18,35,36 poly(ε-caprolactone) (PCL)^{26,34,37–41} and poly(p-dioxanone) (PPDO).²⁰

Aliphatic polycarbonates have been widely explored as a promising class of materials for biomedical applications due to their low inherent toxicity, biodegradability, biocompatibility and slow biodegradation times compared to PLLA analogs.^42–44 Among the synthetic aliphatic polycarbonates reported in the literature, poly(trimethylene carbonate) (PTMC) has been extensively explored as a drug delivery device and scaffold for tissue engineering.^45–48 PTMC is an excellent candidate due to its in vivo biodegradation behavior, biocompatibility, high flexibility and toughness.^47,48 However, PTMC is a predominantly amorphous polymer with a low glass transition temperature (T_g = ca. −20 °C) and is only semi-crystalline in the stretched state.^49,50 Its amorphous nature limits its application as a crystalline core-forming block for CDSA. Changes to the chemical structure have been demonstrated to allow for the crystallization of a previously amorphous core-forming block.⁵¹ Previously, we have identified a PTMC derivative, poly(fluorene trimethylene carbonate) (PFTMC), as an excellent crystalline core-forming block for use in living CDSA to form a wide variety of functional nanofibers.^50,52–58 Recently, we have reported low dispersity PFTMC-b-PDMAEMA (poly(2-(dimethylamino)ethyl methacrylate) nanofibers of controlled length (22–1300 nm) and demonstrated their length-dependent antimicrobial activity as well as their antibiotic and nucleic acid delivery capabilities.^55–58 Additional properties such as stability, enzymatic degradability and low inherent toxicity show the excellent biocompatibility of these nanofibers.^50,55 Despite these promising properties, the hydrolytic biodegradation product of PFTMC, 9H-fluorene-9,9-dimethanol, revealed increased cytotoxicity with IC₅₀ values (the amount of material required to inhibit cell viability by 50%) of 1.01 mM and 0.45 mM against HeLa and WI-38 cell lines, respectively.⁵⁰ Next to well controlled 1D nanofibers, there is only one example of polydisperse 2D nanoribbons of PFTMC-b-poly(ethylene glycol) (PEG) upon increasing the core-to-corona ratio to 1 : 2.⁵⁰ However, to date, no size controlled 2D nanostructures prepared by living CDSA with a polycarbonate-based core-forming block have been reported. Therefore, developing a new crystalline polycarbonate-based core-forming block for living CDSA to yield precisely controlled 2D platelets with less harmful biodegradation products is the inspiration for the present work. We envisioned that alternative polycarbonate-based crystalline core-forming blocks could be developed through the introduction of different side groups to PTMC to improve its crystallinity (Fig. 1).

Fig. 1 Schematic illustration of increasing the crystallinity of poly(trimethylene carbonate) (PTMC) derivatives with different side groups.

Herein, we investigated the steric hindrance effect of different side groups of PTMC derivatives (e.g. methyl, ethyl, n-propyl, n-butyl, cyclopropane and cyclobutane groups) on the homopolymer crystallinity. Poly(2,2-dimethyltrimethylene carbonate) (PDTC) showed improved crystallinity over the PTMC derivatives and was explored as a core-forming block for CDSA. Living CDSA of amphiphilic BCPs of PDTC-b-PEG was performed and it yielded precisely controlled 2D platelets. Additionally, well-defined segmented 2D platelets with spatially localized coronal chemistries were obtained via sequential living CDSA from PDTC-based BCPs with different corona functionalities. As the hydrophilic PEG corona provides colloidal stability of the nanostructures in aqueous media, the toxicity of the 2D platelets towards WI-38 and U-87 MG cells was evaluated. These experiments revealed no discernible cytotoxicity of the nanomaterials towards either cell line, showing their great potential for further biomedical applications such as drug delivery vehicles.

Results and discussion

Crystallization behavior of polycarbonate homopolymers with different side groups

Homopolymers of PTMC derivatives were prepared via ring opening polymerization (ROP). The six-membered aliphatic carbonate monomers were obtained from their corresponding diols (Fig. S1–S6†). Changes to the chemical structure of the diols allow us to introduce different side chains or cyclic groups into the backbone of the six-membered aliphatic carbonate monomers and homopolymers, respectively.

The number-average degree of polymerization (DP_n) of each polymer was determined by ¹H NMR spectroscopy (Table 1, Table S1 and Fig. S7A–S12A†) while gel permeation chromatography (GPC) analysis revealed low dispersity values (Đ_m) between 1.14 and 1.30 (Table 1, Table S1 and Fig. S7B–S12B†). The crystallinity of each homopolymer was investigated by differential scanning calorimetry (DSC). DSC revealed that different side chains have a significant influence on the T_g and melting transition temperature (T_m) of the polycarbonate analogs (Table 1, Table S1 and Fig. S7C–S12C and S13†). All PTMC derivatives with different side chain lengths show increased T_g values in comparison with unsubstituted PTMC (T_g = −19 °C), while the dimethyl-substituted polycarbonate (PDTC) showed the highest T_g value of 27 °C. However, with increasing side chain length, the T_g values decrease from 27 °C to −11 °C. This phenomenon is well known, as longer alkyl chains decrease the frictional interactions between the polymer chains resulting in lower T_g values.⁵⁹ Also, we introduced cyclic side groups such as cyclopropane and cyclobutene; the former exhibited an increased T_g value of 51 °C, while the latter showed no T_g value due to annealing of the homopolymer sample (Table S1†). Additionally, crystallization points at 87 °C and 175 °C were observed for the cyclopropane and cyclobutene derivatives, respectively (Table S1†). Notably, these homopolymers do not follow the same trend as the PTMC derivatives with increasing side chain length, and due to their poor solubility, they were not studied further (Fig. S14†). These findings reveal that the introduction of side chains and cyclic side groups successfully restricts polymer backbone flexibility and improves their crystallinity.⁶⁰ PDTC was the only PTMC derivative with side chains that exhibited a T_c of 98 °C. This suggests that PDTC crystallizes more readily than the other PTMC derivatives with different side chains as the DSC experiment for each homopolymer was performed under the same conditions. Due to its improved crystallinity, PDTC was chosen as the core-forming block for the following living CDSA studies. In addition, complementary CDSA studies with PDEC as the core-forming block were performed to explore the effect of side chain length on the self-assembly properties.

Table 1 Synthesis of polycarbonate homopolymers via organocatalytic ROP

R	Polymer^c	M n ^d (g mol^−1)	Đ m ^d	T m ^e (°C)	T c ^e (°C)	T g ^e (°C)
a TU (N-(3,5-trifluoromethyl)phenyl-N-cyclohexyl-thiourea) was used as the catalyst, and DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) was used as the base for the polymerization. b DPP was used as the catalyst for the preparation. c Determined by ^1H NMR spectroscopy. d Determined by GPC versus polystyrene standards. e Determined from the DSC thermogram. f Two temperatures indicate two distinct peaks in the DSC data, suggesting the coexistence of two crystal forms.
H^a	PTMC65	8630	1.16	—	—	−19
Me^a	PDTC88	14 550	1.30	94, 109^f	98	27
Et^a	PDEC35	7780	1.13	84, 95^f	—	−6
n-Pr^b	PDPC58	11 400	1.16	104	—	−7
n-Bu^b	PDBC26	5210	1.14	47	—	−11

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Synthesis and self-assembly of PDTC-b-PEG and PDEC-b-PEG block copolymers

To investigate the effect of block ratio on the CDSA of PDTC-b-PEG and PDEC-b-PEG polymers, a series of well-defined BCPs with different core-forming block lengths were prepared by ROP (Table 2 and Table S2†). Due to the living characteristic of the ROP, increasing amounts of the cyclic carbonate monomers DTC and ETC yielded higher DP_n for the core forming blocks, respectively (Fig. S15–S23†). For each BCP, the DP_n of the PDTC and PDEC blocks was determined using ¹H NMR spectroscopy, while GPC analysis revealed low dispersity values (Đ_m) below 1.1. These low dispersity BCPs are ideal for studying their self-assembly process in solution as they should yield morphologically pure nanoparticles.^61,62

Table 2 Synthesis of PDTC-b-PEG BCPs via organocatalytic ROP

Block polymer	PDTC DPn ^a	M n ^b (g mol^−1)	Đ m ^b	PDTC : PEG block ratio^a
a Determined by ^1H NMR spectroscopy. b Determined by GPC versus polystyrene standards.
PDTC10-b-PEG124	10	7770	1.05	1 : 12
PDTC16-b-PEG129	16	9600	1.07	1 : 8
PDTC28-b-PEG130	28	11 750	1.07	1 : 5
PDTC49-b-PEG130	49	13 700	1.06	1 : 3
PDTC92-b-PEG132	92	20 310	1.06	1 : 1

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Next, the solution state self-assembly behavior of each BCP was investigated by attempting to form micelles via homogeneous nucleation in mixtures of MeOH and THF (Fig. 2). Under these conditions, MeOH acts as a selective solvent for the PEG block to promote self-assembly, while a small amount of THF acts as a common solvent for both blocks to promote PDTC and PDEC crystallization.²² The solutions for homogeneous nucleation experiments were prepared at 0.5 mg mL⁻¹ in 85 : 15 MeOH : THF (v : v). The resulting solutions were dropcast onto carbon coated copper grids and stained with uranyl acetate (3 wt% in MeOH) for transmission electron microscopy (TEM) analysis. Uranyl acetate stain was used to increase the contrast of low electron density PDTC_m-b-PEG_n- and PDEC_m-b-PEG_n-micelles relative to the carbon film, allowing better visualization of the nanostructures. As a result, the micelle core appears bright against the dark background as uranyl acetate only penetrates the PEG-corona but not the PDTC and PDEC micelle cores, repsectively.⁵⁰ As shown in Fig. 2, the morphologies of the resulting micelles prepared from the PDTC_m-b-PEG_n BCPs exhibited a clear transition from spherical micelles to 1D nanofibers, to 2D platelets as core-to-corona ratios increased. For example, PDTC₁₀-b-PEG₁₂₄ with the lowest core-to-corona ratio of 1 : 12 formed spherical micelles with a diameter of ca. 28 nm (Fig. 2A). However, short nanofibers (ca. 76 and 127 nm) were observed for PDTC₁₆-b-PEG₁₂₉ and PDTC₂₈-b-PEG₁₃₀ with core-to-corona ratios of 1 : 8 and 1 : 5, respectively (Fig. 2B and C). A further increase of the core-to-corona ratio to 1 : 3 and 1 : 1 for PDTC₄₉-b-PEG₁₃₀ and PDTC₉₂-b-PEG₁₃₂ yielded 2D platelets (Fig. 2D–F). Such morphological transition can be qualitatively explained by the chain packing theory proposed by Vilgis and Halperin,⁶³ which means a decrease in crowding of the tethered PEG corona due to the increased length of the PDTC core block will result in a less curved core–corona interface. This phenomenon is consistent with the self-assembly results of other crystalline–coil BCPs.^19,20,64 The resulting micelles prepared from the PDEC_m-b-PEG_n BCPs exhibited a similar transition from spherical micelles to 1D nanofibers, to 2D nanoribbons as the core-to-corona ratio increased (Fig. S24†). Notably, the 2D nanoribbons of PDEC₉₅-b-PEG₉₈ exhibit higher aspect ratios than the 2D platelets of the PDTC analogue. However, due to unfavorable properties such as self-nucleation (Fig. S24C and D†) and poor colloidal stability (Fig. S25†), these BCPs were not further studied. Next, the height of 2D platelets formed by PDTC₉₂-b-PEG₁₃₂ was analyzed by atomic force microscopy (AFM) revealing them to be flat and uniform with a height of ca. 8 nm (Fig. S26A†). By dividing the height of the micelle by the average chain length of the PDTC core, the amount of chain folds can be calculated. In its lowest energy helical conformation, two repeating units of PDTC exhibit a length of 0.95 nm.⁶⁵ This reveals an average unit length of 0.475 nm per monomer. This corresponds to an average chain length for the PDTC₉₂-core of ca. 43.7 nm, which is approximately five times the height of the micelle core, suggesting that it undergoes 4 chain folds. Additionally, X-ray diffraction (XRD) measurements revealed the crystalline nature of these 2D platelets (Fig. S26B†).⁶⁴

Fig. 2 TEM images of micelles prepared via homogeneous nucleation in MeOH : THF (v : v) 85 : 15 at 0.5 mg mL⁻¹ by heating polymer samples to 70 °C for 2 h before cooling to 23 °C over a period of 2 h and aging over 24 h. (A) PDTC₁₀-b-PEG₁₂₄. (B) PDTC₁₆-b-PEG₁₂₉. (C) PDTC₂₈-b-PEG₁₃₀. (D) PDTC₄₉-b-PEG₁₃₀. (E) PDTC₉₂-b-PEG₁₃₂. (F) PDTC₉₂-b-PEG₁₃₂ (MeOH : THF (v : v) 95 : 5). Scale bars = 500 nm. TEM images were obtained with uranyl acetate staining (3 wt% in MeOH).

Living CDSA of PDTC₉₂-b-PEG₁₃₂ block copolymers

After the successful preparation of 2D platelets via CDSA, living CDSA (seeded-growth) was employed to fabricate 2D platelets of precisely controlled size (Fig. 3A). Seed platelets were prepared by sonication of disperse 2D platelets derived from a PDTC₉₂-b-PEG₁₃₂ solution (0.5 mg mL⁻¹ in 95 : 5 MeOH : THF) for 3 h at 0 °C. TEM analysis revealed seed micelles with a number-average area (A_n) of 0.51 × 10⁴ nm² and low area dispersity (Đ_A = 1.22) (Fig. S27†). As we have previously shown, the temperature significantly impacts the living CSDA (seeded growth) process as it influences the rate of crystallization and suppresses unimer aggregation.^20,66,67 Therefore, the effect of temperature on the seeded growth process of 2D platelet seeds of PDTC₉₂-b-PEG₁₃₂ was evaluated. For this, seed micelles were kept at three different temperatures (23, 30 and 40 °C), while the same amount of unimer solution in THF with a unimer-to-seed mass ratio (m_unimer/m_seed) of 20 : 1 was added to each sample. The resulting 2D platelets are of different size and uniformity (Fig. S28†). The 2D platelets obtained after preheating the seeds at 40 °C are of the expected area, suggesting effective suppression of unimer aggregation at elevated temperatures (Fig. S28C†). Next, area control of 2D platelets was employed by varying the m_unimer/m_seed ratio at elevated temperatures. Different amounts of unimer solution of PDTC₉₂-b-PEG₁₃₂ in THF (10 mg mL⁻¹) were added to the preformed seed micelle solutions at 40 °C. Low-area-dispersity 2D platelets were formed with controllable areas up to 105.71 × 10⁴ nm², as shown by TEM (Fig. 3D–G). An excellent linear relationship between the resulting micelle area and the m_unimer/m_seed consistent with the expected theoretical area was observed (Fig. 3H–J), confirming the living CDSA behavior of PDTC₉₂-b-PEG₁₃₂. This represents a rare example of uniform and exceptionally well-controlled biodegradable 2D platelets from the seeded-growth of a 2D seed.

Fig. 3 Preparation of low dispersity platelets of controlled size by living CDSA. (A) Schematic representation of the preparation process of low dispersity 2D platelets from PDTC₉₂-b-PEG₁₃₂. (B) TEM micrograph of disperse platelets prepared in MeOH : THF (95 : 5) by homogeneous nucleation. (C) TEM image of seed platelets prepared by sonication of disperse platelets. TEM images of low dispersity platelets prepared through seeded-growth by addition of the unimer in THF to the platelet seed solution with a m_unimer/m_seed ratio of (D) 20 : 1, (E) 50 : 1, (F) 100 : 1 and (G) 200 : 1, respectively. Scale bars = 1 μm. TEM images were obtained with uranyl acetate staining (3 wt% in MeOH). (H) Summary of platelet area. Đ_A is the area dispersity of the platelets. σ is the standard deviation of the area distribution. (I) Contour area histograms of low dispersity 2D platelets. (J) The plot of the experimental and theoretical A_nversus m_unimer/m_seed.

Triblock comicelles prepared by sequential living CDSA

To illustrate their potential as building blocks for hierarchical assembly, triblock comicelles with distinct coronal segments were prepared by the sequential addition of BCPs with different coronal chemistries to preformed 2D platelets (Fig. 4A). To achieve this, a PDTC-based BCP with a poly(2-vinylpyridine) (P2VP) corona block was prepared by a combination of ROP and reversible addition–fragmentation transfer (RAFT) polymerization. The DP_n of each block was identified by ¹H NMR spectroscopy and GPC analysis revealed low dispersity (Đ_m) values of 1.10 for PDTC₁₂₈-b-P2VP₆₁ (Fig. S29†). Due to the good solubility of both PEG and P2VP corona-segments in MeOH, it was anticipated that block comicelles could be prepared by adding the PDTC₁₂₈-b-P2VP₆₁ unimer to preformed PDTC₉₂-b-PEG₁₃₂ platelets in MeOH. Briefly, the PDTC₁₂₈-b-P2VP₆₁ unimer (10 mg mL⁻¹ in THF) was added to preformed PDTC₉₂-b-PEG₁₃₂ low dispersity 2D platelets (A_n = 10.09 × 10⁴ nm², Đ_A = 1.07), resulting in uniform AB core–shell block 2D platelets (A_n = 45.58 × 10⁴ nm², Đ_A = 1.07) (Fig. 4B, Fig. S30A and B†). TEM analysis of the dropcast and negatively stained samples showed two clearly defined segments, due to the higher electron contrast of the outer segment P2VP corona than that of the inner segment PEG corona.

Fig. 4 (A) Schematic representation of the preparation process of uniform triblock comicelles by sequential living CDSA steps of preformed 2D platelets of PDTC₉₂-b-PEG₁₃₂. The PDTC core is depicted in blue, the PEG corona in red, the P2VP corona in green and the BODIPY-FL dye as green stars. (B) TEM image of PEG-b-P2VP block comicelles prepared from PDTC₉₂-b-PEG₁₃₂ platelets (A_n = 10.09 × 10⁴ nm², Đ_A = 1.07) and PDTC₁₂₈-b-P2VP₆₁ unimers. (C) TEM image of PEG-b-P2VP-b-PEG-Dy triblock comicelles prepared from the resulting block comicelles and PDTC₉₂-b-PEG₁₃₂-Dy unimers. Magnified TEM image of a triblock comicelle. (D) CLSM images of the three-layer 2D platelets. Scale bars = 2 μm. TEM images were obtained with uranyl acetate staining (3 wt% in MeOH), Dy = BODIPY-FL.

In addition, the use of fluorescent dye labeled BCPs allows micelle morphologies to be studied in situ using confocal laser scanning microscopy (CLSM). Therefore, fluorescent-dye labeled PDTC₉₂-b-PEG₁₃₂-Dy was prepared by the condensation of a carboxylic acid functionalized BODIPY-FL dye with the hydroxy chain end of the PDTC core segment. The attachment of the fluorescent dye was confirmed by UV-vis spectroscopy revealing the appearance of a strong absorbance band at 500 nm after dye functionalization (Fig. S31†). Further addition of the dye functionalized PDTC₉₂-b-PEG₁₃₂-Dy unimer (10 mg mL⁻¹ in THF) to the preformed AB core–shell block 2D platelets yielded uniform ABC three-layer 2D platelets (A_n = 66.79 × 10⁴ nm², Đ_A = 1.08) as revealed by TEM (Fig. 4C and Fig. S30C†). The outermost fluorescent layer was confirmed with CLSM, which appears to be hollow 2D platelets (Fig. 4D).

Aqueous stability and compatibility of PDTC₉₂-b-PEG₁₃₅ 2D platelets

To explore the biocompatibility of the prepared 2D platelets, their aqueous stability and cytotoxicity were evaluated. A sample of uniform 2D platelets (BCP = PDTC₉₂-b-PEG₁₃₂, A_n = 4.02 × 10⁴ nm², Đ_A = 1.13) was prepared at a concentration of 2 mg mL⁻¹ (Fig. 5A and Fig. S32A†). The aqueous colloidal stability of these 2D platelets was explored by dialysis against water. This resulted in a clear, colloidally stable solution with no observable change in the micelle morphology via TEM analysis (A_n = 3.85 × 10⁴ nm², Đ_A = 1.14) (Fig. 5B and Fig. S32B†). Dynamic light scattering (DLS) experiments revealed no change in the hydrodynamic radius (R_hca. 200 nm) after solvent exchange from MeOH to water (Fig. 5C). To assess the colloidal stability of the 2D platelets in water over time, TEM and DLS analyses were performed on 2D platelets stored in water for 1 month, which revealed negligible changes in size and dispersity (Fig. 5B, C and Fig. S32C†). These results confirmed the excellent stability of the uniform 2D platelets in aqueous media and allowed further examination of the potential biocompatibility of these platelets. For this, the preformed 2D platelets were diluted in Dulbecco's modified Eagle's medium (DMEM) and no changes in the hydrodynamic radius were revealed by DLS (Fig. 5D). WI-38 fetal lung fibroblasts and U-87 MG glioblastoma cells were incubated with the preformed 2D platelets at concentrations ranging from 1 to 100 μg mL⁻¹. The analysis of the cell populations showed that 2D platelets of PDTC₉₂-b-PEG₁₃₂ exhibit no discernible cytotoxicity towards either cell line (Fig. 5E and Table S3†). These results reveal excellent biocompatibility, similar to PFTMC-based nanomaterials; however, the cytotoxicity of the PFTMC-biodegradation product is of concern. Therefore, the biocompatibility of the hydrolytic biodegradation product of PDTC, 2,2-dimethylpropane-1,3-diol, was evaluated in both cell lines (Fig. S33 and Table S4†).^42,43 The biodegradation product exhibits no discernible cytotoxicity towards either cell line up to a concentration of 250 μg mL⁻¹ proving that PDTC-based BCPs form less harmful biodegradation products in comparison with PFTMC-based BCPs.

Fig. 5 (A) TEM image of PDTC₉₂-b-PEG₁₃₂ platelets before dialysis from MeOH into water. (B) TEM image of PDTC₉₂-b-PEG₁₃₂ platelets after dialysis. Platelets are in proximity due to a drying effect on the carbon coated TEM grid. (C) TEM of PDTC₉₂-b-PEG₁₃₂ in water after 1 month of storage. Scale bars = 1 μm. TEM images were obtained with uranyl acetate staining (3 wt% in MeOH). (D) R_h of 2D platelet micelles before and after dialysis, stored in water for one month and in DMEM cell culture media. (E) Cell viability was measured after 24 h exposure of WI-38 and U-87 MG cells to different concentrations of 2D platelet micelles (A_n = 3.85 × 10⁴ nm², Đ_A = 1.14) in water.

Conclusions

In summary, we have developed a series of crystalline aliphatic polycarbonates through the introduction of different side chains and cyclic side groups to PTMC. The dimethyl substituted polycarbonate PDTC was selected as a core-forming block due to its improved crystallinity, compared to other PTMC derivatives. By harnessing the living CDSA of PDTC₉₂-b-PEG₁₃₂, we were able to access uniform 2D platelets and segmented triblock comicelles with controlled size and spatially defined corona compositions. The core-forming block undergoes exceptionally well-controlled living CDSA from 2D seeds in contrast to other biodegradable cores. This represents the first example of size controlled 2D platelets from a BCP with a crystalline polycarbonate-based core-forming block. More importantly, these synthetic 2D platelets exhibit great colloidal stability in aqueous solutions and excellent biocompatibility. Next, we were able to show that PDTC forms less harmful degradation products than PFTMC revealing its enhanced biocompatibility. Future work will focus on applying these 2D platelets with tailored corona chemistries in biomedical applications.

Author contributions

Hannah Schnicke: conceptualization (supporting), formal analysis (supporting), investigation, visualization, and writing – review & editing (lead). Chuanqi Zhao: conceptualization (lead), formal analysis (lead), investigation, methodology, visualization, and writing – original draft. J. Diego Garcia-Hernandez: methodology (monomer synthesis). Jiandong Cai: visualization (AFM and CLSM imaging). Yifan Zhang: conceptualization (triblock comicelles). Charlotte E. Boott: writing – review and editing. Ian Manners: conceptualization, funding acquisition, resources, supervision, and writing – review and editing.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

The work was funded by the Canada 150 Research Chair supported by the Canadian Government and NSERC Discovery Grants (to I. M). I. M. also thanks the University of Victoria for start-up funds and the Canada Foundation for Innovation (CFI), the British Columbia Knowledge Development Fund (BCKDF), and NSERC for equipment and instrumentational support.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5py00078e

‡ These authors contributed equally.

§ Present address: Key Laboratory of Photochemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.

¶ Deceased December 3^rd, 2023.

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