Method for Preparation, Programming, and Characterization of

Feb 24, 2014 - Miniaturized Particulate Shape-Memory Polymer Matrices. Christian Wischke* ... tone) [PPDL], with a particular focus on the effects of ...
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Method for Preparation, Programming, and Characterization of Miniaturized Particulate Shape-Memory Polymer Matrices Christian Wischke* and Andreas Lendlein Institute of Biomaterial Science and Berlin-Brandenburg Center for Regenerative Therapies, Helmholtz-Zentrum Geesthacht, Kantstrasse 55, 14513 Teltow, Germany ABSTRACT: Their capability to change their shape on demand has created significant interest for shape-memory polymers (SMPs) in minimally invasive surgery. To evaluate the miniaturization of SMP matrices for small-sized implants or controlled release systems, a strategy to prepare and evaluate microsized SMP model particles is required. This methodological study reports the emulsion-based preparation of ∼30 μm microparticles (MPs) from a phase-segregated SMP, poly(ε-caprolactone) [PCL] and poly(ω-pentadecalactone) [PPDL], with a particular focus on the effects of process parameters such as polymer solvents or stabilizer type/ concentration on formation and size distribution of SMP MPs. Processes for the preparation of SMP MP-loaded water-soluble polymer films with tailored mechanical properties were developed and applied for programming the SMP MP to a temporary ellipsoid shape by film stretching. For the functional evaluation of shape recovery of MPs, a light microscopy-based setup with temperature control is proposed by which the stimuliinduced switching of the microsized SMP matrices could be confirmed. Overall, by applying this methodological strategy to various thermoplastic SMPs, a routine to identify and characterize the microscale functionality of SMPs in miniaturized applications will be broadly accessible.

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INTRODUCTION The capability to respond to external stimuli by well-defined movements makes shape-memory polymer (SMP) matrices candidate materials for medical implants.1 Their self-sufficient active movements reverse an external mechanical deformation applied during a programming step. On the molecular level, the shape-memory effect (SME) requires netpoints in the polymer, which define the permanent shape and are stable during the programming or shape recovery process. Upon mechanical deformation of the material for programming to its temporary shape, stress is introduced into the material. This stress can be stored by the formation of temporary netpoints such as crystallites or vitrified amorphous domains in the case of temperature-sensitive SMPs. Upon exposure to the suitable external stimulus, in this case heat, the temporary netpoints are removed allowing for an entropy-driven recoil of polymer chains to their relaxed state and, simultaneously, a shape recovery of the SMP device to its initial (permanent) shape. Quantitative data to characterize the SME can be obtained by cyclic thermomechanical testing of macrosized dumbbellshaped test specimens in tensile testers, which allow clamping of samples, their programming to a permanent shape, and the evaluation of their shape fixity and shape recovery based on the determined sample length at the different steps of this process.2 By such methodologies, the shape-memory functionality of temperature-sensitive SMPs has been extensively explored during the past decade for macrosized devices such as films, rods, or more complex three-dimensional shapes.3 © 2014 American Chemical Society

Considering the strong trend toward miniaturization in all technical fields, there is a need to evaluate the suitability of SMPs to also serve as enabling technology on the microscale, for example, as actuators, valves, or the like. Importantly, corresponding to dumbbell-shaped test specimens on the macroscale, concepts are required for a systematic screening of the various families of SMPs on the micro-/nanoscale. So far, most approaches involved indentation of SMP films.45 When indentation is combined with subsequent removal of the topmost polymer layers (“polishing”), films with smooth surfaces as temporary shapes and protrusive permanent shapes can be obtained.6 However, the formation of cavities by indentation is accompanied by a substantial multidirectional displacement of polymer sideward around the indentation to form a circular protrusive rim,78 which adds complexity to a quantitative description of the shape-memory functionality. Even more important, the shape recovery of micro-/nanoindented films appears to be driven by the underlying macrosized bulk material, which can also be seen from the mentioned example of shape recovery to protrusive permanent shapes after polishing indented films.6 In conclusion, approaches that quantify the recovery of indentation depth of films9 may not be suitable to predict the SME of free-standing miniaturized Received: July 9, 2013 Revised: February 21, 2014 Published: February 24, 2014 2820

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Synthesis of SMP. Poly(ω-pentadecalactone)diol (PPDL) was synthesized by ring-opening polymerization from 1-oxa-2-cyclohexadecanone at 130 °C using octandiol as initiator and dibutyltin oxide as catalyst.15 A PPDL−PCL multiblock copolymer was obtained by reaction of PPDL (40 wt%, 3 kDa) and PCL (60 wt%, 3 kDa) with hexamethylene diisocyanate at 85 °C using dibutyltin dilaurate as catalyst.14 Multidetector GPC analysis with an RI detector Shodex RI101 (Showa Denko, Japan) and the dual detector T60A (Viscotek Corp., USA) performed in chloroform with universal calibration revealed a number average molecular weight Mn of 33.8 kDa and a polydispersity PD = 2.4 for the obtained product. Differential scanning calorimetry revealed melting temperatures Tm of crystalline domains at 49 °C (Tm,PCL; switching domain) and 83 °C (Tm,PPDL; hard domain). SMP Microparticle Preparation. Microparticles were prepared by an oil-in-water (o/w) emulsion technique using solutions of different concentrations of PPDL−PCL (e.g., 80 mg/mL) in different organic solvents as o-phase and emulsifying 1 mL of this solution in 2 mL of an aqueous stabilizer solutions (such 2 wt% Mowiol 4-88). For emulsification, either vortexing (2500 rpm, MS 1; IKA, Staufen, Germany) or rotor-stator homogenization with S25-8G tool (UltraTurrax T25, IKA) in sample tubes (diameter 14 mm) was employed. Subsequently, samples were transferred into a hardening bath of typically 30 mL aqueous stabilizer solution in 100 mL beakers (diameter 47 mm) with magnetic stirring for 3 h to allow for solvent extraction/evaporation. The microparticles were collected by centrifugation and lyophilized at 0.080 mbar (Alpha 1-2LD plus, Christ, Osterode, Germany). Characterization of Microparticle Size, Shape, and Morphology. The particle size was determined by static light scattering using aqueous suspensions of microparticles and the Fraunhofer approximation for data evaluation (Mastersizer 2000, Malvern, Herrenberg, Germany). The surface morphology of the microparticles was characterized by scanning electron microscopy (SEM) with or without sputtering with Pt/Pd (exclusion of sputtering artifacts) using a Gemini Supra 40 VP SEM (Carl Zeiss NTS, Oberkochen, Germany). Additionally, a Keyence VHX digital microscope with a VH-Z 500 zoom lens was used to study particle morphology. PVA Films and Particle Loaded PVA-Based Phantoms. For the preparation of PVA (Mowiol 3-85) films, the powdered material was first dissolved at different concentrations (20−40 wt%) in hot water and supplemented with different amounts of glycerol (0−5 wt% in the PVA solution) to reduce brittleness. Gas bubbles entrapped in the viscous solutions were removed either at 40 °C by sonication in a water bath or by incubation at 4 °C for 16 h. By using a casting knife (0.5 mm), the PVA solutions were spread on glass plates at 25 or 40 °C and allowed first to air-dry at ambient conditions for 3 days. Subsequently, the films were isolated and incubated at a relative humidity (r.h.) of 28% at room temperature. The water content of PVA films was determined by Karl Fischer titration with an Aqua 40.00 instrument with headspace module (Analytik Jena, Germany). Tensile tests of PVA films at different temperatures were performed with an elongation rate of 5 mm·min−1 with a tensile tester (Z1.0 with 200 N load cell, Zwick, Ulm, Germany). To enable a simultaneous deformation of numerous SMP particles, a multimaterial system, herein named phantom, was prepared by embedding SMP particles into PVA as described above using a 1 mg· mL−1 microparticle suspension in the PVA solution. Programming and Recovery of SMP Micromatrices. Programming of particles with a permanent spherical shape to prolate ellipsoids as temporary shape was performed by stretching particle loaded phantoms at Thigh = 70 °C and 5 mm·min−1 with a tensile tester (Z1.0 with 200 N load cell, Zwick, Ulm, Germany) with subsequent cooling to Tlow = 0 °C. Particles could be isolated by dissolving the phantom in water at room temperature. The shape recovery was studied in heating experiments with 5 K·min−1 from 25 to 70 °C and in some cases further to 90 °C using a light microscope (Axio Imager.A1m, Carl Zeiss Microimaging, Gö ttingen, Germany) equipped with a LTS 350 stage chamber (Linkam, Tadworth, UK).

SMP devices without an underlying bulk. Similarly, in the case of microstructured SMP surfaces with subsequent bending and recovery of protrusive structures such as pillars,10 the observed SME cannot be exclusively assigned to the SMP microstructure. This is because bending of pillars involves significant deformation particularly at their base, where the polymer chains of the pillars are anchored in the underlying bulk. Additionally, the pillars’ base may be intruded into the underlying film at the side of the bending direction, again resulting in contributions of the bulk phase to shape recovery. Therefore, this study aimed to provide an alternative approach for microscaled SMP model matrices to study SMP functionality. Ideally, such model matrices should be scalable and should not require complex preparation steps. Considering the well-established particle formation by emulsion/solvent evaporation techniques,11−13 particles from a multiblock copolymer from poly(ε-caprolactone) [PCL] and poly(ωpentadecalactone) [PPDL] segments were recently explored for their polymer morphology and crystallinity if scaled to the micrometer and nanometer size.14 On the basis of this result, it can be assumed that microparticles of ∼30 μm with all three dimensions in the microscale may be suitable free-standing matrices for studying SMP functionality after miniaturization. However, for broadly exploring the capability of microparticles as a model system, relevant process parameters that effect microparticle formation and properties such as particle size and size distribution should be identified and methodologies and experimental conditions for particle programming and analysis of recovery should be explored in detail in this study. The selected PPDL−PCL multiblock copolymer served as an example of a thermoplastic SMP because of the challenging stabilization of this polymer during particle formation, which required a systematic strategy later on applicable also to other materials. Additionally, it is a general advantage of suitable semicrystalline multiblock copolymers to form “permanent” netpoints by crystallization, in this case of PPDL domains, as demanded for the shape-memory effect without the need to perform covalent cross-linking. Furthermore, the switching temperature Tsw may be adjusted by the length of the PCL segments and by the programming condition, which makes this model polymer an example of a family of materials with adjustable shape-memory properties.15 The concepts and methodologies established in this work should be also suitable for other thermoplastic SMP materials and therefore should serve as a guide for systematic physicochemical evaluation of SMP micromatrices for smallsized implants or controlled release systems.

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EXPERIMENTAL SECTION

Materials. Poly(ε-caprolactone)diol (PCL; number average molecular weight Mn = 3 kDa) was purchased from Perstorp (Cheshire, UK) as an oligomeric building block for the SMP, hexamethylene diisocyanate was from Fluka, and dibutyltin oxide was from Sigma-Aldrich. Polyvinyl alcohol (PVA) was either Mowiol 3-85 (for films; 85% hydrolyzed, Mn = 5.6 kDa, pure material with Tg = 66 °C and Tm = 163 °C) or Mowiol 4-88 (for emulsion stabilization; 88% hydrolyzed, Mn = 12.6 kDa, Tg: 69 °C; Tm: 163 °C) kindly donated by Kuraray, Frankfurt a.M., Germany. Other emulsion stabilizers included Poloxamer 188 (Pluronic F 68, AppliChem, Darmstadt, Germany), Polysorbat 80 (Tween 80, SigmaUltra; Sigma-Aldrich, Taufkirchen, Germany), or polyvinyl pyrrolidone K25 (Fluka, Germany). The reagent for Karl Fischer titration was Hydranal-Coulomat AG. All other chemicals were of analytical grade. 2821

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Figure 1. Methodology to prepare SMP microparticles by o/w emulsion solvent evaporation technique.

Figure 2. Effect of experimental conditions in o/w emulsification on the size distribution of PPDL−PCL microparticles after hardening. (A) Alteration of PPDL−PCL concentration in dichloromethane as o-phase (PVA concentration 2 wt%). (B) Impact of PVA concentration in w-phase (the PPDL−PCL concentration of the o-phase was 8 wt%). (C) Effect of homogenization technique either by vortexing (also used in panels A and B) or by rotor-stator homogenization (Ultra-Turrax) at different revolutions per minute (rpm) for 8 wt% PPDL−PCL (2 wt% PVA). In all cases, the PVA concentration of the hardening bath was kept at 0.5 wt%.

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RESULTS AND DISCUSSION Preparation of Microparticles As Miniaturized SMP Matrices. Miniaturized model matrices that provide a substantial size reduction compared to common macroscopic test specimens and remain assessable for characterization by light microscopy ideally should be prepared at average microparticle sizes of 20−30 μm. The oil-in-water (o/w) emulsion/solvent evaporation technique enables tailoring of particle sizes by the employed substances and conditions during dispersion of a polymer solution into a second solvent phase such as water with poor solvent strength for the SMP (Figure 1). Besides the geometry of the setup, which was kept constant in the present study, the viscosity of the polymer solution, the energy input for homogenization, and the stabilizing agent in the water phase typically have the largest impact on particle properties. A systematic approach to establish the most suitable experimental conditions for SMP microparticle preparation should first involve the selection of a stabilizer that is compatible with the polymer employed as particle matrix. When screening different aqueous stabilizer solutions often used in microparticle preparation, namely, PVA (0.5−5 wt%), poloxamer 188 (1 wt%), polysorbat 80 (1 wt%), and polyvinylpyrrolidone (1 wt%), only PVA allowed obtaining individual PPDL−PCL particles. In all other cases, strong aggregation of nascent microparticles during solvent evaporation was observed, indicating a poor interaction of the other

evaluated stabilizers with PPDL−PCL. Therefore, PVA was subsequently used as stabilizer in the continuous phase. Second, the effect of experimental parameters on the particle size should be identified. As a standard emulsification procedure, vortexing of the water phase and polymer solution was employed since this process often results in particles sizes as aimed for in the present study.12 When increasing the PPDL−PCL concentration in dichloromethane as o-phase solvent until reaching its maximum solubility of about 10 wt%, larger particle sizes were obtained due to increasing o-phase viscosities (Figure 2A). The most narrow size distribution matching the desired range of particle sizes was obtained for a PPDL−PCL concentration of 8 wt% in dichloromethane as the o-phase, which subsequently served as standard conditions. Still, in some cases alternative o-phase solvents may be needed, for example, if the solubility of the shape-memory polymer in the o-phase is very low. A requirement of such polymer solvents would be low miscibility with the continuous phase for slower solvent exchange at the interphase leading to a controlled particle hardening. Table 1 illustrates the mutual solubility of potential alternative o-phase solvents and water in comparison to dichlormethane used so far as standard o-phase solvent. In the case of PPDL−PCL, a lack of relevant solubility limited the applicability of ethyl acetate, which is a very suitable solvent for microparticle preparation from other polymers. Chloroform, 1,2-dichloroethane, and toluene were explored as further alternative o-phase solvents; while chloroform or 1,2dichloroethane allowed stable microparticle formation, toluene 2822

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analyzed by SEM. Furthermore, samples were analyzed by digital optical microscopy, which again provided evidence of rough surface morphology (Figure 3B). The observed roughness of the microparticle surface was considered to be an inherent feature of the PPDL−PCL material originating from the emulsion based techniques. At least, a screening study with different o-phase solvents and altered experimental parameters during solvent evaporation indicated that such surface structure remains unaffected by preparation conditions (data not shown). Similar surface patterns have been observed before for other (multi)block copolymers and have been assigned to water uptake and subsequent collapse of water-rich domains.17,18 On the basis of the very slow hydrolytic degradation of PPDL−PCL, the contact with water during the particle preparation was not considered to affect the polymer integrity but should be critically evaluated in the case of other SMPs containing more labile bonds or poor removal of residual water after micromatrix preparation. Tuning the Properties of PVA-Based Phantoms. PVA has shown its capability to stabilize PPDL−PCL microparticle suspensions due to its compatibility with the matrix polymer and its anchoring on the rather hydrophobic microparticle surface. Because of its good interaction with microparticles, PVA should additionally be employed for embedding microparticle into a multimaterial system, herein named phantom, which allows parallel manipulation of numerous particles. Because of its water solubility, the PVA matrix of the phantoms can subsequently be removed for collection of non-water soluble SMP particles. The PVA solubility at ambient conditions generally is highest at a degree of deacteylation in the range of ∼85% and increases with decreasing Mn. Sufficient solubility was observed for a low molecular weight PVA (Mn = 5.6 kDa), which subsequently was used for particle embedding. The requirements for phantom properties include suitable thickness for particle inclusion as well as the absence of voids originating from gas bubbles. Casting of viscous aqueous PVA solutions with casting knifes was performed at ambient conditions but also at elevated temperatures, whereby thermal transitions of the SMP switching domains should not be exceeded (Tm,PCL = 49 °C). Since the addition of antifoaming agents should be avoided, the removal of air bubbles as generally observed in PVA solutions needed to be included in the phantom preparation concept. This point can be first evaluated for particle-free PVA films. Casting at different PVA concentrations (20−40 wt%) was performed either at 40 °C (after degassing at 40 °C with ultrasound) or at 25 °C (after degassing at 4 °C). However, bubble-free films could only be obtained at concentrations ≤25 wt%. Since the casting regime at 25 °C allowed for more reproducible and homogeneous films than the process at 40 °C, which was sensitive to slight changes in environmental temperature, the casting procedure at 25 °C with 22.5 wt% PVA solutions was subsequently used. Employing this PVA solution and a casting knife with a 0.5 mm clearance resulted in the targeted dry-state phantom thickness of 80 μm and an absence of gas bubbles (Figure 4A). Since PVA is a hygroscopic material, high residual water content may be expected in PVA films even after drying. Since high water contents generally may induce the hydrolytic degradation of embedded SMP microparticles at least for materials with high hydrolytic degradation rates during longterm storage, the residual water should be kept at a minimum level. At day 1 after casting, about 9 wt% water remained in

Table 1. Properties of Solvents (S) Explored for PPDL−PCL and Their Suitability for Microparticle Preparation solubility [wt%] solvent

S in water

water in S

stabile MP

ethyl acetate dichloromethane chloroform 1,2-dichloroethane toluol

8.70 1.32 0.80 0.81 0.06

3.30 0.20 0.20 0.15 0.05

a √ √ √ b

a

No solubility of PPDL−PCL in this solvent. b After o/w emulsification, nascent MPs strongly aggregated during solvent evaporation.

resulted in microparticle aggregation during solvent evaporation, which illustrates that the combination and compatibility of solvent and stabilizer need to be addressed in each specific case. When continuing with dichloromethane as standard solvent in combination with PVA and altering the PVA concentration used during emulsification in the range of 0.5−5 wt%, no strong impact on the average particle size was observed. However, the width of the size distribution was clearly affected with the most narrow size distribution being observed at 2 wt% PVA for the selected preparation setup (Figure 2B). It appears that at low PVA concentrations the o-phase droplets were not well stabilized due to low diffusivity or long diffusion length of the polymeric stabilizer and partially underwent coalescence. At too high PVA concentrations, different mechanisms might have contributed to an incomplete separation of nascent droplets such as an elevated w-phase viscosity or a bridging of particles.16 If particles obtained by this procedure may not match the desired size range, in a third step higher shear forces may need to be applied for dispersion during particle preparation. As illustrated in Figure 2C, by rotor stator homogenization at increasing stirring speeds, that is, increasing local power densities for dispersion, the size of the particles could be systematically decreased. A small particle fraction, which often can be observed for rotor stator homogenization as well as for other techniques, increased with increasing shear forces and may be removed from the sample by wet sieving. Morphology of PPDL−PCL Microparticles. To confirm the particle size data and explore the morphology of the SMP micromatrices, samples were subjected to SEM analysis (Figure 3A). Rough surfaces of microparticles were observed, which was also the case when native samples without sputtering were

Figure 3. Analysis of the morphology of SMP microparticle prepared from PPDL−PCL multiblock copolymer (8 wt% polymer solutions employed during preparation). (A) SEM analysis with Pt/Pd coating. (B) Image of microparticles obtained in aqueous suspension with a digital microscope. 2823

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Figure 4. Properties of PVA (Mowiol 3-85) films depending on preparation conditions and subsequent treatment. (A) Thickness of dry PVA films depending on PVA concentration and the clearance of the casting knife (casting at 25 °C). Subsequently, 22.5 wt% PVA solutions and 0.5 mm casting knifes were employed. (B) Effect of drying conditions and duration of residual water content. (C) Impact of supplementation of PVA solution for casting with different concentrations of glycerol on the residual water content after 28 days of drying. (D, E) Results of tensile tests of PVA films dried at 27% r.h. for 28 days showing the temperature dependency of the E (Young’s) modulus (D) or the elongation at break εB (E) between 40 and 70 °C (mean, S.E.M., n = 4−6).

Figure 5. (A) Principle of synchronized programming of numerous spherical SMP micromatrices to a prolate ellipsoid shape with subsequent particle collection. (B) Local differences in elongation after stretching with highest elongations in the center of the film. (C, D) Microscopic images of two different PVA films loaded with SMP microparticles from PPDL−PCL (C) as prepared and (D) after programming.

PVA films as determined by Karl Fischer titration. The water content was halved when films were incubated at ambient conditions with 60% r.h. for 28 days (Figure 4B). In contrast,

when stored at 27% r.h., a decreasing water content with a plateau at about 3 wt% after 3−4 weeks could be observed. To ensure equilibrium water content for further testing, PVA films 2824

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deformation also for SMP micromatrices with elevated E moduli. However, particle collection from these PVA films by dissolution might only be possible at elevated temperatures due to an expected low solubility at room temperature. This may interfere with the switching temperature of the embedded programmed SMP micromatrices, which demonstrates that a balance between the mechanical properties and the solubility of PVA films is critical for their usage in programming of SMP micromatrices. Because of the common necking of stretched samples, slightly different local elongations of the phantoms were observed with highest elongations in the center as visible by marks applied to the film before stretching (Figure 5B). Accordingly, the aspect ratio AR (length of the longest axis l divided by the shortest axis d) of the produced particles in their temporary prolate ellipsoid shape will differ to some extent when collected from different parts of the films. This can be easily overcome by putting a grid of markings on the surface of the PVA film and only collecting those sections with a local elongation equaling the overall elongation of the film.20 In this way, particles with reproducible AR can be obtained. Efficient deformation even in the case of slightly aggregated particles (Figure 5C,D) indicated continuity of displacement and good traction at the interfaces of particles and the PVA matrix. The AR is a widely used shape factor for nonspherical particles. Here, it should be used as a quantitative measure to describe the SME of microparticulate SMP matrices by the recovery rate of the aspect ratio Rr, AR as calculated by eq 1

were subsequently incubated at defined humidity for 21−28 days at 27% r.h. To use as phantoms for particle programming, also the mechanical properties of the matrix need to be taken into account. Programming of temperature-sensitive SMPs typically requires deformation at temperatures above the thermal transition temperature of the switching domains (here PCL, Tm,PCL = 49 °C) as well as cooling to low temperatures for fixing the temporary shape. Therefore, the mechanical properties of PVA and possible effects of water as an efficient plasticizer need to be considered. When the residual water was reduced to a minimum level, the thin PVA films showed high brittleness at room temperature, which strongly impeded their handling or usage for phantoms to deform embedded SMP micromatrices. Therefore, PVA solutions were supplemented at different concentrations with glycerol, which is well-known to be a good plasticizer for PVA and has before been used as additive for stretching particle-loaded films.19 While advantageously not increasing the residual water content by its hygroscopicity (Figure 4C), the strong plasticizing effect of glycerol became obvious during mechanical testing particularly at 40 °C as the lowest temperature, at which all samples including glycerol-free PVA films could be analyzed. At 40 °C, a decrease of the E moduli from 2300 to 67 MPa was observed upon increase of the glycerol concentration in the casting solution from 0 to 5 wt% (Figure 4D). Since the used PVA was a semicrystalline polymer with melting temperatures Tm >100 °C, crystalline domains will stabilize the PVA matrix when increasing the temperature above the glass transition temperature Tg, which was, for example, Tg = 56 °C for films prepared from PVA solutions substituted with 1 wt% glycerol (Tg = 66 °C for pure PVA). Because of the higher chain mobility above the Tg, the E moduli of the different films strongly decrease and adapted values between 83 and 35 MPa at 70 °C depending on the glycerol concentration. With increasing temperature, also the elongation at break εB of the films increased steadily as expected (Figure 4E). Programming and Analysis of Shape-Recovery. For programming simultaneous deformation of PVA embedded particles20 has been performed at Thigh = 70 °C, which was above the Tm of the PCL switching domains but below the Tm of the PPDL domains forming the permanent netpoints. Typically, 1 mg·mL−1 microparticles were suspended in PVA solution for phantom preparation but also higher phantom loadings (e.g., 10 mg·mL−1) are possible without impeding homogeneous particle deformation. During phantom stretching (Figure 5A) to predefined elongations εph, particles with a spherical permanent shape were transferred into prolate ellipsoids, which were fixed as temporary particle shape by cooling to Tlow = 0 °C. It should be noted that the stretching of the matrix will be most efficiently transferred into a deformation of its particulate inclusions when the particles are softer than the surrounding matrix. Therefore, conditions should be preferred, at which the E modulus of the PVA matrix (compare Figure 4D) should be larger or at least similar to that of the SMP particles. This condition was fulfilled at Thigh for PPDL−PCL (E = 6 MPa) and films from PVA solutions with 1 wt% glycerol (E = 51 MPa). Higher glycerol contents did not provide a further reduction of E modulus at this condition and were therefore not considered for programming of PPDL−PCL microparticles. Generally, by increasing the molecular weight or degree of hydrolysis of the PVA, films with larger mechanical strength might be obtained, possibly supporting effective

R r,AR =

AR programmed − AR recovered AR programmed − AR original

(1)

where ARoriginal corresponds to the permanent spherical shape (AR = 1), ARprogrammed represents the AR of the temporary prolate ellipsoidal shape, and ARrecovered corresponds to the AR of the same particle after stimuli-induced shape recovery, respectively.14 To study the shape-memory effect of individual particles triggered by an external stimulus, a microscopy-based strategy appeared most suitable to precisely assess and ideally monitor online the induced recovery of their permanent shape. Accordingly, light microscopy was performed on a heating stage in a setup as illustrated in Figure 6A. When dissolving pieces of the phantoms on the microscope slides by adding droplets of water, bending and curling of the films were observed resulting in stacking of formerly dispersed particles, which impeded their analysis. This issue could be overcome by covering the dry PVA films with a coverslip and afterward adding water, which flow underneath the coverslip driven by capillary forces. When heating with a low heating rate was applied to study the shape recovery, the aqueous dispersion medium evaporated over the course of the experiment. This induced a current to the edges of the coverslip, resulting in a strong migration of particles and eventually led to the complete drying of the samples. Therefore, immersion oil was applied around the edges of the coverslip (Figure 6A) efficiently impeding evaporation of the water even at temperatures as high as 90 °C. As exemplarily illustrated for PPDL−PCL particles programmed by phantoms stretching to ε ph = 50% (ARprogrammed = 2.8), a gradual shape recovery with Rr,AR = 90% at 90 °C14 could be monitored upon heating (Figure 6B). Despite not quantitatively comparable due to the different geometries, recovery rates of macroscopic samples of the same 2825

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recovery and making assessable recovery kinetics on the micrometer-scale, this report may serve as a guide for screening the SME for various families of thermoplastic SMPs when reduced to matrix sizes with all three dimensions in the micrometer range. If monodisperse model particles are needed for mechanistic studies, microparticle preparation may be realized by microfluidic methods in the future.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the technical support by A. Pfeiffer, Dr. H.-J. Ziegler, and Dr. H.-J. Kosmella. This study was supported by the Deutsche Forschungsgemeinschaft (No. WI 3637/1-1). The authors are grateful to Helmholtz Association for financial support.

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Figure 6. Shape-recovery experiments of microparticulate SMP matrices. (A) Schemes of microscopy setup and sample preparation. (B) Representative microscopic pictures of a SMP microparticle from PPDL−PCL multiblock copolymer after programming and at different temperatures during the shape recovery experiment.

material in accompanying cyclic thermomechanical analysis in tensile testers will indicate if miniaturization to micrometer matrices affects shape-memory functionality, which was not the case for PPDL−PCL microparticles. Overall, this time- and temperature-resolved method may not only be applicable to study the general capability of micromatrices from various SMPs for shape-recovery but could also assess recovery kinetics depending on heating rates and a quantitative description of the SME by shape factors such as the aspect ratio for each individual micromatrix.

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CONCLUSION For studying the capability of SMPs for miniaturization, microparticles were suggested in here as small-sized model matrices that would overcome limitations of previously applied approaches based on microstructures created on top or in a larger bulk of the shape-memory polymer that may be prone to interference of the underlying bulk polymer phase to shaperecovery. The studies with PPDL−PCL as a thermoplastic shape-memory polymer illustrated that SMP micromatrices can be formed by the applied emulsion based particle preparation and which experimental parameters are particularly relevant for obtaining well-stabilized particles in the desired size range. On the basis of the suggested systematic approach for creating particle loaded PVA phantoms with suitable properties, a method for simultaneous programming of numerous particles to an ellipsoid temporary shape could be established. Along with the established methodology for monitoring shape 2826

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