Spray drying of polylactic acid (PLA) with the Nano Spray Dryer B /2011

Transcription

1 Spray drying of polylactic acid (PLA) with the Nano Spray Dryer B /2011

2 1 Introduction Applications of PLA PLA (polylactic acid) is a colourless, glossy and stiff thermoplastic polymer that has received great attention from the biomedical field and the packaging area over the last few years, as a biocompatible, Figure 1: Structure of PLA non tissue reactive and biodegradable material [1, 2]. PLA belongs to the family of aliphatic polyesters (Figure 1) and is derived for example from corn starch, tapioca or sugarcanes. PLA is used especially for medical applications, such as for sutures, stents, dialysis media, drug delivery devices, tissue repairing and engineering as well as equipment [2, 3]. Types of PLA Mechanical and degradation properties of PLA depend mainly on molecular weight, crystallinity and the insertion of possible copolymers [4]. Due to the chiral nature of lactic acid, several distinct forms of polylactic acid exist: poly-l-lactic acid (PLLA), poly-d-lactic acid (PDLA) and the hybrid form poly(l-lactide-co-d,llactide) (PLDLLA) [4, 5]. For this Application Note the Resomer type R 202 H formulation was used, a biodegradable polymer made on the basis of DL-lactic acid and a registered trademark of Evonik Röhm GmbH (former Boehringer Ingelheim). It is a white, amorphous, nearly odourless powder with neutral taste and has a molecular weight of kda (inherent viscosity of 0.16 to 0.24 dl/g measured at 0.1% in CHCl 3, 25 C) [6]. PLA was selected because of its favourable glass transition temperature, which is at around C [6]. The glass transition temperature is an important parameter of the polymer, defined as the temperature at which an amorphous material transits from a hard and relatively brittle state into a molten or rubber state. Spray dried particles are typically amorphous due to the fast evaporation times and due to the lack of ability to form crystalline structures [7]. The solvents Before creating particles by spray drying, the PLA has to be dissolved. Dichloromethane (DCM) was selected as dissolvent due to its high solvation capacity for PLA [8], its low boiling point of 40 C and because it is considered to be the least toxic of the halogenated solvents [9]. Additionally, ethanol (EtOH) which is miscible with water and many organic solvents was used as a versatile solvent. A mixture of DCM and ethanol in a ratio of 70:30 (v/v) was previously determined as the best solvent mixture for spray drying PLA [10]. Use of spray drying In pharmaceutical applications spray drying is often used for drying amorphous drugs [11, 12], to generate mucoadhesive microspheres or to design novel controlled drug release systems [13]. The investigation on the formation of PLA microspheres by spray drying is justified by interesting results presented in relevant literature [14-17]. A variety of PLA R 202 H solutions were spray dried, leading to spherical microspheres resulting in a very specific surface area [14-17]. Nano Spray Dryer B-90 The Nano Spray Dryer B-90 utilizes three main technologies: A piezoelectric nozzle for spray Figure 2: Principle of droplet generation by piezoelectric driven mem- generation A laminar flow brane vibration of drying gas for gentle dryingan electrostatic particle collector for submicron- and nano particle collection providing high yields Its main advantages are: a high separation efficiency of up to 99%,a simple particle collection, andintegrated outlet gases filter helping to protect users and the environment. Droplets are generated by a piezoelectric driven actuator, vibrating a thin, perforated, stainless steel membrane in a small spray cap (see Figure 2). The membrane (spray mesh) features an array of precise, micron sized holes (4.0, 5.5 or 7.0 µm). The actuator moves at an ultrasonic frequency, causing the membrane to vibrate, thereby ejecting millions of precisely sized droplets every second with a very narrow droplet size distribution [18]. Goals The goal of this Application Note is to test the feasibility of the Nano Spray Dryer B-90 to spray dry PLA solutions in DCM and ethanol. The spray cap size was varied during the experiments in order to see its influence on particle size and morphology. The highest sprayable concentration and optimal inlet temperature were evaluated in order to receive high yields. Application Note 002/2011 Version A, Copyright 2011 BÜCHI Labortechnik AG 2/7

3 2 Methods Equipment Nano Spray Dryer B-90 with Dehumidifier B-296 and aspirator in open mode (BUCHI Labortechnik AG) Analytical balance (accuracy +/- 0.1 mg, New Classic MF, Mettler Toledo) Scanning electron microscope (Tabletop TM- 1000, Hitachi) Moisture Analyzer B-302 (BUCHI Labortechnik AG) Magnetic stirrer (MR Mini, BUCHI Labortechnik AG) 50 ml glass beaker (Duran) Magnetic stir bars (made of PTFE, Semadeni) Chemicals and Materials Resomer R 202 H (poly D,L-lactide, mat no , lot no , Boehringer Ingelheim Pharma Gmbh & Co. KG) Dichloromethane (mat. no., 34856, lot no. 665P6MMV, Sigma Aldrich) Ethanol 99% (Brenntag AG) Sample preparation Samples with a concentration of 1.0 % (w/w) were prepared as starting solutions. 1.0 g of PLA was weighted and mixed with 100 g of a 30:70 v% ethanol/dichloromethane solvent mixture. For some experiments the PLA solution was further diluted. The solution was stirred for about 10 minutes until a clear solution was received. For every experiment 50 g of solution were transferred into a 50 ml glass beaker including a magnetic stir bar. The solution was stirred continuously during the spray drying run. The beaker was sealed with Parafilm foil to prevent the dichloromethane from evaporating during the drying process. Experimental procedure The selected process parameters have an important influence on the success of the spray drying process. The parameters were kept as stable as possible during the experiments in order to obtain reproducible results. Each combination was tested in triplicate. The here presented results are the mean values of these three runs. 1. Compressed air was used as drying gas. 2. The drying gas flow rate was of 140 L/min resulting an the inside pressure of 60 mbar. The laminar drying gas flow and piezoelectric atomization lead to a gentle evaporation. 3. The inlet temperature was varied between 20, 25, 30, 35 and 40 C. Depending on the selected spray cap size the outlet temperature and the spray head temperature varied accordingly. 4. The feed rate of the Nano Spray Dryer B-90 depends on the size of the spray membrane applied. For the experiments a spray rate of 60% was used. The viscosity of the solution is a further factor that influences the sample throughput. 5. After reaching the set inlet temperature, a solution with 30:70 ethanol to dichloromethane ratio was sprayed in order to stabilize the outlet temperature. Then the sample was sprayed. In every run 50g of the sample were sprayed. 6. The dried particles were collected in an electrostatic particle collector. Before collecting the particles manually with a rubber spatula, the collector cylinder was left to cool down to room temperature. The resulting powder was weighted and filled into a vial. 7. The relative yield (%) was calculated by dividing the weight of the collected powder by the amount of sample sprayed, then multiplied by The morphology and particle size of the spray dried PLA powder was studied through a scanning electron microscope (SEM). The moisture content was determined by an infrared Moisture Analyzer B Results and discussion The influence of sample concentration (Table 1, in blue), inlet temperature (Table 1, in yellow) and spray cap mesh size (Table 1, in green) on particle size and morphology, yield and residual solvent content was tested during the experiments. Influence of total solution concentration centration on particle size Two different concentrations (0.2% and 1% of the total solids amount) were tested with a 7.0 µm spray cap. The experiments were performed at 17 C to 20 C inlet temperature using the Dehumidifier B-296 to dehumidify and cool down the inlet drying gas. The influence on particle size and distribution detected was minimal. Compared to the higher concentrations, the lower concentrated samples showed a clearly different morphology (shrivelled and transparent spheres). The results are displayed in Figure 2. Application Note 002/2011 Version A, Copyright 2011 BÜCHI Labortechnik AG 3/7

4 Experiment Nr Spray head size (µm) Solid concentration (w%) T inlet ( C) Aspirator speed (Hz) T outlet ( C) T spray head ( C) Spray drying time (min) Yield (mg) ( n.a Yield (%) n.a Particle size (µm) 1 to to to to to to to to 4 spherical, Particle morphology transparent spherical spherical spherical spherical spherical spherical spherical Residual solvent content (w%) Table 1: Spray drying parameters and results (constant parameters: 70:30 DCM to EtOH, 50g of solution, air as drying gas, 140 L/min drying gas flow, 60% spray feed rate, 60 mbar inside pressure, 47 Hz aspirator speed, n.a. means not measured) Figure 2: SEM pictures of the experiments with different total solution concentrations (0.2% on the left and 1% on the right), 7.0 µm Influence of the temperature on the parti- cle size and residual solvent content The influence of the inlet temperature on formed particles. was tested at 25 C, 30 C, 35 C and 40 C A spray cap size of 7.0 µm and a concentration of 1% of solids were used for the experiment. A marginal reduction of residual solvent content from 0.15% (at 20 C, 25 C and 30 C) up to 0.10% (at 30 and 35 C) was observed. An influence of inlet temperature on particle size and distribution could not be detected. However, a clear linear correlation between the inlet and the outlet temperature was detected (Figure 3). With an increasing inlet temperature the relative yield also increased. Outlet temperature [ C] T out = 0.44 T in Inlet temperature [ C] Figure 3: Correlation between inlet and outlet drying gas temperature in the Nano Spray Dryer B-90 spraying DCM solutions. Application Note 002/2011 Version A, Copyright 2011 BÜCHI Labortechnik AG 4/7

5 25 C 30 C 35 C 40 C Table 1: SEM pictures of experiments with different inlet temperatures Influence of the spray cap size on the particle size and morphology ogy The influence of different spray drying membranes (7.0, 5.5 and 4.0 µm) on the particle size and morphology was evaluated during the experiments. Figure 5 (top) shows homogeneous spherical particles of sizes ranging from 1 to 10 µm. With the use of the 5.5 µm spray cap (middle) most of the particles were of a size ranging between 0.7 to 7 µm. The mean size of particles sprayed with the 4.0 µm spray cap (bottom) lies between 0.4 and 4 µm. The conducted study demonstrates that: All the particles observed were spherical. A bigger membrane size leads to a bigger particle and wider size distribution. The yield was quite high, typically between 50% and 88% for 50 mg of sample sprayed. Feed rate [g/h] µm 5.5 µm 7 µm Spray cap size Figure 4: Feed rate of DCM samples in correlation to spray cap size Application Note 002/2011 Version A, Copyright 2011 BÜCHI Labortechnik AG 5/7

6 Conclusions Microspheres of different mixing ratios of PLA can be easily prepared by spray drying. The results of this study suggest that: 1. the selection of the spray cap size affects the particle size and its distribution; 2. the total solution concentration can control the amount of PLA in the final particles; 3. the control of processing variables, especially inlet temperature and feed rate, allows for the production of micro particles of low moisture content with high yields of up to 88%. References [1] Garlotta D. et al. (2002): A Literature review of Poly (Lactic Acid), Journal of Polymers and the Environment, Vol. 9, Nr. 2. [2] Kulkarni R. K. et al. (1966): Polylactic acid for surgical implants, Technical rept., Army medical biomechanical research Lab, Washington DC, AD aprefix=html&identifier=ad [3] Trimaille T. et al. (2006): Poly(hexyl-substituted lactides): Novel injectable hydrophobic drug delivery system, Wiley InterScience, DOI: /jbm.a [4] Resomer Brochure (March 2011, V 1.0): Evonik Röhm GmbH, Darmstadt, Germany, wnloads/brochures/pages/default.aspx. [5] Arpagaus C. and Schafroth N. (2007): Spray Dried Biodegradable Polymers as target material for Controlled Drug Delivery, best@buchi no. 46, BÜCHI Labortechnik AG, Flawil, Switzerland, [6] Supplier of Resomer R 202 H: Sigma-Aldrich (2011), Figure 5: SEM pictures of PLA particles prepared with different spray cap sizes 4.0 µm (top), 5.5 µm (middle) and 7.0 µm (bottom). [7] Schöttle I. A. (2006): Spray Drying of PLA and PLGA microparticles for controlled release of pulmonary dosage forms, Dissertation, University Mainz. [8] Youan B.C. (2007): Microencapsulation of Superoxide Dismutase into Biodegradable Microparticles by Spray-Drying. Drug Delivery, Vol. 11, p [9] Blanco M.D. (2005): 5-Fluorouracil-loaded microspheres prepared by spray-drying poly(d,l-lactide) and poly(lactide-co-glycolide) polymers: Characterization and drug release. Journal of Microencapsulation, Vol. 22, No. 6, p Application Note 002/2011 Version A, Copyright 2011 BÜCHI Labortechnik AG 6/7

7 [10] Schafroth N., Arpagaus C., Jadhav U. Y., Makne S., Douroumis D. (2010), Nano and Microparticle Engineering of Water Insoluble Drugs Using a Novel Spray-Drying Process. Colloids and Surfaces B: Biointerfaces, doi: /j.colsurfb [11] Corrigan O. I. et al. (1983): Physicochemical properties of Spray Dried drugs: phenobarbitone and hydroflumethiazide, Drug Development and Industrial Pharmacy, Vol. 9, p. 1. [12] Corrigan O. I. et al. (1984): Amorphous spray dried hydroflumethiazide-polyvinylpyrrolidone systems: physiochemical properties, Journal of Pharmacy and Pharmacology, Vol. 36, p [13] Palmieri G. F. et al. (2001): Spray Drying as a Method for Microparticulate Controlled Release Systems Preparation: Advantages and Limits. I. Water- Soluble Drugs, Drug Development and Industrial Pharmacy, Vol. 27 (3), p [14] Zebunnissa R. et al. (2010): Investigation of interaction of biodegradable micro- and nanoparticulate drug delivery systems with platelets, Journal of Pharmacy and Pharmacology, Vol. 63, p [15] Farahidah M. et al. (2008): Engineering Biodegradable Polyester Particles With Specific Drug Targeting and Drug Release Properties, Wiley Inter- Science, DOI: /jps [16] In-Sook K. et al. (2005): Physicochemical characterization of poly(l-lactic acid) and poly(d,l-lactideco-glycolide) nanoparticles with polyethylenimine as gene delivery carrier, International Journal of Pharmaceutics, Vol. 298, p [17] Elke W. et al. (1999): Microencapsulation of DNA using poly(dl-lactide-co-glycolide): stability issues and release characteristics, Journal of Controlled Release, Vol. 61, Nr 3, p [18] Nano Spray Dryer B-90 (2009): Operation Manual, BÜCHI Labortechnik AG, Flawil, Switzerland. Application Note 002/2011 Version A, Copyright 2011 BÜCHI Labortechnik AG 7/7