Viktor
Eriksson
a,
Jules
Mistral
ab,
Ting
Yang Nilsson
c,
Markus
Andersson Trojer
c and
Lars
Evenäs
*ad
aDepartment of Chemistry and Chemical Engineering, Chalmers University of Technology, 412 96, Gothenburg, Sweden. E-mail: lars.evenas@chalmers.se
bUniv Lyon, CNRS, UMR 5223, Ingénierie des Matériaux Polymères, Université Claude Bernard Lyon 1, INSA Lyon, Université Jean Monnet, F-69622, Villeurbanne Cédex, France
cDepartment of Polymers, Fibers and Composites, Fiber Development, RISE, 431 53, Mölndal, Sweden
dWallenberg Wood Science Center, Chalmers University of Technology, 412 96, Gothenburg, Sweden
First published on 14th February 2023
Functional textiles is a rapidly growing product segment in which sustained release of actives often plays a key role. Failure to sustain the release results in costs due to premature loss of functionality and resource inefficiency. Conventional application methods such as impregnation lead to an excessive and uncontrolled release, which – for biocidal actives – results in environmental pollution. In this study, microcapsules are presented as a means of extending the release from textile materials. The hydrophobic model substance pyrene is encapsulated in poly(D,L-lactide-co-glycolide) microcapsules which subsequently are loaded into cellulose nonwovens using a solution blowing technique. The release of encapsulated pyrene is compared to that of two conventional functionalization methods: surface and bulk impregnation. The apparent diffusion coefficient is 100 times lower for encapsulated pyrene compared to impregnated pyrene. This clearly demonstrates the rate-limiting barrier properties added by the microcapsules, extending the potential functionality from hours to weeks.
A means to control the release of the actives, and thus means to avoid the issues stated above associated with uncontrolled release, is to encapsulate them into delivery vehicles. Several delivery vehicles for achieving sustained release are presented in the literature, including mesoporous silica nanoparticles8,9 and polymeric microcapsules2,5 among others. For the last decades, polylactides have been used extensively as biodegradable microcapsule materials.10,11 This family of polymers can be produced from renewable resources and degrade into safe degradation products, making them attractive alternatives as microcapsule materials.12,13
In many applications, sustained release is desired from a macroscopic object such as a paint film or an implant where the microparticles are embedded in for example a coating matrix or a hydrogel.14–16 An additional and growing product segment is functional fiber materials, ranging from textiles in agriculture and ropes or nets for marine applications to wound dressings, implants, and fibrous scaffolds for tissue engineering.17 Fibers are, as compared to coatings and hydrogels, generally more difficult to functionalize with microcapsules – especially polymeric capsules that are biobased and thermosensitive18 when compared to inorganic particles. The use of biobased capsules does, however, add value to the materials by allowing the design of systems based on renewable resources and that can be biodegraded. Given the current trends toward a circular economy and biobased materials, regenerated fibers from the most abundant biopolymer cellulose are interesting for these functional fiber materials.19 Several methods are available for preparing different types of fiber materials based on regenerated cellulose. The most well-known techniques for preparing continuous regenerated cellulosic fibers are the viscose and lyocell wet-spinning processes.19,20 If instead nonwovens are desired, electrospinning is a popular approach where nanoparticle functionalization previously has been studied.21,22 Noteworthy here is solution blowing, which has recently been proposed as an alternate way of producing cellulose nonwovens, addressing the problems of upscaling associated with electrospinning.23–25 Compared to fossil based fiber materials, spinning fibers from regenerated cellulose often requires harsh chemicals, further complicating any functionalization by microcapsules. We have recently published a paper describing a wet-spinning and corresponding solution blowing method that allows for a range of microcapsules to be efficiently incorporated into continuous textile filaments or nonwovens based on different biobased polysaccharides.18
The sustained release from particles in the nanometer range is usually fast and complete within hours to days due to short diffusional pathways. This is especially true for inorganic nanoparticles onto which the actives are physiosorbed (such as in the nanoparticle-functionalized electrospun fibers mentioned above). In such a case, the diffusivity barrier added by the fiber matrix would also affect the release to a significant amount, leading to difficulties in predicting and controlling the release profile. In addition, any swelling of the fiber would affect the release profile. It is instead desirable to have a controlled release system that is rate-determining. This would completely decouple the release properties of the fiber matrix from the produced release profile, leading to improved predictability and control by tuning the microparticle size.
In this work, we demonstrate how the microcapsule barrier properties dominate, and therefore facilitate the control of, the release rate from cellulosic nonwovens. The model substance pyrene has been loaded into solution blown cellulose fibers in three different ways: (i) surface impregnation, (ii) bulk impregnation, and (iii) microencapsulation into microcapsules embedded within the fibers. Following this, the release of pyrene from these materials has been investigated and fitted with Fickian diffusion models to determine the apparent diffusion coefficients of pyrene and hence the barrier properties of the materials.
Solution blowing was performed at room temperature in a Biax-Fiberfilm solution blowing spinneret containing 9 nozzles of 220 μm diameter. The spinneret was connected to a cylinder pump set to an extrusion speed of 2.5 mL min−1 and pressurized air at 0.3 bar. The solution was sprayed onto a rotating drum immersed in a deionized water coagulation bath where the nonwoven cellulose was collected. Following solution blowing, the nonwoven was thoroughly washed in deionized water to remove residual EMIMAc.
Samples for surface impregnation of pyrene were prepared by solution blowing pure cellulose fibers. After washing and drying the nonwovens, a solution of pyrene in DCM was added dropwise at 2 mL g−1 cellulose. This was just below the maximum amount of liquid that the nonwovens could hold without anything dripping from the material. Finally, the DCM was allowed to evaporate completely overnight in a fume hood.
The loading efficiency of pyrene in the samples was determined by immersing the fibers in methanol overnight to leach out all pyrene and then determining the pyrene concentrations by UV-visible spectrophotometry.
(1) |
(2) |
Here, K is the pyrene partitioning coefficient between release medium and sphere, and Vsink and Vsphere are their respective total volumes. Finally, qs,n is the n: th non-zero positive root of
(3) |
The samples of microcapsules are polydisperse as described by the size distribution p(r). The final expression for the fractional release is then given by
(4) |
By similar means, release from the fiber samples with pyrene either bulk impregnated or encapsulated into microcapsules, could be described by diffusion in a cylindrical geometry. The Fickian diffusion models used here are presented in the ESI.†
L929 mouse fibroblast cells (NCTC clone 929: CCL-1 American Type Culture Collection) were propagated in a tissue culture flask to obtain subconfluent monolayers of cells in a 96 well plate (100 μL/104 cells per well). The extracts were added to the six replicate wells containing the subconfluent cell monolayer and the plate was incubated for 24 hours at 37 °C. After the incubation, the extracts were removed and an MTT solution was added to each well after which the plate as incubated for two hours at 37 °C in 5% CO2. The MTT solutions was subsequently removed, after which 2-propanol was added under rigorous shaking. The viability was finally assessed by quantifying the resulting formazan product using UV-vis spectrophotometry at 570 nm.
For preparing surface impregnated fibers, pyrene was dissolved in dichloromethane and added dropwise to a cellulose nonwoven material after which the solvent was evaporated rapidly. In this case, pyrene was detected on the surface of the fibers using fluorescence microscopy (Fig. 1a2). A homogeneous film of pyrene was not observed but it was rather found in small and separated areas with larger concentrations of pyrene. Fibers with bulk impregnated pyrene were prepared by dissolving pyrene directly in the dope solution before solution blowing. In this case, homogeneous fluorescence intensity was seen throughout the entire fiber (Fig. 1b2), indicating an even distribution of pyrene. It should also be noted that cellulose exhibits autofluorescence in the same spectral region as pyrene, making it hard to differentiate a low pyrene concentration from cellulose based only on fluorescence micrographs.
In the microcapsule-functionalized fibers, the microcapsules were well-dispersed throughout the entire fiber cross-section (Fig. 1c1 and 2b). Fluorescence micrographs revealed that pyrene was highly partitioned towards the microcapsules rather than the surrounding fiber (Fig. 1c2 and Fig. S5, ESI†). The microcapsules were also seen to remain intact without any fragmentation during the solution blowing process. This was further confirmed by scanning electron microscopy, Fig. 2a. Some fibers appeared to be slightly flattened, most likely a consequence of collecting not fully coagulated fibers onto the rotating drum in the coagulation bath. For all three samples, the fibers were loaded with equal amounts of pyrene. The biocompatibility of microcapsule-functionalized fibers without pyrene was determined. The material was non-cytotoxic towards L929 mouse fibroblast cells as shown in the ESI.†
As can be seen in Fig. 3 there was a significant difference in the release profiles from the different materials. From fibers with a simple surface impregnation of pyrene, a large burst release of about 50% was observed at the first data point after 40 s (0.01 h). Additionally, a very rapid time-dependent release was observed, with a complete release reached after approximately 1 hour. For the samples with bulk impregnated pyrene, an improvement was observed in terms of a significantly lower burst release compared to that of the surface impregnated samples. However, a rapid release was still seen with almost all pyrene released after approximately 20 hours. A considerable decrease in the release rate was observed from fibers where pyrene was encapsulated in microcapsules. From these fibers, a plateau in the release was not reached until after approximately two weeks (300 hours). Adding to this, the observed burst release fraction was negligible. Another benefit of the encapsulation compared to bulk impregnated pyrene, as previously described by us,18 was the substantial increase in efficiency with regard to fiber uptake of pyrene during preparation.
After approximately three months (2500 hours) in the release medium, there was a distinct change in the appearance of the microcapsules. Before starting the release measurement, the pristine microcapsules were observed as smooth spheres (seen in the insets of Fig. 4 and 1c). However, after three months in the release medium they had started to degrade and fragment, as seen in Fig. 4. A more detailed comparison for microcapsules immersed directly in the aqueous release medium is shown in Fig. S8 in the ESI.† Since the motivation for using polylactides is that the polymer eventually will degrade, this was the intended outcome. Given enough time, the microcapsules would degrade completely, thus preventing the accumulation of microplastics in the environment. No macroscopic changes in the appearance of the fiber materials were however observed over this experimental time frame (see ESI†). At equilibrium in the release measurement, a small amount of pyrene was still seen to be remaining in the microcapsules. This was detected from the fluorescence micrograph in Fig. 4b. A corresponding confocal micrograph of the sample is shown in Fig. S9 in the ESI.† When immersing the fibers in methanol, which is a far better solvent for pyrene compared to the release medium, to leach out the residual pyrene inside the spheres at equilibrium this amount was found to be 3% of the total loading.
For the composite fibers loaded with microcapsules, the modelled diffusion coefficient D includes – in contrast to the effective diffusion coefficient described previously – diffusional contributions from all compartments of the composite material as well as the distribution of the active between the compartments. This parameter is therefore termed the apparent diffusion coefficient. A comparison of the apparent and effective diffusion coefficients is a powerful tool to investigate the impact of adding different components to a composite material on the release. In addition, the sample geometry used in the model equation can be chosen to monitor the influence of different compartments separately on the diffusion coefficient. In this paper, two different geometrical approaches have been employed to fit the release data as seen in Fig. 5. (1) A cylindrical geometry was used to model the release from cellulosic fibers. Here, the diffusion coefficient is in principle the permeability of the active in the fiber (with or without microcapsules) and consequently a direct measure of the effect of encapsulation on the release. (2) A spherical geometry was used to model the release from the microcapsules. Using this approach, the effect of the surrounding medium (cellulose matrix and aqueous phase for the fiber material or aqueous phase for the microcapsule suspension) on the release from the microcapsules could be investigated. To put this in a slightly different context, approach (2) allowed the rate-determining release properties of the microcapsule to be assessed.
Fig. 5 Fitted diffusion coefficients D from models based on both a cylinder (fiber) and a sphere (microcapsule). |
When comparing the apparent diffusion coefficient of the microcapsule-functionalized fibers fitted by a cylindrical geometry to the fitted effective diffusion coefficient in fibers containing bulk impregnated pyrene in Fig. 5, a reduction by almost two orders of magnitude could be observed. As seen in Fig. 3 this corresponded to an increase in the lifetime of the material from hours to more than a week. This was a notable difference, especially considering that the average fiber diameter was around ten times greater than the average microcapsule diameter (Fig. S1 in the ESI†). By employing a simplified steady-state model29 it was found that microcapsules with radii as small as 30 nm would still be the main rate-limiting barrier, discussed more in detail in the ESI.† When instead comparing the diffusion coefficient for microcapsules embedded within the fibers to those free in aqueous suspension there was only a minor difference in the fitted values. This was a clear indication that the microcapsules acted as the rate-limiting barrier in the release system and that the barrier effects of the surrounding fiber were of minor importance in comparison. If this would have not been the case and there was a significant rate-limiting barrier added by the fiber, the fitted effective diffusion coefficient for microcapsules in fibers, fitted on a spherical geometry, would have been significantly smaller than the corresponding value for free microcapsules in aqueous suspension.
Release from the fiber sample with a surface impregnation of pyrene was fitted using a different approach than the one used for bulk impregnation. Two distinct contributions to these release profiles could be seen from the data: an immediate burst release and a diffusive release. During impregnation, a fraction of pyrene was likely also dissolved into the outermost parts of the fiber in addition to the pyrene that was deposited on the outer surface. Therefore, pyrene impregnated onto the fiber surface was released rapidly, resulting in the large burst fraction of around 50% detected at the first measurement after 40 s (0.01 h). This burst was here assumed to be of zero-order kinetics (simply due to lack of data points) for mathematical modelling, shown by the dashed line for surface impregnated pyrene in Fig. 3. The diffusive part of the release was significantly slower. This part was modelled as diffusion from a plane sheet with the same effective diffusion coefficient as the sample containing bulk impregnated pyrene. Hence, the diffusion coefficient was not fitted for this material. Instead, this allowed us to determine the thickness of the pyrene-containing layer to 7 μm, which was about half of the average fiber radius.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2tb02485c |
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