Introduction
Solution blow spinning (SBS) is gaining traction as a rapid technique to produce nano- and micro-fibres, using simple equipment with no electric fields [
1‐
6]. The fibre networks produced typically exhibit more open inter-fibre porosity than electrospun mats [
7,
8]. A range of materials have been successfully processed to fibres by SBS to date, including polymers [
2,
3], composites [
9,
10], ceramic fibres either by incorporating inorganic components in a matrix followed by burn-off of the organic matrix and sintering [
11‐
13], and those derived by sol–gel precursors and subsequent calcination [
14,
15]. Highly porous bioactive composite fibres have been produced by solution blow spinning poly(
D,
L-lactide) solutions containing nanobioactive glass particles directly into a cryogenic bath followed by lyophilisation [
9]. Macroporous bioactive nanofibre networks were further created using ice as an in situ porosifier, by spraying water droplets to the same point at which fibres contact the cryogenic phase [
9]. These fibres were shown to exhibit rapid formation of hydroxycarbonate apatite (HCA) and release soluble silica ions [
9]; however, due to the highly porous structure they exhibited high apparent water-in-air contact angles ~140° [
9]. Such relatively hydrophobic materials frequently require surface modification for subsequent in vitro studies, such as plasma treatment to improve their wettability [
16,
17].
Whilst melt and flame sprayed bioactive glasses are attractive as therapeutic ion delivery vehicles [
18‐
23] they can be challenging to process into composite fibres, especially at high weight fractions due to issues with sedimentation and increased viscosity [
9]. Sol–gel derived bioactive glasses can be produced at low temperatures and formed into monolithic scaffolds in situ by direct foaming [
24‐
27], freeze-casting [
28‐
30], and even fibrous structures via electrospinning and air-spraying (a process analogous to SBS) [
31,
32]. Notably, bioactive glasses with compositions that contain calcium and phosphate exhibit more bioactivity than pure silica based glasses [
33,
34]. However, these materials can be brittle and it can be challenging to incorporate therapeutic ions into the glass network, especially when low/non-toxic materials are desired [
35,
36]. Sol–gel inorganic–organic hybrids expand the range of material properties achievable and have been comprehensively reviewed by Jones [
18]. Hybrids can behave as homogeneous materials and have a range of mechanical properties that can be tailored to soft tissue [
18,
25,
37‐
39]. Their degradation rate and mechanical properties can be tuned by judicious selection of the inorganic and organic phases as well as degree of crosslinking [
18,
25,
38]. Hybrids are broadly split into two classes, class I where the inorganic and organic phases are bonded by weak intermolecular interactions, such as hydrogen bonds and van der Waals forces, and class II hybrids which have covalent bonds between the inorganic and organic phases [
37,
40,
41].
In 2000, Orefice et al. [
32] produced sol–gel bioactive fibres using an air-spraying process containing both calcium and phosphate ions followed by sintering. The air-spraying process employed is similar to SBS process, using an air-brush as the spraying device [
32,
42,
43]. The spinnability was optimised by limiting the water content to produce quasilinear chains of silica rather than a highly connected network [
32]. Electrospinning has also been used to form neat silica and calcium containing silica sol–gel fibres [
31]. Most methods utilise solvent evaporation to tune the rheological properties to reach a spinning window [
31,
44,
45]. Increased evaporation typically leads to higher colloidal concentration, which raises the rate of network formation. As evaporation continues viscosity is raised, and past a critical point the polycondensing sol–gel will no longer spin into fibres. The electrospinning technique has been extended to inorganic–organic hybrids formulations, commonly using tetraethyl orthosilicate (TEOS) as the inorganic phase with various natural and synthetic organic phases, including PLA [
46], chitosan/poly(ethylene oxide) (PEO) [
47], and gelatin functionalised with (3-glycidyloxypropyl)trimethoxysilane (GPTMS) [
48,
49]. GPTMS can be used to provide the inorganic network [
48], as well as act as a crosslinker between inorganic and organic phases [
49].
Gelatin is attractive as organic phase particularly as it is recognised and adhered to by many cell types, can impart toughening, and is relatively hydrophilic [
38]. Solvents such as hexafluoroisopropanol [
50], trifluoroethanol [
51,
52], and acetic acid [
53,
54] have been used to fabricate gelatin nanofibre scaffolds. Whilst more challenging, aqueous electrospinning of gelatin at elevated temperatures has been achieved using a heated chamber and concentrations of or up to 30–40 wt% [
55], or alternatively using binary PBS (phosphate-buffered saline)-ethanol solutions to tune ionic strength and spinning at room temperature [
56]. Importantly, gelatin exhibits thermoreversible properties in aqueous solutions, existing as a random coil above sol–gelation point (circa 20 °C) and forms triple helices below the gelation temperature [
57,
58].
In the current work, the sol–gel inorganic–organic hybrid formulations are developed to enable rapid fibre formation by SBS, leveraging the capability of cryogenic SBS (cryo-SBS) [
9] to form class I and II hybrid porous fibres, the thermoreversibility of gelatin in aqueous solutions is also exploited to tune the hybrid precursor solution viscosity. Porous hybrid fibres are realised using cryo-SBS, with
tert-butanol (TBA) in place of ethanol in the formulations, as it facilitates thermally induced phase separation (TIPS) and is more facile to lyophilise due to its relatively high melting point and suitable vapour pressure. Porous hybrid spheres are also produced here using these formulations, which can have application as injectable systems and for cell expansion in bioreactors. The combination of hybrid processing with cryo-SBS enables porous fibres to be produced with higher inorganic contents than particulate filled polymer fibres. The silica–gelatin porous hybrid fibres produced here are inherently hydrophilic and do not require further surface treatment, and are a platform for tuning mechanical and degradation properties, as well as ion release downstream.
Materials and methods
Materials
Gelatin derived from porcine skin (Type A, gel strength ~300 g bloom), Dulbecco’s phosphate-buffered saline (PBS, pH 7.4), tetraethyl orthosilicate (TEOS, purity 98%, reagent grade), (3-glycidyloxypropyl)trimethoxysilane (GPTMS, purity 97%), tert-butyl alcohol (TBA, purity ≥99.0% ACS reagent), ethanol (EtOH, purity ≥99.5%), and HCl (1 M standard solution) were all obtained from Sigma Aldrich, UK.
Precursor preparation, fibre spinning
Sol–gel and hybrid fibre precursor formulations for conventional SBS
For the production of neat sol–gel silica fibres a sol was prepared using formulations based on the work of Poologasundarampillai et al. [
31]. Briefly, TEOS, EtOH (mutual solvent), water and 1 M HCl (acid catalyst for hydrolysis) were added in that order to a final molar ratio of 1:2:2:0.01. Typical volumes used to make all the sol-precursors are given in Table S1. The solution was mixed for 24 h at room temperature in a sealed 50-mL polypropylene container (Falcon
® tube), using a roller mixer (Stuart SRT6, Bibby Scientific, USA). This solution was then transferred to a water bath and then heated to 70 °C with the lid-removed to assist evaporation, until ~65% of the mass remained and a suitable viscosity for solution blow spinning (SBS) obtained, SBS is described further below. Evaporation of EtOH has been shown to increase solution viscosity by catalysing the polycondensation of the hydrolysed TEOS [
31].
Sol–gel silica–gelatin class I hybrid solutions for conventional SBS were prepared in two parts. Aqueous gelatin solutions at 10% w/v were prepared by dissolving gelatin (typically 1 g gelatin in 10 mL deionised water) at 40 °C under magnetic stirring in a sealed container, using a temperature controlled water bath. Separately a sol was prepared as described above to give a molar ratio of 1:0.7:1.6:0.03 (TEOS:EtOH:H2O:HCl) and mixed until the solution was clear. Hybrid solutions were typically made by mixing 6 mL of gelatin solution to 7 mL silica sol in a 100-mL sealed container under continuous magnetic stirring and the pH adjusted to 3.6 by drop-wise addition of 1 M HCl in 50 µL increments. The hybrid solution was then heated in the sealed container under continuous stirring in a water bath at 60 °C. At intervals of 5 min, samples of the stock solution were taken for solution blow spinning, in order to identify the reaction time/window to enable fibre production (determined later to be between 30 and 40 min, discussed below) as opposed to droplet spray or blocking.
Class I and Class II hybrid fibre precursors for cryogenic SBS
The sol formulation described above was modified to enable effective freezing and lyophilisation of the hybrids fibres, by the use of
tert-butyl alcohol (TBA) in place of ethanol. TBA was selected as it has a melt point of 25 °C, has been used to form silica cryogels [
59], is relatively facile to lyophilise, and forms azeotropes with water and ethanol. TEOS, TBA and 1 M HCl were added in that order to give a molar ratio of 1:5:2.7:0.05 (TEOS:TBA:H
2O:HCl), typical volumes shown in Table S1. The sol was then roller mixed as described above for 24 h. Class I hybrid solutions were then prepared by mixing a 10% w/v solution of gelatin in PBS (prepared as described above) with the hydrolysed sol. Class II hybrid precursor solutions were made by functionalising gelatin (already dissolved at 10% w/v in PBS) by addition of GPTMS to 92 µL g
−1 of gelatin, and subsequent incubation at 40 °C for 13 h under continuous orbital shaking (150 rpm, using a Stuart Orbital incubator SI50, Bibby Scientific Limited, UK), following a method adapted from [
60]. Class I and II hybrid solutions were typically made by mixing 7.26 mL of the hydrolysed sol to 8 mL of the gelatin solutions, to achieve a target theoretical inorganic content of ~40% (based on assumed SiO
2 content). Hybrid solutions were sealed in Falcon
® tubes, then placed in a water bath at 40 °C. The gelatin was initially observed to precipitate upon mixing and then solubilise within 30–40 min. The solutions were held for a further 20 min prior to cryo-SBS and rheological characterisation, described below.
Fibre production via conventional and cryogenic SBS
Conventional and cryogenic SBS was conducted based on set-ups previously described for the production of bioactive smooth and porous nanobioactive glass-filled PLA composite fibres [
9]. For conventional SBS, neat sol–gel and hybrid precursor solutions (described in “
Sol–gel and hybrid fibre precursor formulations for conventional SBS” section) were injected to the inner coax of the SBS head at a rate of 200 μL min
−1 using a precision syringe driver (N-300, New Era Pump Systems, Inc., Farmingdale, NY, USA). The inner nozzle was surrounded by a high-pressure air sheath, or outer nozzle, through which compressed air was delivered at 0.207 MPa (30 psi) via a Bambi VT150D oil-free and water-free air compressor (Bambi, Italy) and in-line regulator 5 cm from the SBS head air inlet. The internal diameter (ID) of the SBS inner nozzle was 0.8 mm, outer diameter (OD) 1.8 mm, the outer nozzle had an ID of 2 mm and OD of 3 mm. The inner nozzle had a tapered end (truncated cone) and set to protrude 2 mm from the outer nozzle. Fibres were collected at a distance of 30 cm from the SBS head on a round (~15 cm diameter), static stainless steel collector with spokes 2 mm diameter (adapted from a computer case fan grill).
Cryo-SBS [
9] was conducted using class I and II hybrid formulations described in “
Class I and Class II hybrid fibre precursors for cryogenic SBS” section. The solutions were injected separately into the inner nozzle of the SBS head as described above at a rate of 230 μL min
−1, using outer nozzle pressure supply of 0.138 MPa (20 psi). Fibres were sprayed directly into a bath of liquid N
2 at a working distance of 10 cm and at an angle of ~45° to the bath. The spraying angle and sheath pressure were selected to limit liquid N
2 loss. The bath container was made from an expanded-polystyrene foam box. During cryo-SBS, the level of liquid N
2 in the bath was constantly topped-up to ensure that the working distance remained ±2 cm of the target value (10 cm). The temperature of the solution was measured at the SBS nozzle, by direct contact with a thermostatic probe, to be 22 °C. Frozen fibre membranes were collected in 50 mL Falcon
® tubes, transferred to lyophilisation flasks (pre-cooled in liquid N
2) and subsequently lyophilised using a VirTis BenchTop Pro Freeze Dryer (NY, USA). The external glass portion of the lyophilisation flask was kept under liquid N
2 using the polystyrene box until a vacuum of ~50 mTorr had been reached, after which the outside of the flask was left exposed to ambient conditions. Lyophilisation was continued for 72 h, after which the fibres were collected for characterisation. Additionally, porous class I and class II hybrid spheres were formed based on a method previously described by Blaker et al. [
9,
61], and briefly solutions were directly dripped into liquid nitrogen and lyophilised as described above.
Rheological characterisation of precursor spinning solutions
Rheological properties were conducted on precursor spinning solutions as function of shear rate and temperature using an AR-G2 Rheometer (TA Instruments, USA) operated using a cone-plate set-up. Samples of 1–2 mL were tested. Shear rate measurements were conducted over the range (\( \dot{\gamma } \)) (10–500 s−1) at 40, 30 and 20 °C for class I, class II and gelatin precursor solutions. Temperature-dependent viscosity measurements made at a constant shear rate of 500 s−1 (as all samples exhibited Newtonian behaviour) as a function of temperature for class I and class II. The cryo-SBS hybrid precursors from 40 to 12 °C over a 10 min period (equivalent rate 2.8 °C min−1). The effect of temperature cycling between 20 and 30 °C was investigated to assess thermoreversible properties. The temperature cycling measurements consisted of a conditioning step at 30 °C for 2 min to allow the sample to equilibrate, a peak hold step under constant shear rate (500 s−1) at 30 °C for 1 min, followed by a conditioning step at 20 °C for 2 min, and a peak hold step at 20 °C (500 s−1). Samples were left to relax between each temperature ramp. During the peak hold step, a 10 s sampling time was used. The temperature cycle was repeated several times for both cryo-SBS class I and II precursor solutions. The viscosity was also measured as a function of time for approximately 75 min at 40 °C for class I and II solutions, as well as gelatin-GPTMS solution to identify the time point at which extensive crosslinking between the organic and inorganic phase occurred. The class II solution was also measured at 20 °C over 75 min to assess the effect of temperature and time on viscosity.
Surface tension measurements of cryogenic spinning precursor solutions
Surface tension measurements of class I and II hybrid precursors were obtained using a KRÜSS DSA 100 (GmbH, Germany) at room temperature (22 °C), mimicking spinning conditions. The surface tension of 10% w/v gelatin (in PBS) and gelatin functionalised with GPTMS (“Class I and Class II hybrid fibre precursors for cryogenic SBS” section) were obtained at 30 °C as the both were below their gelation point at room temperature. Surface tension was computed for six separate droplets per sample, using Drop Shape Analysis software (KRÜSS GmbH, Germany).
Physical and structural characterisation of sol–gel and hybrid fibres
Fibre morphology and diameters
The morphologies of fibres and porous spheres were assessed using scanning electron microscopy on a FEI Sirion FEG-SEM (USA), as well as Hitachi S300 N microscope (USA) for screening. Porous spheres were sectioned using a double-edged Wilkinson Sword
® razor blade, as previously described [
9,
61]. Mounted samples were blown with compressed air to remove and loose material and sputter coated with platinum using a Gatan Model 682 Precision Etching Coating System (USA) to an average thickness of 7 nm. Fibre diameters were analysed using ImageJ (version 1.48, NIH, Bethesda, MD, USA) software, from a minimum of 70 fibres.
29Si magic angle spinning nuclear magnetic resonance (MAS NMR)
The solid state
29Si MAS NMR spectra were obtained using a Bruker AVANCE (USA) 400 MHz spectrometer operating at 79.48 MHz. The spectra were recorded with a 20° pulse and a repetition delay of 20 s (4000 scans) using 4 mm zirconia rotors spinning at 6 kHz. The spectra were referenced against TMS (0 ppm). The degree of condensation (
D
C) was the following equation [
25]:
$$ D_{\text{C}} = \left( {\left[ {\frac{{ 2T^{2} + 3T^{3} }}{3}} \right] + \left[ {\frac{{2Q^{2} + 3Q^{3} + 4Q^{4} }}{4}} \right]} \right) \times 100\% $$
(1)
where
T
n
and
Q
n
are the relative abundances of silicon atoms bound to a carbon and
n bridging oxygens and silicon atoms bridging to
n oxygens, respectively [
25].
Thermogravimetric analysis (TGA) for inorganic content determination
TGA was conducted using a TGA-Q500 (TA Instruments, USA) using a dynamic heating regime in air to ensure removal of the organic phase. Samples (2–30 mg) were first equilibrated at 40 °C for 60 s to remove trapped water then heated at 10 °C min−1 to 75 °C, and held 15 min to perform an isotherm. The temperature was then raised using a high-resolution ramp at 50 °C min−1 to 400 °C and held for 30 min, the isotherm ensured complete combustion of the organic phase. Heating was then continued in high-resolution mode at 50 °C min−1 to 800 °C, at which point the residual mass is representative of the silica content as the gelatin has undergone thermal degradation.
Surface area analysis
Brunauer–Emmett–Teller (BET) surface area measurements of the fibres were made using a surface area and porosity analyzer (TriStar II, Micromeritics, Neu-purkersdorf, Austria). Prior to the gas adsorption experiments, impurities were removed via a “degassing” step using a FlowPrep 060 instrument (Micromeritics). Approximately 100 mg (about 1 cm3) of each sample was placed inside a glass sample chamber, which was purged with nitrogen at ambient temperature overnight. Afterwards, the nitrogen adsorption isotherms were measured at 77 K.
Wetting of the hybrid fibre membranes by contact angle analysis
Class I and class II hybrid fibre membranes produced via cryo-SBS were mounted separately on glass microscope slides using thin strips of Scotch®Magic™ Tape (3 M) applied at the sample edges to obtain reasonably flat surfaces for measurement. Contact angle measurements were taken on a KRÜSS DSA 100 (GmbH, Germany). Single droplets of water were dropped onto the substrates and the wetting behaviour recorded by video recorded with a built-in high-speed camera.
Conclusion
Sol–gel silica and inorganic–organic silica–gelatin hybrids have been formed into fibres via solution blow spinning. The spinning window, however, is rather short, and the process is challenging to control. Formulations have been developed by replacing ethanol with tert-butanol (TBA), which has a higher melting point, forms azeotropes with water and ethanol, and is relatively facile to freeze and remove via lyophilisation. TEOS/gelatin hybrid formulations containing TBA could be dripped directly into liquid nitrogen and lyophilised to realise highly porous class I and II hybrid spheres. The reversible thermally activated conformation transition of gelatin from random coil to triple helix was exploited to tune formulation viscosity and therefore enable cryogenic solution blow spinning to be conducted. The resultant class I and II fibres exhibited porous structures, which should extend the degradation, ion release and mechanical property ranges of these materials. The in vitro behaviour and inclusion of calcium and other therapeutic ions is the focus of current research.