Patent Application
20160206745
Rapid prototyping, such as solid free-form fabrication, is a technique used to form porous three-dimensional (3D) scaffolds for tissue engineering applications. A major advantage of this fabrication method is the ability to control pore structures and geometries. 3D-Bioplotting is a method in which a viscous or a paste-like slurry is extruded by compressed air-induced pressure, onto a surface in air or submerged in a liquid medium (1-4). The continuous injection of a material using layer-by-layer deposition generates a fully interconnected pore structure. Although solid free form fabrication is commonly applied to synthetic materials, 3D printing and plotting of natural biopolymers such as proteins and polysaccharides is not as common. One challenge in printing soft materials is that they have a wide range of intrinsic properties such as viscosity, which can vary greatly between batches during sample fabrication.
Animal and plant-based proteins are ideal biomaterials due to inherent bioactivity, degradation properties, and natural binding sites that can be tailored to control cell adhesion and growth both in vitro and in vivo (5). Specific optimization and design of printing parameters and conditions have been developed for various proteins (6-10) and composites (11, 12). 3D printing using a powder mixed with a binding solution was used to fabricate a blend of corn starch, gelatin, and dextran (11). Gelatin alone has also been plotted at 3% and 10% concentrations in water (6, 7). Landers et al. discussed the important fabrication parameters involved in plotting soft gel materials including viscosity, swelling in plotting medium, density, and thermal behavior of the plotted material, using gelatin and agar as examples (7). Bovine collagen has been fabricated using an indirect printing technique, where the slurry is printed into a negative mold, and the mold is subsequently dissolved away after freeze-drying to leave a scaffold with a predefined structure (8-10). The 3D Bioplotter was also used to fabricate 3D collagen-chitosan-hydroxyapatite hydrogels in a study characterizing the angiogenic and inflammatory response in vivo in comparison with a plotted PLGA scaffold (12).
Two main challenges with printing natural biopolymers include controlling strut solidification upon extrusion (6) and electrostatic interactions between the biopolymer and liquid media if plotting into a solution. Scaffolds formed in air would require that the slurry dries at an appropriate rate during extrusion to support subsequent printed layers. Plotting into a liquid medium requires that the density of the solution be the same as the material being injected to preserve strand shape and to prevent dissolution of the printed strands (2, 7). Standardizing the plotting method for individual soft materials is essential since variability in moisture content of slurries and drying as a result of variable environmental conditions can affect reproducibility during mass production.
Three dimensional porous soy protein-containing scaffolds comprising denatured soy proteins are provided. The soy protein chains in the denatured soy proteins can be crosslinked with enzymatic crosslinkers or with non-enzymatic chemical crosslinkers. The scaffolds can be tailored to have pore sizes suitable for promoting cell growth and proliferation within the pores and/or robust mechanical properties, as determined by their compressive moduli.
In one aspect, a porous soy protein-containing scaffold comprises a plurality of layers configured in a vertical stack, each layer comprising a plurality of strands comprising denatured soy proteins.
In some embodiments, the scaffold has a porosity of at least 50% and a pore interconnectivity of at least 90%.
In some embodiments, the scaffold is configured such that within each layer the plurality of strands are spaced apart and aligned along their longitudinal axes and the angle, θ, defined by the longitudinal axes of the strands in adjacent layers is in the range of 0°<0≦90°, such that pores are defined by the strands in adjacent layers of the vertical stack. In some embodiments, the angle θ is in the range of 45°≦θ≦90° or in the range of 75°≦θ≦90°. In some embodiments, the angle θ in the scaffold is in the range of 85°≦θ≦90°; the soy protein chains in the denatured soy proteins are crosslinked; the crosslinking density of the soy protein chains within the scaffolds is at least 0.3; and the scaffold has a compressive modulus of at least 3500 Pa.
In some embodiments, the scaffold has a compressive modulus of at least 1000 Pa or a compressive modulus in the range from about 2000 Pa to about 5000 Pa.
In some embodiments, the soy protein chains in the denatured soy proteins are crosslinked and the relative crosslinking density of the soy protein chains within the scaffolds is at least 0.1 or in the range from about 0.1 to about 0.35.
In some embodiments, the pores have a median pore diameter in the range from about 200 μm to about 1000 μm or in the range from about 300 μm to about 400 μm.
In some embodiments, the median x-axis strand thickness for the strands is no greater than about 600 μm.
In some embodiments in which the scaffold is crosslinked, the crosslinks comprise carbodiimide crosslinks.
In some embodiments, the scaffold further comprises at least one of a growth factor or a drug incorporated into one or more of the strands.
In another aspect, a tissue growth scaffold comprises any of the disclosed porous soy protein-containing scaffolds and tissue-forming cells, or cells that are precursors to tissue-forming cells, integrated within the pores of the porous soy protein-containing scaffold.
In another aspect, a method of growing tissue on a tissue growth scaffold comprises culturing the scaffold in a cell growth culture medium, wherein the scaffold comprises a plurality of layers configured in a vertical stack, each layer comprising a plurality of strands comprising denatured soy proteins; and tissue-forming cells, or cells that are precursors to tissue-forming cells, integrated within the pores of the scaffold.
In another aspect, a method of forming a porous soy-protein containing scaffold comprises extruding a slurry comprising denatured soy proteins in the form of a first layer, the first layer comprising a plurality of strands; and extruding the slurry in the form of one or more additional layers, each additionally layer being vertically stacked upon the previously extruded layer and comprising a plurality of strands.
In some embodiments, the strands in each layer are spaced apart and aligned along their longitudinal axes, and the angle, θ, defined by the longitudinal axes of the strands in adjacent layers is in the range of 0°<θ<90°.
In some embodiments, the slurry further comprises water and a plasticizer.
In some embodiments, the method further comprises dehydrating the scaffold in an alcohol.
In some embodiments, the method further comprises removing water from the scaffold via a dehydrothermal treatment. In some embodiments, the method further comprises freeze-drying the scaffold, whereby water is removed via the sublimation of water frozen on the strand surfaces, prior to the dehydrothermal treatment.
In some embodiments, the method further comprises chemically or enzymatically crosslinking the soy protein chains in the denatured soy proteins.
In some embodiments, at least 9 additional layers are extruded.
In some embodiments, the slurry comprises at least one of a growth factor or a drug.
In another aspect, a method of forming a porous biopolymer-containing scaffold comprises extruding a slurry comprising a biopolymer in the form of a first layer, the first layer comprising a plurality of strands; and extruding the slurry in the form of one or more additional layers, each additionally layer being vertically stacked upon the previously extruded layer and comprising a plurality of strands; wherein the mass flow rate of the slurry is maintained at a constant rate during extrusion by adjusting one or both of the extrusion pressure and extrusion speed during extrusion.
In some embodiments, the strands in each layer are spaced apart and aligned along their longitudinal axes, and the angle, θ, defined by the longitudinal axes of the strands in adjacent layers is in the range of 0°<θ<90°.
Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.
Illustrative embodiments of the invention will hereafter be described with reference to the accompanying drawings.
Porous soy protein-based scaffolds and methods for making the scaffolds using 3D printing techniques are provided. Also provided are tissue growth scaffolds comprising the porous soy protein-based scaffolds and methods for growing tissue on the tissue growth scaffolds.
The porous soy protein-containing scaffolds can be used as tissue growth scaffolds by integrating tissue-forming cells, or cells that are precursors to tissue-forming cells, within the pores or within the struts of the porous soy protein-containing scaffolds. Tissue can be grown within the tissue growth scaffolds by incorporating tissue-forming cells, or cells that are precursors to tissue-forming cells, into the porous soy protein-containing scaffolds (as by, for example, seeding); and culturing the seeded scaffolds in a cell growth culture medium. Human mesenchymal stem cells, hematopoetic stem cells, embryonic stem cells, and induced pluripotent stem cells are examples of precursors to tissue-forming cells. Examples of tissue-forming cells include osteoblasts, chondrocytes, fibroblasts, endothelial cells, and myocytes.
The porous soy protein-containing scaffolds can be printed via layer-by-layer extrusion of a soy protein-containing slurry (or “ink”) through a print head in a printer, such as an inkjet-type printer or bioplotter. In addition to the denatured soy proteins, the slurry may comprise a carrier liquid, such as water and additives, including plasticizers (e.g., glycerol) and antibiotic or antimycotic agents. In some embodiments, the slurry comprises a growth factor or a drug (i.e., a pharmaceutical compound), such that the growth factor or drug can be incorporated directly into one or more of the strands during the printing of the scaffold. Various amounts of denatured soy protein may be used in the slurries. In some embodiments, the amount of denatured soy protein in the slurry is equivalent to the saturation point of the denatured soy protein. In some embodiments, the amount of denatured soy protein in the slurry is in the range from about 15 weight % to about 20 weight %.
The use of 3D printing for the fabrication of the scaffolds is advantageous because it provides for regular geometric patterning of the layers that make up the scaffold, which makes it possible to control and tailor the porosity, pore size and pore interconnectivity of the scaffold. The printing can be carried out at relatively low extrusion temperatures, including temperatures in the range from about room temperature (i.e., 21° C.) to about 40° C. The mass flow rate of the slurry during extrusion is a parameter for achieving optimal, reproducible scaffolds. The optimal mass flow rate generally depends upon the composition of the slurry, e.g., the concentration of the soy protein. For a particular composition, an optimal mass flow rate can be determined by first optimizing the extrusion speed (i.e., the time over which the extrusion occurs) and next, optimizing the extrusion pressure (i.e., the pressure applied to extrude the slurry) as discussed in the Example, below. The flow rate measured at the optimized extrusion speed and optimized extrusion pressure corresponds to an optimal mass flow rate which can be maintained at constant rate during extrusion by adjusting one or both of the extrusion pressure and extrusion speed during extrusion. This constant mass flow rate approach to 3D printing can also be applied to the printing of porous scaffolds comprising other biopolymers, such as other proteins or polysaccharides.
One embodiment of a method of fabricating a soy protein-containing scaffold via a 3D printing process comprises the following steps: extruding a slurry comprising denatured soy proteins in the form of a first layer, the first layer comprising a plurality of strands comprising the denatured soy proteins; extruding the slurry comprising denatured soy proteins in the form of a second layer over the first layer, the second layer comprising a plurality of strands comprising the denatured soy proteins. This layer-by-layer printing technique can be repeated by the stepwise extrusion of one or more additional layers until a scaffold having the desired thickness comprising the desired number of layers has been fabricated. For example, in some embodiments the vertical stacks comprise at least 10 layers, at least 20 layers or at least 100 layers. The strands within each layer may be printed in a regular, repeating pattern, may be printed in a random arrangement, or in some combination of both.
The term “strand” as used herein refers to an elongated, continuous and unbranched structure that is delineated and distinguishable as a distinct unit within the scaffold (e.g., as seen in a scanning electron microscope (SEM) image), although strands in neighboring layers in the stacked structure may be merged at their interfaces. Within the scaffolds, strands deposited in a given layer remain substantially within that layer, although there may be sagging of the strands in a layer into the underlying layer. Thus, the strands do not form a mesh that is entangled in three dimensions with strands extended through multiple stacked layers of the scaffold.
The strands in a given layer are spaced apart, defining a strand spacing between adjacent strands. (See “w” in
The strands may be characterized by a thickness. The thickness may refer to the largest dimension (e.g., diameter) across a lateral cross-section of a strand. Alternatively, the thickness may refer to an x-axis thickness or a z-axis thickness. A x-axis thickness may refer to the largest dimension across a lateral cross-section of a strand as measured along an axis running parallel to a substrate over which the strand is printed. (See “t” in
The strands may be characterized by the shape of a lateral cross-section of a strand. Strands having various lateral cross-sectional shapes may be used, including circular and elliptical shapes. As discussed above, the printing methods make it possible to form uniform or substantially uniform strands, such that the lateral cross-sectional shape is maintained along the length of the strand.
The thickness and/or shape of the strands in a given layer can be, but need not be, the substantially the same as the thickness and/or shape of the strands in another layer. For example, the thickness of the strands in the scaffold may transition (e.g., continuously) from a first thickness value for stands in a layer at or near the bottom of the scaffold to a second (e.g., smaller) thickness value for strands in a layer at or near the top of the scaffold. Similarly, the lateral cross-sectional shape of the stands in the scaffold may transition from a first shape (e.g., circular) for strands in a layer at or near the bottom of the scaffold to a second shape (e.g., elliptical) for strands in a layer at or near the top of the scaffold. Other parameters of the scaffolds, e.g., strand spacing, may be substantially the same or different between different layers.
The 3D printing process is advantageous because it allows the user to control the porosity and pore interconnectivity of the scaffold. For example, for tissue growth scaffolds, the pore size, porosity and pore interconnectivity can be controlled to allow for the seeding of the cells and for efficient delivery of nutrients into and removal of waste from the interior of the scaffold. The optimal pore size, porosity and pore interconnectivity will depend on the cells being seeded and grown. Generally, it is desirably for the pores to have diameters of at least 2 times (e.g., 2-5 times) the diameter of the cells being seeded. For the purposes of this disclosure, pore diameter refers to a two-dimensional cross-sectional diameter, as viewed from above. Thus, the pore diameter for a given lateral cross-section of the scaffold could be determined by taking a lateral cross-section through the scaffold and imaging the surface of that section using, for example a high-resolution SEM. The average/median pore diameter for that layer could then be calculated from the image. An average/median pore diameter for the scaffold could be calculated by imaging a sufficient number of cross-sectional cuts through the scaffold. If the shape of a pore is not regular and symmetric, the diameter can be taken as the largest dimension across the pore. By way of illustration, in some embodiments of the present scaffolds, the average/median pore diameter is in the range from about 50 to about 1000 μm. This includes embodiments in which the average/median pore diameter is in the range from about 100 to about 500 μm. This further includes embodiments in which the average/median pore diameter is in the range from about 200 μm to about 1000 μm or from about 300 μm to about 400 μm. The shape and pore diameter of individual pores within the scaffold may be substantially the same (although they need not be).
The porosity of the scaffold refers to the percentage of void space within the scaffold. Some embodiments of the present scaffolds have a porosity of at least 50% (e.g., from about 50% to about 60%). This includes embodiments in which the porosity is at least about 60%, at least about 70%, at least about 80% or at least about 90% (e.g., from about 60% to about 95%). For the purposes of the present disclosure, the porosity values are measured via mercury intrusion porosimetry using a mercury porosimeter, following the method described in reference (13).
After the scaffold structure has been printed, it can undergo various post-extrusion treatments. For example, the soy proteins can be dehydrated by exposing the scaffold to an alcohol, such as ethanol. Other post-extrusion treatments include removing water from the scaffold via a dehydrothermal treatment; freeze-drying the scaffold such that water is removed via the sublimation of water frozen on the strand surfaces; and/or chemically or enzymatically crosslinking the soy protein chains in the denatured soy proteins. In some embodiments, the soy proteins are crosslinked with a relative crosslinking density of at least 0.1. This includes scaffolds in which the soy proteins have a relative crosslinking density of at least 0.2 or 0.3. The soy protein chains of the denatured soy proteins can be crosslinked by molecules capable of reacting with functional groups on the soy protein chains, such as —NH2, —OH and —SH groups to form covalent linkages between the chains. In some embodiments, the crosslinkers are enzymatic crosslinkers. In some embodiments, the crosslinkers are non-enzymatic chemical molecules. Transglutaminase is one example of an enzymatic crosslinker. Organic molecules, such as carbodiimides (e.g., 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide) and N-hydroxysuccinimide are examples of non-enzymatic, chemical crosslinkers.
In some embodiments of the scaffolds, the strands in each layer are printed in a regular pattern, wherein the strands are spaced apart and aligned along their longitudinal axes. By rotating the scaffold structure between the extrusion of adjacent layers in such structures, the angle, θ, defined by the longitudinal axes of the strands in adjacent layers can be controlled such that it is in the range of 0°<θ≦90°. This allows for the formation of longitudinal pores in the form of a channel having a desired median pore diameter in the scaffold. The formation of such regular strands and channels within a printed scaffold is illustrated schematically in
Typical x-axis and z-axis strand thicknesses for the present scaffolds include, but are not limited to, those in the range from 100 μm to 1000 μm (e.g., 100 μm to 600 μm; 150 to 300 μm). Typical strand spacings (w) in a scaffold with regularly patterned strands include those in the range from about 50 to about 1000 μm (e.g., from about 100 to about 500 μm, including from about 200 to about 300 μm).
A porous material intended for use as a tissue growth scaffold is also desirably sufficiently robust to support the viability of cells cultured thereon and, as such, is desirably elastic or viscoelastic. Robustness can be determined by the compressive modulus of the material. The compressive modulus of the scaffolds recited herein are measured as describe in the examples, below. The scaffolds can be made mechanically robust by, for example, increasing the degree of cross-linking between the soy proteins and/or increasing θ. In some embodiments, the scaffolds have a compressive modulus of at least 100 Pa. This includes embodiments having a compressive modulus of at least 1000 Pa, at least 2000 Pa and at least 4000 Pa. For example, the compressive moduli for the scaffolds can be in the range from about 1000 to about 5000 Pa. This includes embodiments in which the compressive moduli is in the range from about 100 Pa to about 1000 Pa or from about 100 Pa to about 200 Pa. The compressive moduli can refer to scaffolds in which the soy protein chains of the denatured soy proteins have not been crosslinked by enzymatic crosslinkers or non-enzymatic crosslinkers. In some embodiments, the compressive moduli refers to scaffolds in which the soy protein chains of the denatured soy proteins have not been crosslinked by a non-enzymatic crosslinker such as 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide)/N-hydroxysuccinimide and as such, the soy protein chains will not be crosslinked via the amide linkages provided by these crosslinkers.
The Example below illustrates the methods of making and using the scaffolds.
This Example focuses on the Bioplotting of a denatured soy protein, chosen due to the thermoplastic nature of the protein components (14), as well as the initial biocompatibility demonstrated both in vitro with different cell types (15-17) and in vivo (18).
Soy protein is a plant-based material with a wide range of structural and mechanical properties depending on the processing treatment (14). The degradation properties of soy biomaterials can be altered through different crosslinking treatments or fabrication techniques (16, 19, 20). Flexibility in material properties allows soy to be fabricated into a variety of structures including thin films (21), granules (17, 18, 22), hydrogels (23, 24), and scaffolds (16, 25). This Example illustrates the ability of the present methods to fabricate soy protein scaffolds in a reproducible manner with optimized printing parameters.
Methods and Materials
Mass Extrusion
Denatured soy protein isolate containing 87.6% protein as determined by combustion method (26) (Solae LLC, St. Louis, Mo., USA) was mixed with milliQ (MQ) water in varying wt. % to form a slurry. 1 wt. % antibiotic/antimycotic solution (Invitrogen, Carlsbad, Calif., USA) was added to all slurries. Glycerol (Sigma-Aldrich, St. Louis, Mo., USA) was added to 20 wt. % soy protein slurries as a plasticizer. Slurries were sieved sequentially through an autoclaved 297 μm pore size sieve (#50 mesh) and then an autoclaved 105 μm pore size sieve (#140 mesh) to remove non-dissolved particulates, forming a homogeneous paste. Sieved pastes were spread into a polyethylene (PE) syringe cartridge attached to a 200 μm PE tip and loaded into a 3D-Bioplotter (EnvisionTec GmbH, Germany) at room temperature. Slurries were extruded into a petri dish of known weight at a pressure of 3 bars for 2 seconds, and the ejected masses were weighed (N=3 measurements per temperature). Mass flow rate was determined by dividing the weight of the extruded slurry with the amount of time of extrusion (2 seconds). The temperature of the cartridge was then raised and held for 5 minutes prior to the next set of extrusions. Extrusions were performed for the temperature range of 22° C. to 70° C. Optimal mass flow rate for the soy protein slurry was determined by setting the pressure range between 0.5 bars to 3 bars at a fixed temperature of 27° C. To observe the effects of disulfide bond removal, 7.5 mM dithiothreitol (DTT, Sigma-Aldrich) was added to the 20 wt. % soy protein/4 wt. % glycerol solution. To visualize the macroscopic surface morphology of the printed strands, 2 wt. % trypan blue solution (Invitrogen) was added.
Scaffold Fabrication and Post-Treatments
Soy protein slurry composed of 20 wt. % soy protein/4 wt. % glycerol was prepared using the method described above. Individual boxes of 10 mm (length)×10 mm (width)×5 mm (height, 20 layers total) with 0=45° and 90° layer rotations (
Surface Morphology Characterization
A Quanta 600F sFEG scanning electron microscope (SEM, FEI, Hillsboro, Oreg., USA) was used to visualize the scaffold structure and surface morphology of 90° scaffolds (N>2 per group). Cross-sections were obtained by cutting scaffolds using a scalpel. Hydrated scaffolds (NT and EDC groups and individual soy strands with and without addition of DTT) were immersed in 100% ethanol for at least 20 minutes and critical point dried. All scaffolds were coated with 9 nm of osmium prior to imaging. Dimensions of the various scaffolds including strand thickness t and pore width w for top and bottom layers as well as strand thickness z were measured using ImageJ with at least five measurements per sample and N=3.
Determination of Crosslink Density and Mass Loss
Relative crosslink density (XD), the inverse of swelling ratio directly related to the volume fraction of dry soy, was determined for the 45° scaffolds of the different groups (N=5-6) using a previously described method (28). Hydrated scaffolds were immersed in 90° C. water for 2 minutes. The scaffold was then pressed under a 1 kg weight between 11 μm filter papers for 20 seconds to remove water within the pores and weighed to obtain the wet mass (Mw). The wet scaffolds were dried overnight at 110° C. Density of crosslinks was then calculated using the wet and dry masses (Mw and Md respectively) of the soy scaffold, the density of soy (ρs), and the density of water (ρw=1.00 g/cm3) using the following equation:
The density of soy was determined for FD-DHT scaffolds using an AccuPyc 1340 helium pycnometer (Micromeritics, Norcross, Ga., USA) with N=3 taking the average of 6-8 independent measurements per sample. Average density calculated from FD-DHT scaffold results were used to reduce the likelihood of closed cell pores.
Mass loss of the scaffolds upon rinsing was determined by calculating the percent change between dry weights before and after rinsing. Hydrated NT and EDC scaffolds were freeze-dried before measuring starting dry weight (Mb). All scaffolds were individually rinsed 3× for 5 minutes in either PBS or MQ water, freeze-dried, then weighed to obtain mass after rinsing (Ma). Total percent mass loss was calculated as (Mb-Ma)/Mb*100%.
Mechanical Properties
Compression testing on 45° and 90° scaffolds was performed using a mechanical tester (JLW Instruments, Chicago, Ill., USA) using a previously described method (16). Briefly, scaffolds (N=5 per group) were compressed at 0.2 mm/min up to 45% strain, and the compressive modulus was determined by calculating the best fit slope of the linear (elastic) regime starting from the lowest strain outside the toe region (0.8% strain) to R2=0.95 (R2=0.99 for EDC scaffolds).
Cell Seeding
Scaffolds with 45° geometry were used to characterize cell attachment and growth. All scaffold groups were hydrated in PBS for at least 1 day prior to cell seeding. Scaffolds were sterilized in 70% ethanol and exposed to UV simultaneously for 30 minutes. After sterilization, scaffolds were dried on 11 μm filter paper before and after rinsing 3× in PBS with 5 minutes per rinse and incubated overnight in low glucose phenol red free DMEM media supplemented with 25 mM HEPES buffer, 2 mM L-glutamine, 10% FBS, and 1% antibiotic/antimycotic solution (all from Invitrogen, Carlsbad, Calif., USA).
Human mesenchymal stem cells (hMSC) from Lonza were passaged up to Passage 5 in MSC basal media with mesenchymal stem cell growth supplement, L-glutamine, and penicillin/streptomycin (Lonza, Walkersville, Md., USA). All cells were cultured at 37° C. in a 5% CO2 humidified environment. Cells were trypsinized using 0.05% trypsin-EDTA (Invitrogen, Carlsbad, Calif., USA) and seeded onto the top layer of the printed scaffold at varying densities including 1×104, 5×104, and 1×105 cells per scaffold. Cells were allowed to adhere to the scaffold for 90 minutes at 37° C. with 5% CO2 before the addition of the phenol red free DMEM media described above. Scaffolds were cultured for 1 and 7 days.
Cell Seeding Efficiency, Proliferation, and Morphology on Various Scaffolds
Harvested scaffolds were cut into 4 pieces using microscissors and sonicated for 20 minutes in 1 mL of 0.02% (v/v) Triton X-100 (Bio-Rad, Hercules, Calif., USA) in MQ water. Samples were centrifuged at 15,000 rpm for 10 minutes at 4° C. A PicoGreen assay kit (Invitrogen, Carlsbad, Calif., USA) was used to quantify the amount of DNA in the supernatant. Aliquots of 100,000 cells taken during seeding were used to determine the amount of DNA per cell to convert DNA quantity to cell count. Cell seeding efficiency was calculated for the scaffolds seeded with 1×105 cells. The seeding efficiency was defined as the percent difference between the numbers of cells present in the culture after 1 day (C) to the assumed amount of total cells seeded. The formula used was Seeding Efficiency (%)=C/(1×105)*100%. Cell seeded scaffolds were fixed for one hour using 2% glutaraldehyde (Sigma-Aldrich) and 3% sucrose (J. T. Baker, Avantor, Center Valley, Pa.), dehydrated through graded ethanol, and critical point dried prior to imaging using SEM.
Statistical Analysis
All quantitative data from scaffold characterization analyses were reported as the mean with the positive error bar representing one standard deviation. Cell data was reported as the mean with the positive error bar representing standard error mean (SEM=standard deviation/n1/2). Independent t-tests assuming unequal variance was used. One-way analysis of variance (ANOVA) with a Scheffe posthoc analysis (α=0.05) was performed with the post-treatment group as the fixed factor. P values <0.05 were considered statistically significant.
Results
Mass Extrusion of Soy Protein Slurry
To determine an optimal soy slurry composition that could be used to fabricate 3D soy protein scaffolds with controlled pore architecture, the mass flow rate was measured for different slurry compositions up to the saturation point of soy protein at around 20 wt. % (FIG. 2). Increasing the total weight percent of soy protein decreased the mass flow rate. The addition of glycerol decreased the mass flow rate even further for all temperatures regardless of the percent weight of glycerol added up to 20 wt. % (data not shown). The mass flow rate of 20 wt. % soy and 4 wt. % glycerol slurry with DTT was decreased at all temperatures compared to slurry without DTT except at 70° C. (
Scaffold Characterization
Fabricated scaffolds had well-defined pore geometries and varied surface morphologies depending on post-treatment type. The coloration of the scaffolds as viewed in the macroscopic images was darker yellow for the FD-DHT and DHT groups (
Post-treatment of the scaffolds affected the structural and mechanical properties and hence, the robustness of the scaffold (
Cell Seeding, Proliferation, and Morphology in Printed Scaffolds
Cell seeding efficiencies and proliferation rates were varied between the post-treatment groups (
Discussion
This work demonstrated for the first time the ability to fabricate 3D soy protein porous structures via Bioplotting. A major challenge for Bioplotting natural biopolymers is the difficulty in reproducibly printing struts that hold shape and maintain scaffold structure. Pressures needed to print slurries are variable due to moisture content differences from batch to batch. This study used the mass flow rate (measured using the Bioplotter) as a reliable parameter for determining the ability to produce scaffolds with consistent self-supporting architectures, independent of soy slurry conditions. A mass flow rate of 0.0072±0.0002 g/s of soy protein produced well-defined scaffold constructs. To determine how this parameter varied with different slurry compositions, the mass flow rate was measured for different soy protein/glycerol concentrations. For all soy protein concentrations, mass flow rate increased and then decreased at temperatures between 40-60° C. (
To understand the molecular forces involved during printing for this system, DTT, a disulfide bond reducing agent, was added to 20% soy/4% glycerol slurries. The addition of DTT caused a lower mass flow rate, and the texture of the printed strands were smoothened (
Printing parameters including printing speed, layer spacing, and printing temperature were optimized for soy protein slurries to produce well-defined porous constructs. As mentioned previously, the relationship between pressure and mass flow rate may shift along the pressure axis due to varying slurry moisture content between batches. For instance, slurries with slightly less moisture content would require greater pressure to achieve a flow rate of 0.0072 g/s. However, regardless of the pressure needed for extrusion, the mass flow rate necessary to produce optimal scaffolds remains constant in a narrow range. Soy protein was unable to be deposited onto the Teflon foil at pressures lower than optimal, and at higher pressures (even just 0.2 bars away from the optimal pressure) the strands overflowed into the pores (
To fabricate soy protein scaffolds with well-defined pores, the layer spacing during printing was set to be greater than the strand size (nozzle tip) since the material expanded laterally upon extrusion (
Thermal and chemical post-treatments were applied to further increase scaffold robustness. Differences in structural and mechanical properties across the various post-treatments demonstrated the ability to tune the material properties of the printed soy protein scaffolds. Surface texture and scaffold size were the two main structural properties that underwent morphological changes. DHT treatment alone caused smoothening of microscopic texture through further molecular modifications, namely formation of hydrogen and disulfide bonds under heat (32) (
The 45° scaffolds were chosen as an initial geometry for exploring cell attachment on soy scaffolds since limiting the amount of through pores has been shown to increase seeding efficiency (37). The seeding efficiencies were in similar ranges as reported with other Bioplotted scaffolds (between 25-45% (37)). The seeding efficiency may be further improved by exploring geometries with greater overlap of strands. Freeze-drying with DHT decreased the seeding efficiency due to the increase of small pockets or voids not large enough for cells to infiltrate, which decreased the available surface area for cells to attach. Seeding efficiency was also lower for EDC scaffolds likely due to reduced availability of epitopes for cell binding after crosslinking, which is confirmed by inability for cells to spread when attached to the surface. However, rounded cell morphology did not prevent cells from proliferating in the EDC group. Significant proliferation was observed for FD-DHT and EDC groups, and cell viability was maintained across the NT and DHT groups. DHT modification of the soy protein scaffolds maintained cell viability but resulted in limited cell proliferation. This result implies that although seeding density was lower for the FD-DHT and EDC groups, the cell density was still sufficient to support cell growth. At both days 1 and 7, the cells for all scaffold groups were homogeneously scattered throughout the entire scaffold structure. Differences in adhesion and proliferation between the non-treated and DHT-treated scaffolds are therefore likely due to different cell-surface interactions, mechanical, and degradation properties and not due to the dispersion of cells within the scaffold.
The results from these studies demonstrate that reproducible 3D soy protein scaffolds can be fabricated via 3D Bioplotting. The slurry flow rate was determined to be a parameter that could be used to predict the printability of soft materials, as well as serve as a measurement for quality control purposes. The ability to print soy protein is particularly advantageous since the processing can be done at either room temperature or 37° C. without the use of organic solvents, allowing for the possibility of incorporating drugs or growth factors during the printing process.
The word “illustrative” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “illustrative” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, “a” or “an” means “one or more”. Still further, the use of “and” or “or” is intended to include “and/or” unless specifically indicated otherwise.
The foregoing description of illustrative embodiments of the invention has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and as practical applications of the invention to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.
This application is a divisional of U.S. patent application Ser. No. 14/057,479 filed on Oct. 18, 2013, the entire contents of which are hereby incorporated by reference; which claims priority from U.S. provisional patent application Ser. No. 61/716,122 filed on Oct. 19, 2012, and from U.S. provisional patent application Ser. No. 61/767,616 that was filed on Feb. 21, 2013, both of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61716122 | Oct 2012 | US | |
| 61767616 | Feb 2013 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14057479 | Oct 2013 | US |
| Child | 14980132 | US |
