Supporting Information Self-Supporting Nanoclay as Internal Scaffold Material for Direct Printing of Soft Hydrogel Composite Structures in Air Yifei Jin 1, Chengcheng Liu 2, Wenxuan Chai 1, Ashley Compaan 2, Yong Huang 1,2,3,* 1 Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, USA 2 Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA 3 Department of Biomedical Engineering, University of Florida, Gainesville, FL 32611, USA * Corresponding author, P.O.Box 116250, University of Florida, Gainesville, FL 32611, USA, Phone: 001-352-392-5520, Fax: 001-352-392-7303, Email: yongh@ufl.edu S-1
Supporting Information S1 Analysis of highly viscous material printing in air. To further evaluate the selfsupporting property of Laponite as a scaffold material, a highly viscous material (12% (w/v) NaAlg solution) is prepared and printed per the same printing conditions (path speed: 2 mm/s, nozzle diameter: 250 μm, pressure: 24 psi, and standoff distance: 0.3 mm) for the 6% (w/v) Laponite solution. As shown in Figure S1a-1, the Laponite filament can be extruded with a well-defined geometry on a receiving substrate. When extruding beyond the substrate, the extruded Laponite suspension switches its state from sol to gel rapidly, which results in a relatively rigid filament that holds its shape in air. In contrast, NaAlg is not a thixotropic, yield-stress material, so the highly viscous NaAlg filament can well maintain its shape on the substrate (Figure S1b-1) but not support any tensile stress due to the gravity effect when the filament is extended beyond the edge of the substrate. Instead, it breaks as illustrated in Figure S1b-2. It is the yield-stress property and thixotropic behavior instead of the high viscosity that enables Laponite as a versatile scaffold material. S-2
a-1) Deposited Laponite Nozzle a-2) Continuous filament Substrate Laponite printing on substrate Substrate Laponite printing in air b-1) Deposited NaAlg Nozzle b-2) Broken filament Substrate NaAlg printing on substrate Substrate NaAlg printing in air NaAlg droplet Figure S1. Analysis of highly viscous material printing in air. a-1) Laponite filament printing on a substrate and a-2) in air. b-1) High-concentration NaAlg filament printing on a substrate and b-2) in air. S-3
Supporting Information S2 Mechanical stress analysis. To evaluate the maximum stress distributed in a beam composed of one Laponite filament, several simply supported 6.0% (w/v) Laponite RD beams are printed as shown in Figure 2a, and their force and stress states are modeled accordingly as illustrated in Figure S2a. The reaction forces of the beam at two supporting points are: 1 RA RB pl, where R A and R B are the reaction forces at Points A and B, p is the weight 2 distribution along the beam, p gr 2, ρ is the density, g is the gravitational acceleration, R is the beam radius, and L is the beam length. Per the shear force distribution as shown in Figure S2b, the maximum shear stresses happen at both ends and can be estimated as follows: max V Q Ib 2 3 max gl (1) where V max is the maximum shear force which is equal to R A or R B, Q is the first moment of the beam cross-sectional area, I is the moment of inertia, and b is the beam width (2R herein). Per the bending moment distribution as shown in Figure S2c, the maximum bending moment 1 8 2 ( M max pl ) occurs at the midspan, and the resulting maximum tensile stress at the midspan can be estimated as follows: M y gl I 2R 2 max max (2) where M max is the maximum bending moment, y is the distance from the neutral axis to the surface of interest, which equals to R herein. For illustration, the maximum shear stress max of Beam 1 in Figure 2a is estimated as 37.0 Pa, and the maximum tensile stress max is estimated as 559.5 Pa. The shear yield stress yp of Laponite RD can be determined by fitting the shear stress-shear strain data with the Herschel-Bulkley model. For 6.0% Laponite RD S-4
structures, the estimated shear yield stress is 337.5 Pa. According to the octahedral shear stress theory, the tensile yield stress is found to be 715.5 Pa. Since max yp can be estimated by using yp 3/ 2 yp and max yp beam can support itself in a free-standing way. yp, and it for Beam 1, this 6.0% Laponite RD To determine the Young s modulus of Laponite RD, beam structures are printed between two supports with a spanning distance of L = 25.40 mm as shown in Figure S2d. By measuring the maximum deflection max and the beam radius R, the Young s modulus E can be estimated using 4 5 gl E 96 R max 2. Based on the measured deflections and radii of five beams, the average Young s modulus is determined as 4.3 MPa. By using the estimated E value, the 4 5 gl deflections of Beams 4, 5 and 6 in Figure 2a are further calculated using max 2, 96ER resulting in 0.85, 1.69 and 1.95 mm, respectively, which are close to the measurement results (0.80, 1.70 and 2.43 mm) as shown in Figure S2e. S-5
Deflection δ (mm) a) p x d) δ max A B L y b) A c) A pl/2 Shear force distribution Bending moment distribution M max =pl 2 /8 B pl/2 B e) 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 Calculation Measurement 20 22 24 26 28 30 32 34 Spanning distance L (mm) Figure S2. Mechanical analysis of printed Laponite beam structure. a) Mechanical model of a simply supported beam. b) Shear force and c) bending moment distributions. d) A simply supported beam printed between two supporting structures with a spanning distance of 25.40 mm. e) Beam deflection as a function of spanning distance. S-6
Supporting Information S3 Investigation of the effects of the operating conditions on the filament width. To print a 3D structure with well-defined geometries, the filament diameter must be controlled precisely. Herein, the effects of operating conditions including the nozzle diameter, standoff distance, dispensing pressure, and path speed on the filament diameter are systemically investigated by printing Laponite filaments on a glass slide by varying printing conditions. The width of the printed filaments is measured using a microscope. By varying the nozzle diameter, the filament width can be adjusted accordingly, and some representative resultant filaments are shown in Figure S3a. The standoff distance (Figure S3b), the distance between the nozzle tip and the receiving substrate, dispensing pressure (Figure S3c), and path speed (Figure S3d) all have a significant effect on the filament width. The filament width decreases when the standoff distance and/or the path speed increase and the dispensing pressure decreases. In particular, when the standoff distance is larger than a critical value, the filament may break up into droplets, and no continuous filament can be formed. S-7
Filament width (μm) Filament width (μm) Filament width (μm) Filament width (μm) a) 600 b) 500 600 550 Path speed: 2.0 mm/s Dispensing pressure: 10 psi Nozzle: Gauge 25 400 300 200 100 Gauge 32 Gauge 30 Gauge Gauge 27 25 Nozzle type P d v p (psi) (mm) (mm/s) 15.0 0.3 3.0 12.5 0.3 3.0 10.0 0.4 3.0 10.0 0.4 3.0 10.0 0.5 3.0 10.0 0.6 2.0 Gauge 23 Gauge 22 500 450 400 350 0.3 0.4 0.5 0.6 0.7 0.8 Standoff distance d (mm) c) d) 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400 Path speed: 2.0 mm/s Standoff distance: 0.2 mm Nozzle: Gauge 25 10.0 12.5 15.0 17.5 Dispensing pressure P (psi) 1000 900 800 700 600 500 400 300 Dispensing pressure: 10.0 psi Standoff distance: 0.4 mm Nozzle: Gauge 25 200 0 1 2 3 4 5 6 7 8 9 10 Path speed v s (mm/s) Figure S3. Effects of operating conditions on the filament width. a) Width of achievable welldefined filaments as a function of nozzle type (in terms of diameter: 100 μm for Gauge 32 nozzle, 150 μm for Gauge 30 nozzle, 200 μm for Gauge 27 nozzle, 250 μm for Gauge 25 nozzle, 330 μm for Gauge 23 nozzle, and 400 μm for Gauge 22 nozzle) and some representative filaments printed by each nozzle. b) Filament width as a function of standoff distance. c) Filament width as a function of dispensing pressure. d) Filament width as a function of path speed. (Scale bars: 100 μm a), 200 μm b) and 500 μm c) and d)). S-8
3.68 4.88 11.21 6.62 5.85 3.85 3.85 4.00 12.00 7.00 4.00 3.60 Supporting Information S4 Comparison of the 3D model and 3D printed Laponite structures to assess print fidelity. To assess the print fidelity of the proposed fabrication approach, some complex Laponite nanoclay structures including pyramid, bridge, Z-shaped tube and cup structures are printed. As shown in Figure S4a-d, the geometries of the printed structures are measured and compared with the 3D models as designed. It is found that the measured dimensions of the printed structures are very close to those as designed, showing that Laponite nanoclay can effectively hold the shape of printed features as a self-supporting material. a) 4.00 b) 6.00 c) 1.20 4.00 d) 0.50 90 45 0.60 1.00 4.80 1.52 0.48 93 9.82 3.72 17.41 3.82 41 0.55 3.57 1.22 4.82 Figure S4. Comparison of the 3D models and 3D printed Laponite structures to assess the print fidelity. a) Global view of the 3D model and 3D printed pyramid structure with overall geometries marked. b) Global view of the 3D model and 3D printed bridge structure with measured geometries marked. c) Global view of the 3D model and 3D printed Z-shaped tube with measured geometries marked. d) Global view of the 3D model and 3D printed cup structure with measured geometries marked. (Scale bars: 4.00 mm a), 2.00 mm (b) c) and d)) S-9
Supporting Information S5 Numerical simulation of the geometry limitation of inclined printable Laponite tubes. To investigate the limitation of the proposed fabrication approach for inclined tubular structures, three tubes with three different inclination angles (30, 45 and 60 ) are printed as shown in Figure 2g. To verify the achievable maximum height of the printed tubes, 3D models of the tubes are first set up using SolidWorks (Dassault Systemes SolidWorks Corp., Waltham, MA) as shown in Figure S5a and then simulated in its Simulation Module. By considering the gravity effect only, the achievable maximum height of the inclined tube is a function of the inclination angle, and inner and outer radii. In particular, by varying the inclination angle, the stress distribution along the tubular structures can be obtained accordingly as shown in Figure S5b-1, 5b-2 and 5b-3. It is found that the maximum stress happens at Point A in Figure S5a, which is therefore where failure occurs. As the height of tubular structures increases, the maximum stress at Point A increases too. Herein, we use the maximum normal stress (715.5 Pa) as the yield strength to determine the achievable maximum height of tubes with different inclination angles. S-10
a) R i b-2) R o α=45 R i =1.42 mm R o =2.58 mm H max α G=mg A b-3) b-1) α=60 R i =1.25 mm R o =2.75 mm α=30 R i =1.6 mm R o =2.4 mm Figure S5. Simulation of the stress distribution of inclined tubular structures. a) 3D model of a tubular structure. b-1) Stress distribution of a tube with an inclination angle of 30 and maximum height. b-2) Stress distribution of a tube with an inclination angle of 45 and maximum height. b-3) Stress distribution of a tube with an inclination angle of 60 and maximum height. S-11
Supporting Information S6 Comparison of hydrogel and hydrogel composite filament printing. To investigate the effect of Laponite nanoclay as an effective scaffold material for printing, various hydrogel and hydrogel composite filaments are deposited. The printing path is illustrated in Figure S6a, and a printed Laponite-only filament is shown in Figure S6b, which has well-defined features. As a scaffold material, Laponite also enables various hydrogels (PEGDA, alginate, and gelatin herein) to hold their shape after deposition. Some comparative printing experiments are performed by extruding hydrogels only and hydrogel-laponite composites onto a glass substrate. Figure S6c-1 is about 10% (w/v) PEGDA printing. It is found that 10% (w/v) PEGDA has very low viscosity and spreads significantly after extrusion. However, by adding 6% (w/v) Laponite into PEGDA, PEGDA-Laponite filaments can be successfully printed, having the well-defined geometries as designed (Figure S6c-2). To show that Laponite is a versatile scaffold material to facilitate the fabrication of hydrogels with different crosslinking mechanisms, filaments made of 0.5% (w/v) NaAlg (Figure S6d-1) and its NaAlg-Laponite composite (Figure S6d-2) as well as 10% (w/v) gelatin (Figure S6e-1) and its gelatin- Laponite composite (Figure S6e-2) are deposited, respectively. All the hydrogel composite filaments show a well-defined morphology superior to that of the hydrogel-only filaments. S-12
a) b) Nozzle Laponite filament Deposited filament c-1) d-1) e-1) c-2) PEGDA NaAlg Gelatin With Laponite With Laponite With Laponite d-2) e-2) PEGDA-Laponite NaAlg-Laponite Gelatin-Laponite Figure S6. Comparison of hydrogel and hydrogel composite filament printing. a) Design of the filament printing path. b) Laponite filament printing on a glass substrate. c-1) PEGDA and c-2) PEGDA-Laponite filaments printing on a glass substrate. d-1) NaAlg and d-2) NaAlg- Laponite filaments printing on a glass substrate. e-1) Gelatin and e-2) Gelatin-Laponite filaments printing on a glass substrate. (Scale bar: 5.0 mm) S-13
7.00 6.56 6.13 3.60 3.54 3.23 0.56 3.57 6.80 Supporting Information S7 Comparison of the 3D model and 3D printed hydrogel composite cups. To assess the print fidelity of Laponite as a scaffold material to facilitate soft hydrogel composite structure fabrication, the same cup structures are printed using different hydrogel precursor composites (PEGDA-, NaAlg-, and gelatin-laponite) and then crosslinked accordingly. The geometries of the 3D model and printed cups are shown in Figure S7 for comparison. From these figures, it is found that the measured dimensions of the cup structures are very close to those as designed, showing that Laponite nanoclay is an effective scaffold material for the selfsupporting printing of hydrogel composites. a) 0.50 b) c) d) 0.46 0.41 0.46 0.60 1.00 4.80 3D model 0.541.24 4.91 PEGDA-Laponite 0.472.14 5.21 NaAlg-Laponite 1.29 4.93 Gelatin-Laponite Figure S7. Comparison of the 3D model and 3D printed hydrogel composite cups. a) Global view and geometries of the 3D model of the cup structure. b) Global view and geometries of the 3D printed cup structure made of PEGDA-Laponite. c) Global view and geometries of the 3D printed cup structure made of alginate-laponite. d) Global view and geometries of the 3D printed cup structure made of Gelatin-Laponite. (Scale bars: 2.00 mm) S-14
4.00 Supporting Information S8 Comparison of the 3D models and 3D printed triple-walled tube and concentric cannular structure. To further assess the print fidelity of Laponite as a scaffold material to facilitate the soft hydrogel composite structure fabrication, the geometries of a triple-walled tubular structure are designed and measured as shown in Figure S8a-1 and S8a-2, respectively. In addition, a concentric cannular PEGDA-Laponite structure is also printed and crosslinked. The designed and final geometries are demonstrated in Figure S8b-1 and Figure S8b-2, respectively. From the figures, it is found that the measured dimensions of the structures are very close to those as designed. a-1) a-2) D i =4.00 D m =6.00 D o =8.00 D i D m Do D i = 3.93 D m = 5.89 D o = 7.91 3.91 b-1) D 1 =12.0 t 1 =2.0 D 2 =6.0 t 2 =1.0 h =8.0 h t 2 t b-2) 1 D 2 D 1 =11.82 D 1 t 1 =1.62 D 2 =5.00 t 2 =0.92 h=8.17 Figure S8. Comparison of the 3D models and 3D printed triple-walled tube and concentric cannular structure. a-1) Global view and geometries of the 3D model of the triple-walled tube. a-2) Global view and geometries of the 3D printed triple-walled tube. b-1) Global view and geometries of the 3D model of the concentric cannular PEGDA-Laponite structure. b-2) Global view and geometries of the 3D printed concentric cannular structure. (Scale bars: 2.0 mm) S-15
Filament width (μm) Filament width (μm) Supporting Information S9 Size stability during printing process. To assess the effects of water loss during printing on the size change of printed structures, filaments made of Laponite and PEGDA-Laponite are printed on a petri dish. After printing, some PEGDA-Laponite filaments are cured by UV exposure. Then, all the filaments are left in air, and the filament widths are monitored in the next 180 minutes. As seen from Figure S9a, the filament widths only decrease slightly during 3 hours: 3.20% reduction for Laponite filaments, 0.75% reduction for gelled PEGDA- Laponite filaments, and 2.20% reduction for ungelled PEGDA-Laponite filaments, demonstrating that during printing the filament diameter change caused by water loss can be negligible. Furthermore, the Laponite and gelled PEGDA-Laponite filaments are submerged in a deionized water bath to investigate the effects of excessive water on the geometry change in the next 180 minutes, and the filament widths are measured accordingly. As seen from Figure S9b, the filament widths only increase slightly after 180-minute immersion, proving that Laponite and its hydrogel composite structures can effectively maintain their shapes in water or during aqueous-related post-treatment. a) 1000 b) 1100 900 1000 800 900 700 600 500 Laponite Gelled PEGDA-Laponite Ungelled PEGDA-Laponite 0 20 40 60 80 100 120 140 160 180 Time (minutes) 800 700 Laponite Gelled PEGDA-Laponite 0 30 60 90 120 150 180 Time (minutes) Figure S9. a) Filament width change in air as a function of time and b) filament width change in a water bath as a function of time. (Scale bars: 400 μm) S-16
Supporting Information S10 The effects of hydrogel concentration on the mechanical properties. Uniaxial compression testing was performed to investigate the effects of hydrogel component concentration on the mechanical properties (Young s modulus and fracture strength) of PEGDA-Laponite composites, as shown in Figure S10a-c. The addition of nanoclay leads to a significant increase in both the Young s modulus and fracture strength of hydrogel composites with different PEGDA concentrations. Specifically, for hydrogel composites having PEGDA concentrations of 5.0%, 10.0%, and 15.0%, the Young s modulus increases from 3.87 ± 0.47 to 31.34 ± 1.86 kpa, 61.61 ± 4.91 to 118.75 ± 1.05 kpa, and 192.71 ± 17.96 to 305.48 ± 19.23 kpa, respectively, and the fracture strength increases from 1.65±0.20 to 11.20±0.82 kpa, 23.45 ± 4.24 to 45.18 ± 5.13 kpa, and 57.66 ± 3.15 to 104.64 ± 8.83 kpa, respectively, due to the addition of 6.0% (w/v) Laponite. This mechanical property enhancement is attributed to the nanoclay particle-polymer interaction. Cyclic compression tests, consisting of five cycles of loading and unloading, were also performed, as shown in Figure S10d and S10e. The addition of nanoclays increases the dissipated energy of all PEGDA-Laponite composites. Specifically, for hydrogel composites having PEGDA concentrations of 5.0%, 10.0%, and 15.0%, the total dissipated energy increases from 11.80 ± 3.13 to 511.49 ± 56.29 J/m 3, 188.26 ± 21.87 to 1026.00 ± 27.97 J/m 3, and 817.01 ± 141.85 to 2132.93 ± 337.47 J/m 3, respectively, due to the addition of 6.0% (w/v) Laponite. It is noted that the enhancement effects due to Laponite is less pronounced for highconcentration PEGDA hydrogels. This may be due to the relatively decreasing percentage (from 6 out of 11, 6 out of 16, to 6 out of 21 percent when adding the 6.0% Laponite colloid into the 5.0%, 10.0%, and 15.0% PEGDA solutions) of Laponite nanoclay with the PEGDA- Laponite composites with an increasing PEGDA concentration. At low PEGDA concentrations, the network formed by physical interactions between nanoclays and hydrogel molecular chains greatly enhances the overall mechanical properties of hydrogel composites. S-17
Energy dissipated (J/m 3 ) Energy dissipated (J/m 3 ) Stress (kpa) Young s modulus (kpa) Fracture strength (kpa) On the other hand, at high PEGDA concentrations, the polymer chain network dominates, and the addition of nanoclay has less influence on the overall mechanical properties of hydrogel composites. a) 120 100 80 60 5% PEGDA 10% PEGDA 15% PEGDA 5% PEGDA-Laponite 10% PEGDA-Laponite 15% PEGDA-Laponite b) 350 300 250 200 150 PEGDA PEGDA-Laponite c) 120 100 80 60 PEGDA PEGDA-Laponite 40 100 40 20 50 20 d) 2500 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Strain (mm/mm) 2000 PEGDA PEGDA-Laponite 0 e) 700 600 500 5% 10% 15% PEGDA concentration PEGDA PEGDA-Laponite 5% 10% 15% 0 5% 10% 15% PEGDA concentration 1500 400 1000 300 500 200 100 0 5 10 15 PEGDA concentration 0 1 2 3 4 5 No. of cycles Figure S10. Effects of hydrogel concentration on the mechanical properties of hydrogel composites. a) Uniaxial compression test of PEGDA-Laponite composites. b) Young s modulus and c) fracture strength of PEGDA hydrogels and PEGDA-Laponite composites with different PEGDA concentrations. d) Energy dissipated during each loading cycle and e) the total energy dissipated over five cycles in cyclic compression tests of PEGDA hydrogels and PEGDA-Laponite composites with different PEGDA concentrations. S-18