Unstructured Direct Ink Write 3D Printing of Functional Structures with Ambient Temperature Curing Dual-Network Thermoset Ink

Fabrication of structures in unstructured environments is a promising ﬁeld to expand the application spaces of additive manufacturing (AM). One potential application is to add new components directly onto existing structures. In this paper, we developed a versatile, reconﬁgurable direct ink write (DIW) manufacturing method in tandem with a two-stage hybrid ink designed to fabricate high-strength, self-supporting parts in unconventional printing spaces, such as underneath a build surface or horizontally. Our two-stage hybrid DIW ink combines a photopolymer and a tough epoxy resin. The photopolymer can be cured rapidly to enable layer-by-layer printing complex structures. It also possesses adequate adhesion to allow the fabrication of large volume structures on a diversity of substrates including acrylic, wood, glass, aluminum, and concrete. The epoxy component can be cured after 72 hours in ambient conditions with further increased adhesion strengths. We demonstrated the capabilities of the reconﬁgurable DIW extrusion nozzle method to print complex structures in inverted and horizontal environments. Finally, via the addition of DIW-deposited conductive paths, we created a functional 3D printed structure capable of in-situ deformation monitoring. This work has the potential to be used for applications such as appending new parts to existing structures for increasing functionality, repair, and structure health monitoring.

ToC Figure .This work introduces a versatile, reconfigurable direct ink write manufacturing method in tandem with a two-stage curing hybrid ink designed to fabricate high-strength, self-supporting parts in unconventional printing spaces, such as underneath a build surface or horizontally.

Introduction
Extrusion-based additive manufacturing (AM) such as fused filament fabrication (FFF) and direct ink writing (DIW) are popular choices for 3D printing due to their low cost, simplicity, open framework, and broad material versatility.In recent years, researchers have aimed to broaden potential applications of extrusionbased AM by modifying build hardware, control schemes, and layer structure [1][2][3][4].Furthermore, researchers have also utilized DIW material versatility to create structures with a vast breadth of properties ranging from elastomeric, viscoelastic, to high-strength rigid [5][6][7][8][9].As a result, developing novel AM techniques in tandem with ink feedstock with tuned mechanical characteristics to create application-specific methods have been of great interest [10][11][12][13][14][15][16].
An emerging application of AM is to fabricate new components directly onto existing structures, which can have unstructured surfaces unsuitable for conventional DIW 3D printing.This includes applications where the existing build surface is either too large or sensitive to be moved such as in-situ construction repair and in-vivo bone and tissue repair [17][18][19].However, conventional extrusion-based techniques deposit layer on the XY plane with a downward-facing extrusion nozzle and build parts upwards [20].Due to the limited printing space, 3D printed parts are then removed from the tray to be assembled with others.Thus, the ability to create parts in unstructured manufacturing space by these conventional techniques is limited.
Fabricating on unconventional and nonplanar surfaces has been explored via both redefining motion of the build plate and/or extrusion head [21][22][23].Yet, while previous studies have successfully created geometries unattainable with conventional AM methods using a moving build plate, this approach is unsuitable for printing onto an existing, fixed build surface [24,25].Furthermore, studies utilizing movement of the extrusion head predominantly used FFF to deposit rapidly solidifying thermoplastics rather than DIW.This can be attributed to the sensitivity to gravitational effects on the viscoelastic, shear-thinning DIW inks requiring the extrusion nozzle to be facing downwards during deposition, at the cost of impeding formable geometries [26].To overcome this barrier, a rapidly solidifying DIW ink must be utilized.To achieve this, researchers have investigated inks with polymer networks catalyzed via continuous ultraviolet (UV) irradiation [27,28].Moreover, to further preserve shape integrity and improve properties, researchers have developed multi-stage curing inks that typically consists of a photopolymer and a thermal cure resin [29].However, the secondary thermal curing requires placing printed structure in an oven, which is impossible if the application is to print an object on an existing structure.
In this work, we introduce a novel DIW technique for manufacturing parts in non-traditional environments whereby the extrusion nozzle is repositioned to facilitate fabrication of parts in unstructured manufacturing spaces, such as underneath the build surface or horizontally (Figure 1a).Parts were created using a two-stage photo-epoxy thermoset resin wherein the acrylate-based photo resin enabled rapid shape forming while the epoxy resin developed a high-strength network highly compatible with the photo cured polymer network at room temperature (Figure 1b, c).We investigated various ratios of photo cure and epoxy monomer constituents of the two-stage resin on mechanical characteristics to determine the ideal balance of strength and rapid polymerization.The tough epoxy resin constituent of printed ink rose to 50% of degree of conversion (DoC) after 24 hours, increasing further to 73.2% after 48 hours, and finally achieving 86.3% after 72 hours in ambient and room-temperature conditions.Due to the photo-epoxy two-stage curing resin's ability to form an interpenetrating polymer network, the fully cured printed structures demonstrated a high bonding affinity to a variety of substrates.With a favorable photo-to-epoxy resin ratio, we then demonstrated the capabilities of this technique via 3D printing of several structures under a build platform, including a bio-inspired "beehive", conical structures with large overhangs, and load-bearing arches (Figure 1c).Similarly, horizontal parts were printed via reconfiguring the DIW extrusion nozzle such that the direction ink deposition was parallel to the XY plane whereby structures could be fabricated on a vertical surface.As a demonstration, we printed a zero-support horizontal beam with integrated conductive elements enable in-situ deformation sensing.

Two-Stage Resin Preparation
The two-stage curing resin consists of varying ratios of photopolymer resin and epoxy resin (Figure 1a-c).
The epoxy resin contains a 100:32 ratio of Epon 828 (difunctional bisphenol A/epichlorohydrin (DGEBA); Hexion, Columbus, OH, USA) and Jeffamine D230 curing agent (O,O'-Bis(2-aminopropyl) polypropylene glycol-block-polyethylene glycol-blockpolypropylene glycol) (Sigma-Aldrich).The photo curing and epoxy resins were separately hand mixed, and then the resins were combined to form the two-stage resin, followed by 5 minutes of magnetic stirring to further homogenize the mixture. 1 wt% photo initiator Irgacure 819 (Bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide) (Sigma-Aldrich) and 3 wt% co-curing agent to the two-stage resin mixture to facilitate photopolymerization and catalyze room-temperature curing, respectively.The co-curing agent consists of 70 wt% Triethanolamine, 20 wt% Piperazine, and 10 wt% N-aminoethylpiperazine (Sigma-Aldrich).Finally, 7 wt% fumed silica (Sigma-Aldrich) was mixed into the liquid two-stage resin to enable shear-thinning behavior required for DIW 3D printing.Due to the co-curing agent's ability to catalyze epoxy resin at room temperature, the working life of the two-stage ink was around four hours.
Once prepared, the two-stage resin was loaded into syringes and centrifuged for 20 minutes to completely remove air bubbles, then mounted to a custom built DIW printer [30].The DIW printer uses compressed air delivered by an Ultimus V air pressure controller (Nordson EFD, East Providence, RI, USA) to extrude the created ink through a tapered deposition nozzle (Figure 1a).
Additionally, a conductive DIW ink was prepared with ME603 conductor paste (DuPont, Wilmington, DE, USA) combined with 3 wt% Timical Super C45 carbon black (MTI Corporation, Richmond, CA, USA) to improve conductivity and aid in shape retention post-extrusion [31].Table 1 .Chemical ingredients of the two-stage resin

Sample Preparation and Material Characterization
To characterize the properties of the two-stage resins, tensile, dynamic mechanical analysis (DMA), Fourier transform infrared spectroscopy (FTIR), and rheological tests were performed.Dogbone-style samples for tensile and (DMA) characterization experiments were cast in a silicone/polydimethylsiloxane (PDMS) mold.Samples with varying resin compositions (P, PE30, PE50, PE70, E) were subjected to a 60 second UV curing (intensity = 66 mW/cm 2 ) followed by a 72 hour room temperature cure.Additionally, PE70 tensile samples were subjected to only UV curing for 1, 15, 30, and 60 seconds.FTIR and rheological experiments were conducted using only PE70 resin.
To characterize mechanical properties of the two-stage resin at various resin composition ratios (P, PE30, PE50, PE70, E), we performed tensile tests (n = 3) using a universal mechanical testing machine (Criterion, MTS, Eden Prairie, MN, USA) to acquire Young's Modulus (E) as well as yield and ultimate engineering stresses and strains.Furthermore, tensile tests of PE70 resin samples subjected to 66 mW/cm 2 UV irradiation for 1, 15, 30, and 60 seconds were performed.Additionally, we carried out DMA tests using a DMA tester (Q800, TA Instruments, New Castle, DE, USA) to obtain the glass transition temperature (T g ), storage (E') and loss (E") moduli, and tanδ of the two-stage resin.Rheological measurements of PE70 twostage resin were obtained at room temperature using an ARES G2 rotating rheometer (TA Instruments).Viscosity (η), shear storage (G') and loss moduli (G") were obtained over a range of shear rates (0.1 -200 1/s).Finally, two sets of FTIR experiments were preformed to determine the DoC for both photopolymerization and room-temperature epoxy cure reactions in PE70 resin.For photopolymerization, FTIR scans were taken on resin samples subjected to 66 mW/cm 2 UV irradiation for (0, 1, 3, 5, 7, 15, 30, and 60 seconds).
To understand the bonding strength of printed structures, we printed 5 x 10 x 20 mm pillars onto acrylic, wood, glass, aluminum, and concrete substrates.Substrates with pillars directly after printing (0 hour) as well as after 72 hours were constrained to a plate affixed to a universal mechanical testing machine (Criterion, MTS).Next, the pillars were constrained in a tensile test grip and a load applied until the pillar delaminated from the substrate.The critical stress measured was determined to be the adhesion strength.

3D Printing of Structures
3D models were prepared using SolidWorks (Dassault Systèmes SE, Vélizy-Villacoublay, France) computeraided design (CAD) software.Prior to 3D printing, the models were imported into the computer-aided manufacturing (CAM) software, Repetier (Hot-World GmbH & Co. KG, Willich in North Rhine-Westphalia, Germany), for slicing into discrete layers.The resulting toolpath control code (gcode) was modified using custom MATLAB (MathWorks, Natick, MA, USA) scripts in order to redefine position axis such that the extrusion head would move downwards on the Z-axis, and along the Y-axis for horizontal prints for each layer.Structures were subsequently printed through a 0.58 mm diameter nozzle onto glass substrates at a printing speed of 10 mm/s and extrusion pressure of 75 kPa.UV irradiation (66 mW/cm 2 , 15 seconds) was applied every alternating layer.The conductive paths were printed through a 0.41 mm diameter nozzle at a printing speed of 10 mm/s and extrusion pressure of 190 kPa.

Mechanical and Material Characterization
DMA of thermally cured samples over the range of 30 -170°C revealed that the two-stage resin has a T g of 94.4, 121.2, 114.8, 97.4, and 91.5 °C for P, PE30, PE50, PE70, and E resin compositions respectively, indicated by the peaks in tanδ curves (dashed lines) (Figure 2a).Importantly, the materials exhibit a single distinct T g despite the varying concentrations of monomers in the photo and thermal resin constituents having different T g values.This suggests that the photo and thermal cure networks are highly compatible due to epoxide groups presented in the reactive dilutant GMA component of the photopolymer resin, allowing for the epoxy resin component to form an interpenetrating network with the TMPTA photopolymer network with no macroscale phase separation between the two components.As photo resin concentration increases, the tanδ curves begin to exhibit less and less distinct peaks.As a result, the tanδ peak for P is comparably small in height, but very broad.This indicates there is a longer property transition compared to all other conditions, which can account for the P condition's deviation from the upward trend in T g as photo resin concentration increases.Furthermore, based on the storage modulus behavior of P, it can be surmised this phenomenon is due to the highly crosslinked nature of the photo cure network as it has triacrylate monomers and therefore has more crosslinking points.This high crosslink concentration inhibits movement of the interpenetrating polymer network, resulting in the observed increasing trend in T g versus photo resin concentration.
The results of the tensile tests for each resin composition are presented in Figure 2b.The solely photopolymer resin (P) exhibits highly rigid, albeit low strength, behavior as evidenced by the comparably large Young's modulus versus other conditions (2.21 GPa) coupled with inferior tensile strength and fracture strain.However, increasing epoxy resin concentration resulted in an increase in strength and ductility of the cured resins.Indeed, the addition of epoxy resin -even in the lowest concentration PE30 -affords a 170% increase in tensile strength versus the solely photopolymerized resin (23.6 ± 2.1 versus 63.5 ± 3.5 MPa).Moreover, tensile strength of PE70 is comparable to that of the solely epoxy resin (81.4 ± 5.7 versus 79.5 ± 4.8 MPa), indicating that the epoxy network becomes the primary contributor to strength of the material despite it only comprising of 70 wt% epoxy.The increasing influence of the epoxy network over the rigid photo-cure network also manifests in a 0.18 -0.32 GPa drop in Young's modulus versus P as epoxy concen-tration increases.Furthermore, increasing epoxy resin concentrations leads to a linear increase in fracture strain from 1.4 for P to 6.6% PE70 (R 2 = 0.93).However, despite PE70 and E having comparable tensile strengths, there is a relatively large discrepancy in fracture strain increase from PE70 to E (Δε = 2.7%).This implies that, despite the strength of the epoxy network, the highly crosslinked -and therefore rigidphoto-cure acrylate network impedes deformation of the interpenetrating two-stage network.Furthermore, we performed FTIR experiments to understand the curing kinetics of both the photo cure and epoxy resin networks, DoC with respect to time are presented in Figure 2c, d for photo cure and photo epoxy resin networks, respectively.DoC for both resin constituents was determined through analysis of the absorbance intensity of selected wavenumbers corresponding to bonds which drop in frequency as the respective polymer networks develop (Figure S1a, b).Because our inverted and horizontally printed structures are significantly impacted by gravitational forces, a rapidly solidifying material is essential for parts to maintain their structure.As previously discussed, the high presence of bonding sites of the triacrylate TMPTA monomer allows for rapid polymerization, and therefore the development of a suitably stable/solidified network for the purpose of self-support.When exposed to UV irradiation with light intensity of 66 mW/cm 2 for one second, the photopolymer network constituent began rapidly developing (42.5% DoC) -and therefore solidifying -with a Young's modulus of 0.915 ± 0.224 MPa, thereby laying the groundwork for layers to retain their deposited shape (Figure 2c).After five seconds of exposure, the network reaches 82% DoC, and is nearly fully developed after 15 seconds of UV irradiation.As a result, the solidified layers have a Young's modulus of 4.90 ± 0.24 MPa, a 430% increase within 14 seconds.Based on these results, we elected to expose printed layers to 15 seconds of UV radiation to assure the printed structures would remain stable during fabrication.It should be noted Young's modulus continues to increase to 7.10 ± 0.80 MPa and 9.86 ± 1.76 MPa for 30 and 60 second exposure, respectively due to the cumulative UV exposure from curing subsequent layers in fact strengthens prior layers, thus enabling creation of larger structures.With the modulus of ˜5MPa-10MPa, the previously printed layer can sustain a large structure without exhibit visible deformation.For the epoxy resin, FTIR analysis (Figure S1 in Supplementary Information) revealed that the DoC of the epoxy resin constituent rose to 50% after 24 hours, increasing further to 73.2% after 48 hours, and finally achieving 86.3% DoC after 72 hours in ambient, room-temperature conditions; thereby indicating autonomous network formation behavior under room-temperature conditions (Figure 2d).This curing mechanism proved highly effective in strengthening the material, after 72 hours, PE70 resin underwent a three order of magnitude increase in Young's modulus (9.86 MPa to 1.9 GPa) and a two order of magnitude increase in tensile strength (0.79 to 81 MPa) (Figure 2c, d, Figure S2a, b).
To confirm the suitability of the two-stage resin ink for DIW 3D printing, we examined viscosity, G', G", and tanδ.Viscoelastic behavior of the hybrid ink was determined via oscillatory stress sweep as shown in Figure S3a.The ink exhibited a stable plateau of storage modulus G' over loss modulus G", with a value of 2800 Pa and possesses a critical stress (τ c ) of 186 Pa, indicating a high stiffness of the formulated two-stage DIW ink.This is crucial for inverted and horizontal DIW AM to maintain the printed shape versus gravitational sagging post-extrusion.Additionally, we observed that the two-stage resin combined with 7 wt% fumed silica exhibited desirable shear-thinning behavior with respect to viscosity (Figure S3b).Finally, we conducted an analysis of viscosity versus time in order to determine an approximate a working life wherein the twostage ink was reasonably printable (Figure S3c).Based on the dramatic increase in viscosity at t = 10,000 seconds, corroborated by qualitative experimental experience, we determined the working life of the twostage ink to be approximately three hours.This working time allowed for fabrication of multiple structures, for example, pillar samples used in the following section took approximately 25 minutes to print.

Adhesion
To demonstrate the robustness of the two-stage resin bond, and therefore end-use application viability we printed 5 x 10 x 20 mm pillars on several materials (Figure 3a).The substrates selected were acrylic, wood, glass, aluminum, and concrete due to their ubiquity in construction and consumer applications.Adhesion strength is quantified by the peak stress (MPa) required to separate the as-printed (with 15 second UV cure) and the fully cured (72 hours) printed pillars from the substrates (the test apparatus is shown in Figure 3b).The results of the adhesion experiments are presented in Figure 3c.Printed pillars tested directly after printing exhibited similar adhesion strengths ranging from 0.17 to 0.33 MPa with acrylic substrates exhibiting the lowest adhesion strengths and glass the highest.This range suggests that the photopolymer network can rapidly create a substrate-agnostic bond, indicating that structures of comparable size can be printed in a variety of conditions.Importantly, considering the density of the two-stage resin is 1.2 g/cm 3 , it is possible to create self-supporting structures up to 28.15A cm 3 in volume where A is the area of the base of the printed structure (Equation S1).For example, for the pillar with 5 x 10 mm base, the length can be up to 2 m on a wooden substrate.After 72 hours, the formation of the epoxy resin matrix doubled and even quintupled in the case of acrylic the adhesion strengths of printed pillars.Indeed, 72-hour cure samples, acrylic and wood substrates exhibited the highest adhesion strengths (1.04 ± 0.7 MPa and 0.99 ± 0.7 MPa, respectively).This is potentially due to greater hydrogen bond prevalence caused by hydroxide and ester group interactions as well as the surface roughness of wood.Whereas glass, aluminum, and concrete provided similar, lower adhesion strengths (0.64 ± 0.1, 0.66 ± 0.1, and 0.6 ± 0.02 MPa, respectively).The smoothness of the glass and aluminum substrates is a likely contributor to the lower adhesion strength, while failure on the concrete substrate was due to loosened mineral particles at the pillar-substrate interface.Despite these differences, the fully cured printed two-stage resin structures exhibited impressive adhesion strength on a breadth of substrates.As a result, considering the same 5 x 10 mm base pillar on a wooden substrate, the pillar can grow up to 8 m after four cycles of 72 hour curing.These findings further reinforce the importance of the autonomous ambient temperature epoxy curing mechanism for temperature-sensitive substrates such as wood and acrylic.

Unstructured 3D Printing
To demonstrate the capabilities of our unstructured AM process and two-stage resin, we 3D printed various inverted structures including a bio-inspired "beehive", conical geometries, and load-bearing arches (Figures 4a-d, Video 1).Using the inverted printing technique, we were able to create conical structures with large overhangs without support materials due to a combination of gravitational forces acting "upwards" relative to the structure as well as rapid photopolymerization to solidify layers.The synergy of these two phenomena enabled us to create large structures with comparably small base layers without sagging.Furthermore, the multi-phase polymerization mechanism of the printed ink ensured that the large structures preserve their shape integrity over time, thereby ensuring the larger structures continue to resist gravitational forces and do not delaminate from the substrate.To further exemplify the demonstrated overhang printing capabilities coupled with progressive substrate adhesion strength, we printed 24 mm tall arches (Video 2).Upon fully cured, the arches could support a one-kilogram weight, as corroborated by adhesion strength characterization previously discussed (Figure 4e).This is of particular interest when considering the application space of such a structure.To accomplish this feat using conventional DIW manufacturing, the thin-walled structure would likely need to be printed with support material [26], then removed from the original print substrate and repositioned in an inverted configuration, thereby compromising the bond strength between the structure and end-use surface.Conversely, the proposed unstructured print technique can satisfy this load bearing task with an as-printed, high-strength structure.
Rich media available at https://youtu.be/ogzrWTjVQjY Video 1. 3600 view of the bio-inspired "beehive." Rich media available at https://youtu.be/j9XGd8bPxpc Video 2. Inverted arch being fabricated.Alternatively, this printing technique can be configured to fabricate structures horizontally, effectively enabling production of 900 overhangs with zero support material (Figure 5a).By reorienting the extrusion nozzle, the zero-support horizontal beams could be fabricated such that layers are deposited parallel to the build plate similar to conventional DIW printing.The rapid photopolymerization affords rigidity to the structure, allowing it to resist the increasing bending moment imparted by the cumulative weight of the horizontal beam.To further demonstrate the capabilities of these unstructured/alternative build orientations, we deposited conductive ink paths on the surface of a horizontally printed beam to create a functional 3D printed structure to sense deflection (Figure 5b, Video 3).The results of normalized resistance over time for the 50 mm horizontal beam sensor subjected to cyclic deflection three times at δ = 0.4, 0.8, and 1.2, 1.6, and 2.0 mm are presented in Figure 5c.
Rich media available at https://youtu.be/wjxC58Bv3tE Video 3. Cyclic deflection experiment of the functional horizontal 3D printed structure fixed to a deflector.The plotted resistance shows peaks of increasing amplitude in the sensor signal corresponding to prescribed deformation.Indeed, the percent increase from peak to trough of the signal versus deflection shown in Figure 5d indicates a positive trend wherein Δ peak increases from 9.63% to 29.12% for δ = 0.4 and 0.8 mm, respectively.The trend continues finally to 52.74% at δ = 2.0 mm; thereby demonstrating the printed sensor's potential to represent mechanical deformations in the structure.Based on these trends, this printed sensor has the potential to monitor deformations in a pre-existing structure in-situ, which could be invaluable in structural health monitoring applications.

Conclusions
In this work, we developed a versatile, reconfigurable DIW manufacturing method in tandem with a twostage hybrid ink designed to facilitate fabrication of high-strength, self-supporting parts in unconventional printing spaces, such as underneath the build surface or horizontally.Our two-stage hybrid DIW ink combines a photopolymer and tough epoxy resin which is capable of autonomous curing under ambient temperature conditions, thereby creating complex, high-strength geometries without removal of the structure from the printing surface.The photopolymer component can be cured rapidly to enable layer-by-layer fabrication of complex structures.The photocured resin also possesses adequate adhesion to allow the fabrication of large volume structures on a diversity of substrates including acrylic, wood, glass, aluminum, and concrete.Moreover, the epoxy component cured after 72 hours in ambient, room-temperature conditions with increased adhesion strengths.We demonstrated the capabilities of the reconfigurable DIW extrusion nozzle method coupled with our developed ink by fabricating complex structures free of support structures in inverted and horizontal environments.In addition, via the addition of DIW-deposited conductive paths, we created a functional 3D printed structure capable of in-situ deformation monitoring.This work has the potential to be used for applications such as appending new parts to existing structures for increasing functionality, repair, and structure health monitoring.

Figure 1 .
Figure 1.Graphical depiction of the inverted DIW AM procedure fabricating an arch structure under the printing platform.(a) Layer deposition of uncured two-stage resin ink onto structure.(b) Solidification of deposited layer via development of photopolymer network facilitated by UV radiation.(c) Strengthening of printed structure through development of an interpenetrating polymer network via autonomous ambient temperature conversion of epoxy resin.

Figure 2 .
Figure 2 .Mechanical and material characterization of the engineered two-stage resin DIW ink.(a) tanδ and E' quantified over a range of temperatures via DMA.(b) Features of tensile properties for solely photo cure resin (P), several ratios of photopolymer to epoxy constituents (PE30, PE50, PE70), and solely epoxy resin (E).(c) DoC quantified via FTIR versus time superimposed over Young's modulus of PE70 resin at 1, 15, 30, and 60 seconds of UV exposure.(d) DoC quantified via FTIR versus time superimposed over Young's modulus of PE70 resin at 0 and 72 hours after photo cure.

Figure 3 .
Figure 3 .Adhesion strength characterization of DIW printed fully cured two-stage resin structures.(a) Two-stage resin structures printed onto (i ) acrylic, (ii ) wood, (iii ) glass, (iv ) aluminum, and (v ) concrete substrates.(b) Experimental setup showing the tensile grip clamped to the sample on fixed substrate.(c) Quantified adhesion strengths of printed structures on the aforementioned substrates at 0 hour (solid) and 72 hours (patterned).

Figure 4 .
Figure 4. Structures fabricated via the inverted deposition configuration.(a) A bio-inspired "beehive."(b) Conical structure with large overhangs and small base fabricated with zero support material.(c) Twisted conical structure.(d) Load bearing arch.(e) Fully cured inverted arch supporting a 1 kg weight.

Figure 5 .
Figure 5 .Horizontally printed beam used to create a 3D printed functional structure.(a) 3D printing of the horizontal beam.(b) The deflection-sensing horizontal beam with DIW deposited conductive path, scale bar = 10 mm.(c) Normalized resistance response to 0.4, 0.8, 1.2, 1.6, and 2.0 mm deflections over time.(d) Average peak height versus deflection cycling condition.