The current gold standard technique for treatment of anterior cruciate ligament (ACL) injury is reconstruction with autograft. These treatments have a relatively high failure and re-tear rate. To overcome this, tissue engineering and additive manufacturing are being used to explore the potential of 3D scaffolds as autograft substitutes. However, mechanically optimal polymers for this have yet to be identified. Here, we use 3D printing technology and various materials with the aim of fabricating constructs better matching the mechanical properties of the native ACL. A fused deposition modeling (FDM) 3D printer was used to microfabricate dog bone-shaped specimens from six different polymers—PLA, PETG, Lay FOMM 60, NinjaFlex, NinjaFlex-SemiFlex, and FlexiFil—at three different raster angles. The tensile mechanical properties of these polymers were determined from stress–strain curves. Our results indicate that no single material came close enough to successfully match reported mechanical properties of the native ACL. However, PLA and PETG had similar ultimate tensile strengths. Lay FOMM 60 displayed a percentage strain at failure similar to reported values for native ACL. Furthermore, raster angle had a significant impact on some mechanical properties for all of the materials except for FlexiFil. We therefore conclude that while none of these materials alone is optimal for mimicking ACL mechanical properties, there may be potential for creating a 3D-printed composite constructs to match ACL mechanical properties. Further investigations involving co-printing of stiff and elastomeric materials must be explored.

Understanding how learned fear can be reduced is at the heart of treatments for anxiety disorders. Tremendous progress has been made in this regard through extinction training in which the aversive outcome is omitted. However, current progress almost entirely rests on this single paradigm, resulting in a very specialized knowledgebase at the behavioural and neural level of analysis. Here, we used a dual-paradigm approach to show that different methods that lead to reduction in learned fear in rats are dissociated in the cortex. We report that the infralimbic cortex has a very specific role in fear reduction that depends on the omission of aversive events but not on overexpectation. The orbitofrontal cortex, a structure generally overlooked in fear, is critical for downregulating fear when novel predictions about upcoming aversive events are generated, such as when fear is inflated or overexpected, but less so when an expected aversive event is omitted.

Current materials used to fill bone defects (ceramics, cement) either lack strength or do not induce bone repair. The use of biodegradable polymers such as PLA may promote patient healing by stimulating the production of new bone in parallel with a controlled degradation of the scaffold. This project aims to determine the design parameters maximising scaffold mechanical performance in such materials. Starting from a base cylindrical model of 10 mm height and of outer and inner diameters of 10 and 4 mm, respectively, 27 scaffolds were designed. Three design parameters were investigated: pore distribution (crosswise, lengthwise, and eccentric), pore shape (triangular, circular, and square), and pore size (surface area of 0.25 mm2, 0.5625 mm2, and 1 mm2). Using the finite element approach, a compressive displacement (0.05 mm/s up to 15% strain) was simulated on the models and the resulting scaffold stiffnesses (N/mm2) were compared. The models presenting good mechanical behaviors were further printed along two orientations: 0° (cylinder sitting on its base) and 90° (cylinder laying on its side). A total of n = 5 specimens were printed with PLA for each of the retained models and experimentally tested using a mechanical testing machine with the same compression parameters. Rigidity and yield strength were evaluated from the experimental curves. Both numerically and experimentally, the highest rigidity was found in the model with circular pore shape, crosswise pore distribution, small pore size (surface area of 0.25 mm2), and a 90° printing orientation. Its average rigidity reached 961 ± 32 MPa from the mechanical testing and 797 MPa from the simulation, with a yield strength of 42 ± 1.5 MPa. The same model with a printing orientation of 0° resulted in an average rigidity of 515 ± 7 MPa with a yield strength of 32 ± 1.6 MPa. Printing orientation and pore size were found to be the most influential design parameters on rigidity. The developed design methodology should accelerate the identification of effective scaffolds for future in vitro and in vivo studies.

Understanding how learned fear can be reduced is at the heart of treatments for anxiety 11 disorders. Tremendous progress has been made in this regard through extinction training in 12 which an expected aversive outcome is omitted. However, current progress almost entirely rests 13 on this single paradigm, resulting in a very specialized knowledgebase at the behavioural and 14 neural level of analysis. Here, we used a paradigm-independent approach to show that different 15 methods that lead to reduction in learned fear are dissociated in the cortex. We report that the 16 infralimbic cortex has a very specific role in fear reduction that depends on the omission of 17 aversive events but not on overexpectation. The orbitofrontal cortex, a structure generally 18 overlooked in fear, is critical for downregulating fear when fear is inflated or overexpected, but 19 not when an aversive event is omitted. 20 3 Extinction learning has captivated behavioural and neural science for more than a century. It has done 21 so because it allows for the reduction of behaviours that were once adaptive but are no longer so, and 22 gives the therapist a handle to combat others that were never adaptive in the first place. The most-23 widely used method for supressing unwanted behaviour relies on the omission of the event that drives 24 this behaviour, that is, extinction driven by outcome omission. In the context of fear learning, 25 extinction by omission involves the dramatic reduction in fear-related behaviours typically observed 26 after presenting a previously established signal for an aversive event (i.e., a tone paired with shock; 27 tone→shock) in the absence of that event (tone presented alone; tone→nothing). Given its simplicity 28 and effectiveness in the treatment of anxiety disorders 1-6 , extinction by omission has received 29 significant attention in a quest to understand its underlying behavioural and neural mechanisms 7-15 . 30 Critically, although much progress has been made, this progress is limited to the case of outcome 31 omission, while another equally relevant form of extinction learning that also drives reduction in 32 unwanted behaviour, namely overexpectation, remains largely unexplored. This single-paradigm 33 approach is restrictive because at best it can oversimplify and at worst even misrepresent the function 34 of brain areas implicated in extinction learning. Here, we move beyond this paradigm-specific 35 approach and embarked on an investigation into how the brain learns from extinction using two 36 behavioural designs: extinction driven by outcome omission (described above) and extinction driven 37 by overexpectation (described below). 38In overexpectation, reduction in previously established fear responses ensue, strikingly, despite 39 continued delivery of the aversive event. This is possible because separately established signals of a 40 common aversive event (i.e., tone→shock; light→shock) can summate their fear-inducing properties 41 when encountered simultaneously (tone+light), triggeri...

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