Diseño, manufactura aditiva y caracterización mecánica a la compresión de asientos experimentales para sillas de ruedas con estructuras de celosía basadas en filamentos flexibles.

Abstract

Title: Design, additive manufacturing, and compressive mechanical characterization of experimental wheelchair seats with lattice structures based on flexible filaments.

Introduction: Prolonged sitting in wheelchair users generates high pressure concentrations over bony prominences (ischium and sacrum), restricting tissue blood flow and inducing the development of pressure ulcers (PUs). Traditional contoured seats, manufactured via CNC subtractive carving of polyurethane foams, present functional limitations due to their linear homogeneous deformation behavior and lack of thermal permeability. Additive manufacturing (3D printing) using elastic filaments (TPU) enables the design of lattice structures with functional gradients, offering the ability to locally program cushion stiffness to optimize mechanical unloading. However, experimental characterization of these internal geometries under standardized loads is required prior to clinical validation.

Objective: To design, 3D print, and mechanically characterize under compression experimental seat samples with three distinct lattice geometries, quantitatively evaluating their downward vertical displacement expressed as a deformation percentage under a controlled static load of 40 kg.

Methodology: Three-dimensional unit cell typologies (gyroid, honeycomb, and cubic) were digitally modeled while maintaining a constant relative material density (infill). The experimental specimens ($10 \times 10 \times 10 \text{ cm}$) were fabricated via Fused Deposition Modeling (FDM) 3D printing using a flexible thermoplastic polyurethane (TPU, Shore 95A) with a direct extrusion system. Three replicates were manufactured per configuration. For the mechanical tests, the initial height ($H_0$) at rest was recorded. Subsequently, a standard mass equivalent to a 40 kg load ($392.4 \text{ N}$) was uniaxially applied and uniformly distributed over the top surface. Following five minutes of viscoelastic stabilization, the final height under load ($H_f$) was measured. The analysis parameter was the downward vertical displacement, calculated and expressed as a structural deformation percentage: $\epsilon\% = [(H_0 - H_f)/H_0] \times 100$.

Results: Mechanical testing demonstrated that lattice geometry significantly influences elastic deformation. The gyroid lattice exhibited progressive deformation and homogeneous stress absorption. The honeycomb pattern showed an abrupt cell collapse after exceeding a critical load threshold, whereas the cubic structure presented the highest structural stiffness, limiting vertical displacement. Analysis of variance (ANOVA) confirmed significant statistical differences ($p < 0.05$) among the displacement percentages of the three configurations.

Conclusions: Experimental characterization determined that TPU-printed lattice structures are mechanically viable to replace conventional foams. The gyroid geometry stands out as the optimal configuration for biomechanical applications in rehabilitation engineering, offering a controlled percentage displacement that promotes user immersion and load distribution without reaching material bottoming-out.