Customized insoles with uniform reaction

Customization of medical devices

The customization of clinical devices is one of the most promising industrial and research fields. The design and shaping of insoles customized on the specific requirements of each individual patient can help to alleviate the individual diseases with very high effectiveness. The customized insoles are of great relevance in clinical cases where posture and gait are compromised by pathologies of different nature including, for instance, orthopedic issues, post-brake effects, and diabetes.

With the potentialities of digital design and additive manufacturing (AM), the insole can be produced with personalized stiffness by using elastic thermoplastic polymers and light controlled building processes (stereolithography), as demonstrated in this project with an industrial partner.

Insoles with uniform reaction

The individual pressure distribution of the patient is used to shape the insole geometry and its local stiffness. The insole is made by many cells of different size and shape, which sinergically react against the patient weight and generate a uniform pressure distribution. The patient foot sole is then homogeneously exposed to the same pressure everywhere, independently to the local vertical displacement. The footprint is preserved, but the insole reaction gives different forces at every point, calibrated on the individual patient. This property has been obtained by starting from the real footprint on a "laboratory beach sample".

Footprint on the sand, casting of the foot shape and measurement of the displacement

The Voronoi structure

This Ukrainian scientist, in 1908, formalized a special way to divide one plane in many regions: the method is called "Voronoi diagram" and is currently used in geophysics and meteorology. Its extension to the third-dimension leads to the definition of a very versatile shape of lattice structure. The relative density of this structure is simply identified with a global index instead of the geometrical dimension of each single feature.

Voronoi structures with different densities

Design strategies

There are many types of models usable to calculate the structural response of lattice structures. The most accurate, unfortunately, are also the most complicated and the slowest in the software computation. Hopefully, most of the relevant results are achievable by simpler models, as those based on the Euler-Bernoulli beam theory, so popular with the engineers.

Lattice structures FEM models - 1D model, homogenization model, full 3D model-

The cellular material built with nTopology

The product of the design is like a new material having different properties (or, better, equivalent properties) at every point. The "parent material" is the same, in this case is a flexible photocurable thermoplastic polymer, but the macroscopic density, stiffness, and strength are different. Moreover, these properties are controllable and predictable by the designer. The most accurate models, then, can predict the local stress and strain values at every position of the real structure. In short, the cellular material constitutes the structural organism. The software nTopology is used for built and optimize the lattice: read more here.

Lattice structure Voronoi - FEM stress simulation
Lattice structure Voronoi - geometry and topology