Abstract

# A Low-Cost, Self-Powered, Plantar Pressure Distribution Sensing Insole †

Abdo-Rahmane Anas Laaraibi<sup>1,2,3,*</sup> <sup>□</sup>, Gurvan Jodin<sup>1,2</sup> <sup>□</sup>, Mario Costanza<sup>4</sup> <sup>□</sup>, Damien Hoareau<sup>1,2</sup> <sup>□</sup>, Samuel Margueron<sup>4</sup>, Nicolas Bideau<sup>5</sup> and Florence Razan<sup>1,2,3</sup>

**Abstract:** Energy-autonomous wireless sensors are a promising solution for developing wearable medical, lifestyle- and performance-monitoring systems. This paper presents a low-cost, low-power and self-powered wearable intelligent pressure monitoring system based on flexible piezoresistive sensors. The encapsulated insole with an 8 × 2 sensor matrix is powered by a flexible solar panel and connected to a rigid electronic board. Data acquisition occurs via Bluetooth low-energy transmission (BLE), and the average power consumption of the insole is 113 µW.

**Keywords:** low power; smart insole; Bluetooth low energy; piezoresistive sensors; plantar arch

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**Citation:** Laaraibi, A.-R.A.; Jodin, G.; Costanza, M.; Hoareau, D.; Margueron, S.; Bideau, N.; Razan, F. *A Low-Cost, Self-Powered, Plantar Pressure Distribution Sensing Insole*. *Proceedings 2024*, 97, 29.
https://doi.org/10.3390/proceedings2024097029

Academic Editors: Pietro Siciliano
and Luca Francioso

Published: 15 March 2024

<figure><img src="image_2.png" /></figure>

**Copyright:** © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
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4.0/).

# 1. Introduction

Nowadays, using smart insoles to monitor plantar motion is becoming increasingly important. These insoles are renowned for their importance in the sport and medical fields [1]. Some devices are already commercially available (Moticon, Pedar, F-scan etc.); however, these solutions are lacking in terms of energy consumption, comfort, cost and mechanical robustness. Thus, researchers are making significant advances in the development of this kind of device [2]. In this context, the study of a low-cost, low-power and self-powered wearable intelligent pressure monitoring system based on flexible piezoresistive sensors is presented in this paper. A synoptic diagram of the system is presented in Figure 1.

<footnote n="1">¹ Department of Mechatronics, École Normale Supérieure de Rennes, 35170 Bruz, France; gurvan.jodin@ens-rennes.fr (G.J.); damien.hoareau@ens-rennes.fr (D.H.); florence.razan@ens-rennes.fr (F.R.)</footnote>

<footnote n="2">² SATIE Laboratory, UMR CNRS 8029, École Normale Supérieure de Rennes, 35170 Bruz, France</footnote>

<footnote n="3">³ OASIS, IETR UMR CNRS 6164, Université de Rennes 1, 35042 Rennes, France</footnote>

<footnote n="4">⁴ FEMTO-ST Institute, University of Franche-Comté, CNRS (UMR 6174), ENSMM, 26 rue de l'Epitaphe, 25030 Besançon, France; mario costanza@femto-st.fr (M.C.); samuel.margueron@femto-st.fr (S.M.)</footnote>

<footnote n="5">5 Movement, Sports and Health (M2S) Laboratory, École Normale Supérieure de Rennes, 35170 Bruz, France; nicolas.bideau@univ-rennes2.fr</footnote>

<footnote>* Correspondence: abdo-rahmane-anas.laaraibi@ens-rennes.fr</footnote>

<footnote>† Presented at the XXXV EUROSENSORS Conference, Lecce, Italy, 10–13 September 2023.</footnote>
<figure><img src="image_3.png" /><figcaption>Figure 1. System diagram. (a) Self-powered encapsulated insole using a flexible solar panel; (b) inverse viscoelastic model [3]; (c) plantar pressure distribution sensing.</figcaption></figure>

## 2. Materials and Methods

To measure pressure distribution throughout the foot, we design an insole with an 8 × 2 sensor matrix [3]. The structure of this insole is multi-layered, with a single layer of carbon black (Velostat). The electrode structure is based on lines on one side and on columns on the other side of the Velostat layer, as shown in Figure 1a. We then sufficiently measure the resistance between the lines and columns, mainly at the intersection to measure the change in resistance due to applied pressure. The insole is encapsulated in a 27 cm size shoe mold with flexible and biocompatible polydimethylsiloxane (PDMS), RTV 615, to protect the electrode and insole from external influences, such as moisture, mechanical stress, fluctuating voltage, temperature and vibrations.

The insole is connected to an electronic board. Data acquisition is performed using an Arduino nano 33 BLE device, and the data are transmitted through a BLE radio integrated within the microcontroller. The electrical energy provided by the flexible solar panel (MPT3.6-150 PowerFilm©, 100 mA@3.6 V) is stored in a capacitor called $C_{storage}$, which powers the entire electronic board ($V_{stor}$).

The collected data are processed with a computer using an inverse viscoelastic model [3] to estimate the plantar force. The error of the measurements is below 2% of the full scale.

## 3. Discussion

Figure 2 shows the voltage $V_{stor}$ across the capacitor $C_{storage}$ using a PV panel to provide indoor lighting. The value chosen for this capacitor was 100 µF to allow a margin and extra energy capacity to supply the whole system, and $I_{syst}$ represents the current flowing through the system.

The developed smart system features power consumption $P_q \approx 62.8$ mW, which is given in Equation (1) as follows:

$$P_q = \frac{1}{\tau} \int_{0}^{\tau} V_{stor}(t) \cdot I_{syst}(t) \cdot dt, \qquad (1)$$

where $\tau$ is the time of the measurements and data transmission via BLE.

The average power consumption of the insole is 113 µW in the active mode; in fact, the system takes full advantage of the low-power characteristics of the components used.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Functional evolution of the supply voltage *V*<sub>stor</sub> versus time.</figcaption></figure>

**Author Contributions:** Conceptualization, A.-R.A.L., G.J. and M.C.; methodology, A.-R.A.L., M.C., S.M., G.J. and F.R.; software, A.-R.A.L.; validation, A.-R.A.L., M.C., G.J. and F.R.; formal analysis, A.-R.A.L. and G.J.; investigation, A.-R.A.L.; resources, A.-R.A.L.; data curation, A.-R.A.L.; writing—original draft preparation, A.-R.A.L.; writing—review and editing, A.-R.A.L., G.J., M.C., S.M., D.H., N.B. and F.R.; visualization, A.-R.A.L., G.J. and F.R.; supervision, G.J. and F.R.; project administration, G.J. and F.R.; funding acquisition, G.J. and F.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study is funded by the ANR within the framework of the PIA EURDIGISPORT project (ANR-18-EURE-0022).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request.

**Conflicts of Interest:** The authors declare no conflict of interest.

**References**

1. Drăgulinescu, A.; Drăgulinescu, A.-M.; Zincă, G.; Bucur, D.; Feies, V.; Neagu, D.-M. Smart Socks and In-Shoe Systems: State-of-the-Art for Two Popular Technologies for Foot Motion Analysis, Sports, and Medical Applications. *Sensors* **2020**, *20*, 4316. [CrossRef] [PubMed]

2. Almuteb, I.; Hua, R.; Wang, Y. Smart insoles review (2008–2021): Applications, potentials, and future. *Smart Health* **2022**, *25*, 100301. [CrossRef]

3. Laaraibi, A.-R.A.; Jodin, G.; Hoareau, D.; Bideau, N.; Razan, F. Flexible dynamic pressure sensor for insole based on inverse viscoelastic model. *IEEE Sens. J.* **2023**, *23*, 7634–7643. [CrossRef]

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