Review
Systematic Review of Exoskeletons towards a General
Categorization Model Proposal
Javier A. de la Tejera
1,
* , Rogelio Bustamante-Bello
1
, Ricardo A. Ramirez-Mendoza
1,
*
and Javier Izquierdo-Reyes
1,2
Citation: de la Tejera, J.; Bustamante,
R.; Ramirez, R.; Izquierdo, J. Systematic
Review of Exoskeletons towards a
General Categorization Model Proposal.
Appl. Sci. 2021, 11, 76. https://dx.doi.
org/10.3390/app11010076
Received: 1 December 2020
Accepted: 10 December 2020
Published: 24 December 2020
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license (https://creativecommons.org/
licenses/by/4.0/).
1
School of Engineering and Sciences, Tecnologico de Monterrey, Mexico City 14380, Mexico;
rbustama@tec.mx (R.B.-B.); jizquierdo.reyes@tec.mx or javieriz@mit.edu (J.I.-R.)
2
Microsystems Technology Laboratories, Massachusetts Institute of Technology, Cambridge, MA 02319, USA
* Correspondence: j.delatejera@tec.mx (J.A.d.l.T.); ricardo.ramirez@tec.mx (R.A.R.-M.)
Abstract:
Exoskeletons are an essential part of humankind’s future. The first records regarding the
subject were published several decades ago, and the field has been expanding ever since. Their de-
velopments will be imperative for humans in the coming decades due to our constant pursuit of
physical enhancement, and the physical constraints the human body has. The principal purpose
of this article is to formalize research in the field of exoskeletons and introduce the field to more
researchers in hopes of expanding research in the area. Exoskeletons can assist and/or aid in the
rehabilitation of a person. Recovery exoskeletons are mostly used in medical and research areas;
performance exoskeletons can be used in any area. This systematic review explains the precedents of
the exoskeletons and gives a general perspective on their general present-day use, and provides a
general categorization model with a brief description of each category. Finally, this paper provides a
discussion of the state-of-the-art, and the current control techniques used in exoskeletons.
Keywords: exoskeletons; bioengineering; biomechanics; biomechatronics; rehabilitation robotics
1. Introduction
In the modern era, humanity is in constant pursuit of the improvement of everything
the world, including improvement of itself. As technology evolves, humanity evolves with
it in a systematic, artificial way. One of the strategies to attain such improvements is in the
area of exoskeletons. Exoskeletons are orthoses of a certain kind—devices that work in
harmony and parallel with their users in order to augment physical performance [
1
] and
are able to complete certain tasks or movements. In order to achieve such coordination,
the device must be fitted to the user’s dimensions [1].
While in other research areas, exoskeletons are used for protection [
2
], on humans,
their main use is the augmentation of the physical capabilities. This article will only refer
to exoskeletons for human use.
This article contains information about several aspects of exoskeletons, for this reason,
a graphical abstract is presented to show the structure of this article.
The exoskeletons’ limits are set only by the state of current technology and human
imagination. This means that, with the previous definition, the exoskeletons can be used
in any physical activity of a person. However, these constraints keep this research field
from being fully exploited to improve the life quality of people. Nevertheless, the current
development of exoskeletons is historically growing, creating the necessity to formalize
this research field to accelerate the current developments.
With the necessity of formalizing this research field, and the wide variety of possibili-
ties of exoskeletons, the requirement to differentiate between each exoskeleton, using their
design characteristics, is needed.
One way to categorize exoskeletons is according to their structure (soft or rigid). A rigid
exoskeleton is defined in the literature as an exoskeleton with a structure made of rigid con-
Appl. Sci. 2021, 11, 76. https://dx.doi.org/10.3390/app11010076 https://www.mdpi.com/journal/applsci
