Knee Exoskeletons Design Approaches to Boost Strength Capability: A Review

There are different devices to increase the strength capacity of people with walking problems. These devices can be classified into exoskeletons, orthotics, and braces. This review aims to identify the state of the art in the design of these medical devices, based on an analysis of patents and literature. However, there are some difficulties in processing the records due to the lack of filters and standardization in the names, generating discrepancies between the search engines, among others. Concerning the patents, 74 patents were analyzed using search engines such as Google Patents, Derwent, The Lens, Patentscope, and Espacenet over the past ten years. A bibliometric analysis was performed using 63 scientific reports from Web of Science and The Lens in the same period for scientific communications. The results show a trend to use the mechanical design of exoskeletons based on articulated rigid structures and elements that provide force to move the structure. These are generally two types: (a) elastic elements and (b) electromechanical elements. The United States accounts for 32% of the technological patents reviewed. The results suggest that the use of exoskeletons or orthoses customized to the users’ needs will continue to increase over the years due to the worldwide growth in disability, particularly related to mobility difficulties and technologies related to the combined use of springs and actuators.


Introduction
In recent decades, advances in the development of assistive devices have become relevant in medicine, especially in specific areas related to people's disabilities, increasing its research and technological development. One of the main causes may be the worldwide increase in the population that suffers from a disability. According to the World Health Organization (WHO), more than 1000 million people live with some form of disability; almost 200 million suffer considerable difficulties in their functioning. These mobility difficulties are increased because of the aging of the population, and it will be a cause of more significant global concern than it is now [1].
In this context, the prevalence of disability is increasing, and Mexico does not escape this problem. According to data from the National Institute of Statistics and Geography (INEGI), 7% of the population suffers from a disability, and 42.4% of disabilities are related to mobility [2]. Another factor that affects the increase in the population with disabilities is related to obesity. According to the Organization for Economic Cooperation and Development (OECD) in its report "The Heavy Burden of Obesity: The Economics of Prevention", about 73% of the Mexican population suffers from overweight, and 34% of people suffer from morbid obesity, which is the highest degree of obesity [3]. A great part The research and development of devices that provide solutions to mobility problems have increased in recent decades; such is the case of the development of exoskeletons, which have been a milestone in this area [7]. Exoskeletons can be grouped based on energy use into powered exoskeletons and passive exoskeletons [8]. Powered exoskeletons are systems based on an external energy source and actuators that convert electrical, pneumatic, or hydraulic energy to mechanical work to give support and controlled movement [9][10][11][12][13][14], whereby the rehabilitation tasks turn out to be more suitable and effective than traditional methods. However, a limitation of these exoskeletons is that patients do not have control of the trajectory, which challenges patients to train personalized or appropriate movement patterns for their recovery. Passive exoskeletons do not have an external power source, so their movement is dependent on the user. These exoskeletons consist of springs, shock absorbers, and mechanical clutches to store energy or release energy during walking [15]. They are cheaper, less complex, and are likely to be better accepted by users. Herr (2009) classified exoskeletons and orthotics into four categories: (a) serial limb exoskeletons; (b) parallel limb exoskeletons for load transfer; (c) exoskeletons of parallel extremities to increase torque, and (d) exoskeletons of parallel extremities that increase human endurance. In the present review, the terms exoskeletons and orthotics are used similarly, as mechanical devices of an anthropomorphic nature [16].
This work presents the method used to find refined information obtained in databases for both patents and scientific communications. The results section presents the bibliometric analysis of the literature and the patentometric analysis of technological surveillance. Finally, the conclusions section shows the main metrics analyzed and the technology trend to boost strength capability.

Review Methodology
The search strategy was divided into two parts: (1) obtaining patents and (2) obtaining scientific communications, for which in both cases, the arrangement of keywords and The research and development of devices that provide solutions to mobility problems have increased in recent decades; such is the case of the development of exoskeletons, which have been a milestone in this area [7]. Exoskeletons can be grouped based on energy use into powered exoskeletons and passive exoskeletons [8]. Powered exoskeletons are systems based on an external energy source and actuators that convert electrical, pneumatic, or hydraulic energy to mechanical work to give support and controlled movement [9][10][11][12][13][14], whereby the rehabilitation tasks turn out to be more suitable and effective than traditional methods. However, a limitation of these exoskeletons is that patients do not have control of the trajectory, which challenges patients to train personalized or appropriate movement patterns for their recovery. Passive exoskeletons do not have an external power source, so their movement is dependent on the user. These exoskeletons consist of springs, shock absorbers, and mechanical clutches to store energy or release energy during walking [15]. They are cheaper, less complex, and are likely to be better accepted by users. Herr (2009) classified exoskeletons and orthotics into four categories: (a) serial limb exoskeletons; (b) parallel limb exoskeletons for load transfer; (c) exoskeletons of parallel extremities to increase torque, and (d) exoskeletons of parallel extremities that increase human endurance. In the present review, the terms exoskeletons and orthotics are used similarly, as mechanical devices of an anthropomorphic nature [16].
This work presents the method used to find refined information obtained in databases for both patents and scientific communications. The results section presents the bibliometric analysis of the literature and the patentometric analysis of technological surveillance. Finally, the conclusions section shows the main metrics analyzed and the technology trend to boost strength capability.

Review Methodology
The search strategy was divided into two parts: (1) obtaining patents and (2) obtaining scientific communications, for which in both cases, the arrangement of keywords and Boolean operators were ((knee or (lower limb)) and (exoskeletons or orthoses)). We considered a 10-year search period, that is, from 2011 to 2020 as the first filter (Filter_1). Later, the data obtained were processed for analysis by applying three filters, which are detailed in Sections 2.1 and 2.2.

Patent Search
Five search engines were used (Derwent Innovation, Espacenet, Google Patents, Patentscope, and The Lens), in which the arrangement of keywords and Boolean operators mentioned in the previous paragraph shows a growth in the patent registration of lower limb exoskeletons in the past decade (see Figure 2).
Appl. Sci. 2021, 11, x FOR PEER REVIEW Boolean operators were ((knee or (lower limb)) and (exoskeletons or orthoses)). W sidered a 10-year search period, that is, from 2011 to 2020 as the first filter (Filter_1). the data obtained were processed for analysis by applying three filters, which are de in Sections 2.1 and 2.2.

Patent Search
Five search engines were used (Derwent Innovation, Espacenet, Google Paten tentscope, and The Lens), in which the arrangement of keywords and Boolean ope mentioned in the previous paragraph shows a growth in the patent registration of limb exoskeletons in the past decade (see Figure 2). Subsequently, two other filters were used, denoted as Filter_2 and Filter_3, rela the international patent classification (IPC) A61H3/00 and A61F5/01. These patents b to the group of non-surgical devices or appliances to help people walk. We applie filters directly in the corresponding search engine, as the goal is to determine the sta the passive knee exoskeleton technique. Filter_3 has to do with the inclusion of the knee. Finally, with the results of the databases, obtained after applying the three fil single record was integrated and "data cleaning" was performed using the Open R free software. Later, we disaggregated the data and performed a manual review o record, applying criteria to restrict and select patents of interest. This action const Filter_4.
A. Inclusion criteria: (a) The patent belongs to a type of lower limb exoskeleton or orthosis.  Subsequently, two other filters were used, denoted as Filter_2 and Filter_3, related to the international patent classification (IPC) A61H3/00 and A61F5/01. These patents belong to the group of non-surgical devices or appliances to help people walk. We applied the filters directly in the corresponding search engine, as the goal is to determine the status of the passive knee exoskeleton technique. Filter_3 has to do with the inclusion of the word knee. Finally, with the results of the databases, obtained after applying the three filters, a single record was integrated and "data cleaning" was performed using the Open Refine ® free software. Later, we disaggregated the data and performed a manual review of each record, applying criteria to restrict and select patents of interest. This action constitutes Filter_4.

A.
Inclusion criteria: The patent belongs to a type of lower limb exoskeleton or orthosis.
The patent corresponds to a component of an exoskeleton or lower limb orthosis. (c) The patent describes the design or manufacturing method of an exoskeleton or lower limb orthosis.
The patent can be registered in any patent office in any country.

B.
Exclusion criteria: (a) Related to other devices for members of the body other than the lower one.
Related to complementary systems not related to the knee joint. (c) Patents are found in more than one database; only the patent found in the first database was considered to avoid duplication of information. (d) Patents prior to 2011, considering slow progress in development and management.
This discrimination of the fourth filter (Filter_4) is made by adding all the documents obtained after Filter_3, resulting in 205 documents to which the inclusion and exclusion criteria were applied, leaving only 74 documents identified with the data of the inventor, registry office, year of publication, patent title, and registration key. Table 1 shows the effect of reducing the records when applying each filter.

Search of Scientific Communications
For the analysis of the literature, two search engines (Web of Science and The Lens) were used, in which the disposition of keywords and Boolean operators were applied, as well as the time period of 10 years as the filter (Filter_1). The number articles per year is shown in Figure 3, where a growing trend is observed in the case of the Web of Science (WoS) search engine over the entire time series, but not in the case of the data obtained with the search engine The Lens, where a clear trend is not appreciated; this may be due to the unified system of the search engine that gathers patents and academic documents in a database with the ability to identify intersections between these two types of documents.
(b) Related to complementary systems not related to the knee joint. (c) Patents are found in more than one database; only the patent found in the first database was considered to avoid duplication of information. (d) Patents prior to 2011, considering slow progress in development and management.
This discrimination of the fourth filter (Filter_4) is made by adding all the documents obtained after Filter_3, resulting in 205 documents to which the inclusion and exclusion criteria were applied, leaving only 74 documents identified with the data of the inventor, registry office, year of publication, patent title, and registration key. Table 1 shows the effect of reducing the records when applying each filter.

Search of Scientific Communications
For the analysis of the literature, two search engines (Web of Science and The Lens) were used, in which the disposition of keywords and Boolean operators were applied, as well as the time period of 10 years as the filter (Filter_1). The number articles per year is shown in Figure 3, where a growing trend is observed in the case of the Web of Science (WoS) search engine over the entire time series, but not in the case of the data obtained with the search engine The Lens, where a clear trend is not appreciated; this may be due to the unified system of the search engine that gathers patents and academic documents in a database with the ability to identify intersections between these two types of documents. With the data obtained from each search engine, the results of both were crossed, applying a second filter (Filter_2) using inclusion and exclusion criteria to restrict and select information on the topic: A. Inclusion criteria: With the data obtained from each search engine, the results of both were crossed, applying a second filter (Filter_2) using inclusion and exclusion criteria to restrict and select information on the topic:   Table 2 shows the reduction effect of the records when applying the inclusion and exclusion criteria. A total of 63 records were obtained performing the bibliometric analysis using R-Studio ® and Bibliometrix.

Patentometric Analysis
After filtering the documents, 74 patents related to exoskeletons, orthoses, or lower limb devices were selected; the results are shown in Table 3. The main offices from where patents were registered are the United States (23 patents), the European office (4 patents), the international office (23 patents), China (18 patents), Japan (3 patents), Republic of Korea (2 patents), and Canada (1 patent). As shown in Figure 4, the United States is the country with the highest number of patents.

Patentometric Analysis
After filtering the documents, 74 patents related to exoskeletons, orthoses, or lower limb devices were selected; the results are shown in Table 3. The main offices from where patents were registered are the United States (23 patents), the European office (4 patents), the international office (23 patents), China (18 patents), Japan (3 patents), Republic of Korea (2 patents), and Canada (1 patent). As shown in Figure 4, the United States is the country with the highest number of patents.  Applicants and inventors in the databases were considered. The Beijing Institute of Technology and Otto Bock Healthcare Gumbo are the main applicants, with four and three patents, respectively, from 2011 to 2020. In Figure 5, the main applicants for exoskeletons and orthosis are shown. Applicants and inventors in the databases were considered. The Beijing Institute of Technology and Otto Bock Healthcare Gumbo are the main applicants, with four and three patents, respectively, from 2011 to 2020. In Figure 5, the main applicants for exoskeletons and orthosis are shown. In the analysis of the patents, we found that 59 documents are related to devices for the knee joint and 15 documents are related to devices for two or more joints in the lower limb, as can be seen in Figure 6. In the analysis of the patents, we found that 59 documents are related to devices for the knee joint and 15 documents are related to devices for two or more joints in the lower limb, as can be seen in Figure 6. In the analysis of the patents, we found that 59 documents are related to devices for the knee joint and 15 documents are related to devices for two or more joints in the lower limb, as can be seen in Figure 6. Among the 74 results obtained, 29 results match with orthoses, 16 with braces, 14 with exoskeletons, 7 with devices, 2 with mechanisms, and 1 with exo-suit and prostheses. In addition, three results are associated with methodologies (manufacturing methods, design methods, tests). Figure 7 shows the resulting patents and the distribution with the part of the body of the lower limb that they cover. Among the 74 results obtained, 29 results match with orthoses, 16 with braces, 14 with exoskeletons, 7 with devices, 2 with mechanisms, and 1 with exo-suit and prostheses. In addition, three results are associated with methodologies (manufacturing methods, design methods, tests). Figure 7 shows the resulting patents and the distribution with the part of the body of the lower limb that they cover. From the patent data, 59 refer to patents related only to the knee joint. In 45 patents, their predominant technology is mechanical, using cams, hinges, pivoting arms, elastic bands, springs, and springs as the main elements to carry out the mobility of the joint. In 11 patents, the predominant technology is electromechanical-electronic using actuators, servomotors, motors, and a combination of motor-spring with which they carry out the movement of the device. For three patents, the technology used is pneumatic, in which the movement of the devices is made using soft elements. Table 3 shows the selected patents, the technology used, the type of device, and the part of the lower limb that they cover. Figure 8 shows the technology used in the patents and the main elements with which they carry out the mobility of the device. From the patent data, 59 refer to patents related only to the knee joint. In 45 patents, their predominant technology is mechanical, using cams, hinges, pivoting arms, elastic bands, springs, and springs as the main elements to carry out the mobility of the joint. In 11 patents, the predominant technology is electromechanical-electronic using actuators, servomotors, motors, and a combination of motor-spring with which they carry out the movement of the device. For three patents, the technology used is pneumatic, in which the movement of the devices is made using soft elements. Table 3 shows the selected patents, the technology used, the type of device, and the part of the lower limb that they cover. Figure 8 shows the technology used in the patents and the main elements with which they carry out the mobility of the device. 11 patents, the predominant technology is electromechanical-electronic using actuators, servomotors, motors, and a combination of motor-spring with which they carry out the movement of the device. For three patents, the technology used is pneumatic, in which the movement of the devices is made using soft elements. Table 3 shows the selected patents, the technology used, the type of device, and the part of the lower limb that they cover. Figure 8 shows the technology used in the patents and the main elements with which they carry out the mobility of the device.   Figure 8. Type of technology used in the knee joint.

Scientometric Analysis
The following keywords were analyzed: lower limb (frequency = 12 articles), exoskeleton (frequency = 11 articles), knee (frequency = 9 articles), control (frequency = 5 articles), design (frequency = 4 articles), rehabilitation (frequency = 4 articles), passive (frequency = 4 articles), joint (frequency = 3 articles), gait (frequency = 3 articles), orthosis (frequency = 2 articles), development (frequency = 2 articles), powered (frequency = 2 articles), and assistance (frequency = 2 articles) (see Figure 9). From the information obtained by the scientific documents, the characteristics of study subjects were as follows: average weight of 75 kg, a knee torque corresponding to 95-150 Nm, and the power in the knee between 50-235 Watts in the movement of flexion and extension. Regarding the authors, we find that Mohammed S. is the main author (six articles); however, Meng W. is the most cited author in this field (see Table 4 and Figure 10).  Regarding the authors, we find that Mohammed S. is the main author (six articles); however, Meng W. is the most cited author in this field (see Table 4 and Figure 10).
The United States is the most productive country (14 documents), followed by the United Kingdom (13 documents) and China (7 documents) (see Figure 11).  The United States is the most productive country (14 documents), followed by the United Kingdom (13 documents) and China (7 documents) (see Figure 11). Figure 11. The most productive countries in developing exoskeletons for the knee joint. Table 5 shows the selected articles, used in the scientometric analysis used, results of the databases and selection criteria. The United States is the most productive country (14 documents), followed by the United Kingdom (13 documents) and China (7 documents) (see Figure 11). Figure 11. The most productive countries in developing exoskeletons for the knee joint. Table 5 shows the selected articles, used in the scientometric analysis used, results of the databases and selection criteria.   Table 5 shows the selected articles, used in the scientometric analysis used, results of the databases and selection criteria.

Discussion
In this review of 74 patents and 63 scientific articles, different designs were used to find similarities to guide and facilitate new proposals that may arise in the future. We conclude that using a regulated and slender structure forms the basis of the construction of the exoskeleton design for the knee joint, generally identifying two types of movement: (a) rotation, which achieves flexion and extension of the knee, and (b) rotation and translation, which reproduce more naturally the articular movement of the knee. The design can be scaled to active or passive exoskeletons depending on the force element that is used, and the number of joints of the lower limb that can be included can be expanded.
On the other hand, the number of records per database does not reflect the effectiveness of each search engine. For this research, priority was given to those who provided useful data, such as direct links to patents, the inventor's name, and IPC codes. However, there are some difficulties in processing the records due to the lack of options to filter results or IPC categories. Moreover, some applicants may be included in the name of their companies. This is because some search engines only show the name of the applicant or owner rather than the inventor. In some cases, there is a lack of coherence between the names of the authors in different patents (for example, Chen shuyan and Shuyan, C.). These kinds of inconsistencies were grouped together, but still, the results could not be entirely accurate; the significance of this error does not affect the classification of every system. This work is helpful to researchers and developers concerned with making proposals more functional to patients and therapists. The patients can know in detail the solutions existent in the market and increase confidence in the kind of device they are using or intend to use, comprehending its operating features, which help to assimilation and adopt medical technology.

Structural Support Elements
Based on the selected patents and scientific documentation, it is observed that the design of exoskeletons or knee orthoses consists of a rigid structure that is fixed to the side of the joint attached to the leg by means of elastic bands or straps. This structure can be one or two in parallel, embracing the knee joint from the sides, constituting the main structure in the design. Generally, it is made up of three parts: an upper part that is fixed to the leg above the knee, a lower part fixed below the knee, and a joint axis that allows flexion and extension of the mechanical structure of the exoskeleton (see Figure 12). one or two in parallel, embracing the knee joint from the sides, constituting the main structure in the design. Generally, it is made up of three parts: an upper part that is fixed to the leg above the knee, a lower part fixed below the knee, and a joint axis that allows flexion and extension of the mechanical structure of the exoskeleton (see Figure 12).

Union Types
In patents and research articles, we find two different designs in the joint. The first kind of design allows the flexion and extension of the exoskeleton structure through a hinge with a single axis of rotation in the simplest and most generalized way. It provides the exoskeleton with a movement of a degree of freedom. The second is the union through a mobile axis, which is achieved in three ways: pivoting arms, cams, and polycentric pulleys, providing the exoskeleton with two degrees of freedom due to the linear displacement during the rotation that occurs when the axis moves during rotation (see Figure 13).

Types of Force Elements Used
The two variants in the design of the exoskeleton structure are the basis for building active or passive knee exoskeletons. In the first case, actuators (pneumatic/electric) are added to the structure that provides the force for bending and joint extension; the basic structure is generally with a fixed axis. The second case is by means of springs, cams, or

Union Types
In patents and research articles, we find two different designs in the joint. The first kind of design allows the flexion and extension of the exoskeleton structure through a hinge with a single axis of rotation in the simplest and most generalized way. It provides the exoskeleton with a movement of a degree of freedom. The second is the union through a mobile axis, which is achieved in three ways: pivoting arms, cams, and polycentric pulleys, providing the exoskeleton with two degrees of freedom due to the linear displacement during the rotation that occurs when the axis moves during rotation (see Figure 13). one or two in parallel, embracing the knee joint from the sides, constituting the main structure in the design. Generally, it is made up of three parts: an upper part that is fixed to the leg above the knee, a lower part fixed below the knee, and a joint axis that allows flexion and extension of the mechanical structure of the exoskeleton (see Figure 12).

Union Types
In patents and research articles, we find two different designs in the joint. The first kind of design allows the flexion and extension of the exoskeleton structure through a hinge with a single axis of rotation in the simplest and most generalized way. It provides the exoskeleton with a movement of a degree of freedom. The second is the union through a mobile axis, which is achieved in three ways: pivoting arms, cams, and polycentric pulleys, providing the exoskeleton with two degrees of freedom due to the linear displacement during the rotation that occurs when the axis moves during rotation (see Figure 13).

Types of Force Elements Used
The two variants in the design of the exoskeleton structure are the basis for building active or passive knee exoskeletons. In the first case, actuators (pneumatic/electric) are added to the structure that provides the force for bending and joint extension; the basic structure is generally with a fixed axis. The second case is by means of springs, cams, or

Types of Force Elements Used
The two variants in the design of the exoskeleton structure are the basis for building active or passive knee exoskeletons. In the first case, actuators (pneumatic/electric) are added to the structure that provides the force for bending and joint extension; the basic structure is generally with a fixed axis. The second case is by means of springs, cams, or elastic elements that are added to the design to store and release the energy obtained from the march; the structure can be of a fixed or mobile axis (see Figure 14).
Appl. Sci. 2021, 11, x FOR PEER REVIEW elastic elements that are added to the design to store and release the energy obtain the march; the structure can be of a fixed or mobile axis (see Figure 14). From the documents (patent and research articles), we can determine tha signs, on average, have a range of mobility of the structure from 0 to 120 degrees actuators, on average, provide a torque of 95 to 150 Nm and power from 50 to 23 On average, when it comes to the use of springs, they generate a force of appro 85 N.

Conclusions
Exoskeletons and knee orthoses are rigid structures articulated with one or m grees of freedom, to which elements that provide force on the structure are atta principle, they are of two types: (a) elastic components such as springs or bands, th deformed, store energy to later release it, and (b) electromechanical component are generally based on electric motors that transform electrical energy into me energy. There are design proposals where the force is provided by a soft element pneumatically.
Although in principle, the technologies used remain the same, the trend in th of exoskeletons customized to the needs of users has led to the development o components and the combination of elastic elements with electromechanical e generating semi-active designs which are more versatile. This review shows United States is the country with the highest number of patents and scientific do related to exoskeletons, orthopedic devices, and knee devices. Therefore, some could have been left out of this investigation.
The contribution of these results focuses on knowing, especially among th and patients, the usefulness of classifying the knee exoskeletons and their rela with energy expenditure and comparative energy cost and gait efficiency. In addi work contributes to the formulation of the design presented in Sections 4.1, 4.2, a a topology optimization for new knee orthoses.  From the documents (patent and research articles), we can determine that the designs, on average, have a range of mobility of the structure from 0 to 120 degrees, and the actuators, on average, provide a torque of 95 to 150 Nm and power from 50 to 235 watts. On average, when it comes to the use of springs, they generate a force of approximately 85 N.

Conclusions
Exoskeletons and knee orthoses are rigid structures articulated with one or more degrees of freedom, to which elements that provide force on the structure are attached. In principle, they are of two types: (a) elastic components such as springs or bands, that when deformed, store energy to later release it, and (b) electromechanical components, which are generally based on electric motors that transform electrical energy into mechanical energy. There are design proposals where the force is provided by a soft element actuated pneumatically.
Although in principle, the technologies used remain the same, the trend in the design of exoskeletons customized to the needs of users has led to the development of lighter components and the combination of elastic elements with electromechanical elements, generating semi-active designs which are more versatile. This review shows that the United States is the country with the highest number of patents and scientific documents related to exoskeletons, orthopedic devices, and knee devices. Therefore, some designs could have been left out of this investigation.
The contribution of these results focuses on knowing, especially among therapists and patients, the usefulness of classifying the knee exoskeletons and their relationship with energy expenditure and comparative energy cost and gait efficiency. In addition, this work contributes to the formulation of the design presented in Sections 4.1-4.3 as a topology optimization for new knee orthoses. Funding: The present article uses free software except for Derwent analytics for the patent analysis, provided by the Autonomous University of Mexico State. This research was funded by CONACYT (Consejo Nacional de Ciencia y Tecnología) (grant number 1009754).