Functional electric stimulation ankle-foot orthosis based on 4D printing and multi-sensor fusion and preparation method thereof

Abstract

This application relates to the field of medical rehabilitation device technology, and provides a functional electrical stimulation ankle-foot orthosis based on 4D printing and multi-sensor fusion, as well as its fabrication method. The orthosis includes: a 4D-printed orthosis body, at least a portion of which comprises a shape memory polymer-based composite material, capable of changing shape under preset stimulation to conform to the user's contours; a multi-sensor module for acquiring gait data; a functional electrical stimulation module for outputting electrical stimulation to the user's tibialis anterior muscle; and a main control module. The main control module identifies the user's gait cycle based on the gait data acquired by the multi-sensor module, and when it identifies that the gait cycle is in the early support phase or the mid-to-late swing phase, it controls the functional electrical stimulation module to output an electrical stimulation signal to stimulate tibialis anterior muscle contraction to assist ankle dorsiflexion. This application improves wearing comfort through the adaptive structure of 4D printing and achieves accurate gait recognition and electrical stimulation intervention through multi-sensor fusion, thus realizing effective gait correction.

Description

Technical Field

[0001] This application belongs to the field of medical rehabilitation device technology, and in particular relates to a functional electrical stimulation ankle-foot orthosis based on 4D printing and multi-sensor fusion and its preparation method. Background Technology

[0002] Users often experience gait abnormalities such as foot drop and inversion due to paralysis of lower limb muscles, such as the tibialis anterior muscle, and impaired nerve control. When walking, they have difficulty actively completing ankle dorsiflexion, which often leads to landing on their toes instead of the normal heel strike during the initial support phase. During the swing phase, they are prone to dragging their toes, which in turn triggers compensatory behaviors such as circular gait, affecting walking efficiency and increasing the risk of tripping and falling.

[0003] To address this issue, existing technologies primarily employ ankle-foot orthoses or functional electrical stimulation (fED) devices. However, traditional ankle-foot orthoses are mostly rigid or semi-rigid structures with fixed shapes, making it difficult to perfectly conform to the individualized limb contours of users. This can easily generate localized high pressure at bony prominences such as the ankle and heel, posing a risk of pressure sores for users with reduced sensory function and resulting in poor wearing comfort. On the other hand, existing fED devices lack precision in triggering timing. Some devices rely solely on a single type of sensor (such as plantar pressure sensors or inertial sensors) to determine gait. In complex real-world walking environments, these sensors are easily affected by factors such as uneven ground and speed changes, leading to errors in gait cycle recognition. This results in electrical stimulation being triggered at inappropriate times, failing to achieve the desired corrective effect and potentially even interfering with the user's normal gait.

[0004] Therefore, existing technologies suffer from poor fit and comfort of ankle-foot orthoses, as well as inaccurate timing of functional electrical stimulation triggering and limited corrective effects. Summary of the Invention

[0005] This application provides a functional electrical stimulation ankle-foot orthosis based on 4D printing and multi-sensor fusion, and its preparation method, aiming to solve the problems of poor fit, easy skin compression, and incorrect timing of electrical stimulation caused by inaccurate gait recognition in the prior art.

[0006] In a first aspect, embodiments of this application provide a functional electrical stimulation ankle-foot orthosis based on 4D printing and multi-sensor fusion, comprising: A 4D-printed orthotic body configured to wrap around and support the user's ankle and foot area; At least a portion of the 4D-printed orthotic body comprises a shape memory polymer-based composite material configured to change shape upon receiving a preset stimulus in order to conform to the user’s body contours and avoid bony prominences. A multi-sensor module is installed on the main body of the 4D printed orthosis to collect gait data of the user in real time during walking. A functional electrical stimulation module disposed on the main body of the 4D printed orthosis is used to output electrical stimulation signals to the user's anterior tibialis muscle; A main control module is connected to the multi-sensor module and the functional electrical stimulation module. The main control module is configured to identify the user's gait cycle based on the gait data collected by the multi-sensor module, and when it is identified that the user's gait cycle is in the early support stage or the middle and late swing stage, it controls the functional electrical stimulation module to output an electrical stimulation signal to stimulate the tibialis anterior muscle to contract and assist in ankle dorsiflexion.

[0007] In one possible implementation of the first aspect, the shape memory polymer-based composite material is a thermoresponsive shape memory polymer, and the preset stimulus is human body temperature; When the ankle-foot orthosis is worn on the human body, the 4D-printed orthosis body undergoes an adaptive shape change under the stimulation of the body temperature to conform to the user's ankle and foot contours and avoid bony prominences.

[0008] In one possible implementation of the first aspect, the multi-sensor module includes a pressure sensor array and a gravity orientation sensor; The pressure sensor array is a flexible piezoresistive sensor array. The pressure sensor array is embedded in the sole area of ​​the 4D printed orthotic body. The pressure sensor array is divided into a forefoot pressure acquisition area and a hindfoot pressure acquisition area according to the anatomical structure of the human foot. The forefoot pressure acquisition area and the hindfoot pressure acquisition area are respectively provided with multiple sensor units. The gravity orientation sensor is a MEMS triaxial accelerometer, which is fixed to the outside of the ankle of the 4D printed orthotic body.

[0009] In one possible implementation of the first aspect, the main control module is used to determine the foot landing state based on the pressure distribution data collected by the pressure sensor array, and to determine the ankle flexion and extension state based on the posture data collected by the gravity direction sensor. The pressure distribution data and the posture data are fused and compared to identify the user's gait cycle. The gait cycle includes at least the heel strike phase, mid-stance phase, heel lift-off phase, and swing phase; The main control module is configured to control the functional electrical stimulation module to output electrical stimulation signals when it detects that the user is in the initial stage of support or the middle and late stage of swing.

[0010] In one possible implementation of the first aspect, the main control module has a built-in electrical stimulation calibration module; The electrical stimulation calibration module is configured to: control the functional electrical stimulation module to output an electrical stimulation signal with increasing intensity, collect the plantar pressure change caused by the electrical stimulation signal through the pressure sensor array, and when the plantar pressure change reaches a preset threshold, determine the electrical stimulation intensity corresponding to the plantar pressure change as the effective stimulation intensity. The main control module, based on the effective stimulation intensity, controls the functional electrical stimulation module to output an electrical stimulation signal when it detects that the user's gait cycle is in the early support phase or the middle to late swing phase.

[0011] In one possible implementation of the first aspect, the 4D-printed orthotic body includes an ankle joint hinge, which is disposed at a position corresponding to the ankle joint of the 4D-printed orthotic body to provide dorsiflexion assistance. The ankle joint hinge includes a hinge shaft, an elastic adjustment element sleeved on the outside of the hinge shaft, and a locking knob mechanically coupled to the elastic adjustment element. By rotating the locking knob, the preload of the elastic adjustment element is adjusted to achieve graded adjustment of the dorsiflexion assistance. When adjusted to the no-assist state, the ankle joint hinge does not provide dorsiflexion assistance.

[0012] In one possible implementation of the first aspect, the functional electrical stimulation module includes electrode pads and a stimulation intensity adjustment unit; The electrode sheet is a medical flexible conductive silicone electrode, which is fixed to the 4D printed orthopedic body at the position corresponding to the tibialis anterior muscle of the human body; The stimulation intensity adjustment unit is electrically connected to the main control module, and the stimulation intensity adjustment unit is used to adjust the intensity, frequency and pulse width of the electrical stimulation signal according to the instructions of the main control module.

[0013] In one possible implementation of the first aspect, the 4D-printed orthopedic body is provided with an elastic adjustment strap for adjusting the tightness of the fit. The main control module also includes a wireless communication module, which is configured to establish a communication connection with a mobile terminal to realize real-time display and remote adjustment of gait data and electrical stimulation parameters.

[0014] In one possible implementation of the first aspect, the shape memory polymer-based composite material is made of polycaprolactone, polylactic acid and hydroxyapatite, wherein the mass ratio of polycaprolactone to polylactic acid is 7:3, the amount of hydroxyapatite added is 5%-8% of the total mass of the shape memory polymer-based composite material, and the shape memory response temperature of the shape memory polymer-based composite material is 20-70℃.

[0015] Secondly, embodiments of this application provide a method for fabricating a functional electrical stimulation ankle-foot orthosis based on 4D printing and multi-sensor fusion, including: Using 4D printing technology and shape memory polymer-based composite materials, a 4D printed orthotic body is prepared by integral molding or by printing and assembling parts separately, and a chamber is formed inside the 4D printed orthotic body to accommodate a multi-sensor module, a functional electrical stimulation module and a main control module. The multi-sensor module, functional electrical stimulation module, and main control module are assembled into the 4D-printed orthotic body to obtain the ankle-foot orthotic.

[0016] The beneficial effects of this application's embodiments compared to existing technologies are as follows: By employing a 4D-printed shape memory polymer-based composite material body, the orthosis can adaptively change shape under human body temperature stimulation, achieving precise fit with the user's limbs, effectively avoiding bony prominences, thereby improving wearing comfort and reducing the risk of pressure sores caused by prolonged wear. By integrating data from two different dimensions—plantar pressure and ankle posture—high-precision recognition of the gait cycle is achieved. Based on this, precise triggering of electrical stimulation at two key time points—the initial support phase and the mid-to-late swing phase—effectively corrects two typical abnormal gaits: toe strike and toe dragging, with a correction effect superior to existing technologies relying on a single sensor. Through a built-in electrical stimulation calibration module, personalized effective stimulation intensity can be automatically calibrated based on feedback from changes in plantar pressure induced by electrical stimulation. Simultaneously, the adjustable ankle joint hinge provides mechanical assistance ranging from strong support to no assistance. The combination of these two features allows the orthosis to adapt to the dynamic needs of different users and the same user at different rehabilitation stages. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A schematic diagram of the overall structure of the functional electrical stimulation ankle-foot orthosis provided in the embodiments of this application; Figure 2 for Figure 1 A schematic diagram of the layout of the pressure sensor array in the orthodontic device shown. Figure 3 A flowchart illustrating the workflow of the ankle-foot orthosis provided in this application embodiment; Figure 4 A schematic diagram illustrating the correspondence between gait cycles and electrical stimulation triggering timings provided in embodiments of this application; Figure 5 A system structure block diagram of the functional electrical stimulation ankle-foot orthosis provided in the embodiments of this application. Detailed Implementation

[0019] In the following description, specific details such as particular system architectures and techniques are set forth for illustrative purposes and not for limitation, in order to provide a thorough understanding of the embodiments of this application. However, those skilled in the art will understand that this application may also be implemented in other embodiments without these specific details. In other instances, detailed descriptions of well-known systems, apparatuses, circuits, and methods have been omitted so as not to obscure the description of this application with unnecessary detail.

[0020] It should be understood that, when used in this application specification and the appended claims, the term "comprising" indicates the presence of the described features, integrals, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or a collection thereof.

[0021] It should also be understood that the term “and / or” as used in this application specification and the appended claims means any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0022] As used in this application specification and the appended claims, the term "if" may be interpreted, depending on the context, as "when," "once," "in response to determination," or "in response to detection." Similarly, the phrase "if determined" or "if detected [the described condition or event]" may be interpreted, depending on the context, as meaning "once determined," "in response to determination," "once detected [the described condition or event]," or "in response to detection [the described condition or event]."

[0023] Furthermore, in the description of this application and the appended claims, the terms "first," "second," "third," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0024] References to "one embodiment" or "some embodiments" as described in this specification mean that one or more embodiments of this application include a specific feature, structure, or characteristic described in connection with that embodiment. Therefore, the phrases "in one embodiment," "in some embodiments," "in other embodiments," "in still other embodiments," etc., appearing in different parts of this specification do not necessarily refer to the same embodiment, but rather mean "one or more, but not all, embodiments," unless otherwise specifically emphasized. The terms "comprising," "including," "having," and variations thereof mean "including but not limited to," unless otherwise specifically emphasized.

[0025] Before providing a detailed description of the embodiments of this application, for ease of understanding, some key terms and concepts involved in the embodiments of this application will be explained first. The terms and concepts involved in the embodiments of this application are applicable to, but not limited to, the following interpretations.

[0026] 4D-printed orthotic body: refers to the core structural component of an orthosis manufactured using 4D printing technology. Its 3D aspect refers to its three-dimensional spatial form customized based on individual user data, while the fourth dimension refers to its ability to change its shape or function over time (under specific stimuli). At least a portion of this body contains shape memory material, enabling it to recover from a temporary shape (set during printing or post-processing) to its pre-programmed permanent shape under specific physical or chemical stimuli (such as temperature, light, humidity, electric field, magnetic field, etc.). In a preferred embodiment of this application, this body is used to wrap around the user's ankle and foot area. Its core function is to utilize the shape memory effect to achieve dynamic self-adaptation from passive fitting to active conformation, providing support that combines comfort and stability.

[0027] A gait cycle refers to a complete cycle of walking motion, typically defined as the period from the initial contact of one foot's heel to the second contact of the same foot. A typical gait cycle can be subdivided into a stance phase (approximately 60%) and a swing phase (approximately 40%). The stance phase can be further subdivided into heel strike, mid-stance (full foot contact), heel lift-off, and toe lift-off; the swing phase can be subdivided into early swing, mid-swing, and late swing. Accurate identification and staging of the gait cycle is a fundamental prerequisite for timely intervention of functional electrical stimulation to correct abnormal gait.

[0028] Effective Stimulation Intensity refers to the minimum combination of electrical stimulation parameters (including intensity, frequency, pulse width, etc.) that can induce effective contraction of the target muscle (primarily the tibialis anterior muscle in this application) sufficient to alter relevant biomechanical parameters (such as plantar pressure distribution, ankle angle, etc.). This parameter is highly individualized and depends on various factors such as the user's muscle condition, degree of nerve damage, and skin impedance. Determining and applying this parameter is crucial for achieving personalized and precise rehabilitation, aiming to avoid treatment ineffectiveness due to insufficient stimulation or pain, discomfort, or even muscle fatigue caused by excessive stimulation.

[0029] Dorsiflexion assist refers to the mechanical force, damping force, or torque provided by an orthosis to assist the ankle joint in dorsiflexion (i.e., the toes are pointed towards the lower leg). This assist aims to compensate for foot drop caused by weakness in the dorsiflexor muscles such as the tibialis anterior, helping the user lift the toes during the swing phase of the gait to obtain sufficient ground clearance and achieve a more stable heel strike in the initial stage of support.

[0030] Please see Figure 1 and Figure 5 This application provides a functional electrical stimulation ankle-foot orthosis based on 4D printing and multi-sensor fusion. Existing ankle-foot orthoses (AFOs) generally suffer from a series of technical problems, including poor fit, discomfort leading to low compliance, insufficient robustness due to reliance on a single sensor for gait recognition, inaccurate timing of electrical stimulation resulting in poor rehabilitation effects, and a lack of personalized, adaptive adjustment capabilities to adapt to the user's rehabilitation process. The ankle-foot orthosis proposed in this application, through a highly systematic integration of advanced 4D printing materials science, multi-source heterogeneous sensor fusion technology, closed-loop feedback functional electrical stimulation (FES), and adjustable mechanical assistance, aims to provide foot drop users with an efficient, comfortable, intelligent, and highly personalized comprehensive solution for gait correction and neuromuscular function rehabilitation.

[0031] Specifically, this application provides an ankle-foot orthosis comprising a 4D-printed orthotic body 2. The orthotic body 2 is designed to wrap around the user's ankle and foot, providing necessary structural support and stability. Its function is to provide external fixation for an unstable ankle joint due to nerve damage, preventing accidental sprains caused by inversion or eversion during walking, and providing a stable mechanical platform for subsequent functional electrical stimulation and mechanical assistance. Unlike traditional orthoses made by hand using plaster or thermoplastic sheets, this application employs 4D printing technology, enabling reverse engineering and personalized design using three-dimensional scan data of the user's affected limb (such as point clouds or mesh models obtained through laser scanning or structured light scanning). This ensures that the initial shape of the 4D-printed orthotic body 2 achieves a millimeter-level high match with the user's limb contour, fundamentally solving the problems of poor fit and excessive local pressure caused by the uniform size or large errors in handmade traditional orthoses.

[0032] To further enhance wearing comfort while maintaining a high degree of fit, at least a portion of the 4D-printed orthotic body 2, particularly the areas in contact with soft tissues and bony prominences (such as the medial and lateral malleoli, heel, and navicular bone), comprises a shape memory polymer-based composite material. This composite material is pre-programmed to undergo controllable shape changes upon receiving a preset stimulus (e.g., body temperature), thereby dynamically and actively conforming to the user's body contours and providing fine-tuning to avoid or enclose bony prominences. The principle behind this design lies in utilizing the shape memory effect of the material: the orthotic maintains a slightly loose, temporary shape at room temperature; when worn, under the continuous influence of body temperature, the material is activated and gradually deforms into a final working form that perfectly conforms to the user's limbs with even pressure distribution. This paradigm shift from passive adaptation to active fit alleviates the localized high pressure and shear forces exerted on the skin and soft tissues by rigid structures, thereby reducing the risk of skin redness, damage, or even pressure sores caused by prolonged wear.

[0033] To accurately and in real-time perceive the user's walking intentions and state, the orthosis integrates a multi-sensor module for continuously collecting multi-dimensional gait data during the user's walking process. This multi-sensor module acts as the sensory core of the entire intelligent control system, providing rich and reliable input information for the decision-making algorithm of the main control module 1. This application differs from existing technologies that rely solely on a single inertial sensor or a single pressure sensor, employing a multi-source information fusion strategy. The underlying logic is that different types of sensors have varying sensitivities to gait events and are complementary. Through fusion analysis, the signal drift, misjudgment, or missed judgment problems that easily occur with single sensors in complex walking environments (such as slopes, turns, and uneven ground) or when gait variability is high can be effectively overcome, thereby significantly improving the accuracy and robustness of gait recognition.

[0034] To provide proactive and effective functional intervention for abnormal gait, the orthosis also incorporates a functional electrical stimulation module 3, with electrodes precisely positioned on the surface of the user's tibialis anterior muscle. The tibialis anterior is a key muscle controlling ankle dorsiflexion and foot inversion. Damage to upper motor neurons often prevents this muscle from receiving correct instructions from the brain, resulting in muscle weakness or spasticity, leading to the typical foot drop gait. The purpose of this design is to use the functional electrical stimulation module 3 to externally simulate normal nerve impulses from the central nervous system, transmitting precisely controlled electrical pulses to the motor nerves via surface electrodes. This induces functional contraction of the tibialis anterior muscle during key phases of the gait cycle. This contraction actively assists the user in completing ankle dorsiflexion movements, directly correcting abnormal gait. More importantly, through repeated functional exercise training, it helps promote the plasticity remodeling of neural pathways and the recovery of damaged motor function.

[0035] The core scheduling of all the aforementioned perception, decision-making, and execution modules is accomplished by an embedded main control module 1 (e.g., based on a microcontroller (MCU) or system-on-a-chip (SoC). Optionally, the main control module 1 is fixed inside a control box on the outer side of the lower leg of the orthotic body 2. This main control module 1 communicates and controls the multi-sensor module and the functional electrical stimulation module 3 at high speed. The core task of the main control module 1 is to receive and process gait data streams from the multi-sensor module in real time, accurately identify the user's current gait cycle and its specific stage using a built-in gait recognition algorithm (such as a machine learning model); then, based on preset rehabilitation logic, when it detects that the user's gait cycle has entered a specific target stage, such as the initial support stage requiring heel strike or the mid-to-late swing stage requiring foot lifting to prevent dragging, it immediately sends a command to the functional electrical stimulation module 3 to control it to output an electrical stimulation signal with specific parameters. Through the above design, the application of electrical stimulation is precisely synchronized with the natural movement rhythm of the human body. Triggering stimulation during the initial support phase helps achieve a stable heel-toe rolling landing, reducing foot-slapping. Triggering stimulation during the later stages of the swing ensures sufficient toe clearance, preventing tripping risks caused by toes dragging on the ground. In this way, an improvement is achieved from the traditional blind, open-loop stimulation of FES to precise, closed-loop, and synchronous intervention.

[0036] Furthermore, in an optional embodiment, to maximize the product's convenience and practicality, the shape memory polymer-based composite material is designed as a thermoresponsive shape memory polymer, and its shape memory transition temperature (T_trans) is set near human body temperature, for example, 35-40°C. This means that when the ankle-foot orthosis is worn, no external heating gun, hot water, or other auxiliary equipment is needed. The body's own continuous heat is used as a natural, gentle, and ubiquitous stimulus, automatically triggering the 4D-printed orthosis body 2 to undergo a preset shape adaptive change within minutes. This wear-and-adapt design simplifies the user's operation process, lowers the barrier to entry, and is particularly suitable for home rehabilitation scenarios. By utilizing human body temperature as a natural and continuous stimulus, the 4D-printed orthosis body 2 can closely conform to the user's ankle and foot contours and continuously and dynamically avoid bony prominences, ensuring excellent comfort during prolonged wear. This solves the problem of traditional thermoplastic sheets (such as polypropylene PP sheets) requiring repeated heating and shaping, resulting in a completely fixed shape after cooling and an inability to dynamically adapt.

[0037] In another alternative embodiment, to achieve an optimal balance between biocompatibility, mechanical properties, and shape memory effect, the shape memory polymer-based composite material is prepared by melt blending or other methods using polycaprolactone (PCL), polylactic acid (PLA), and nano-hydroxyapatite (n-HA). Specifically, the mass ratio of PCL (as the shape memory matrix) to PLA (as the reinforcing phase and stiffness modifier) ​​can be 7:3 to obtain suitable transition temperature and mechanical strength. The amount of nano-hydroxyapatite (n-HA) added as a bioactive filler can be 5%-8% of the total mass of the shape memory polymer-based composite material. Furthermore, the shape memory response temperature range of this composite material is set between 20-70°C, and its glass transition temperature (Tg) is designed to be near human body temperature. The design principle of this formulation is based on the following: PCL possesses excellent shape memory properties, good biodegradability and biocompatibility, and has a low melting point (approximately 60°C), making it easy to process and achieve low-temperature thermal response; PLA enhances the material's Young's modulus and overall stiffness, ensuring the orthosis provides sufficient support; and the addition of nano-hydroxyapatite (n-HA) not only further enhances the material's mechanical properties but also improves its bioactivity and compatibility with human tissues, reducing immune rejection. Through precise control of the component ratios, molecular weights, and processing techniques, the composite material can maintain a stable temporary shape at room temperature while responding rapidly and reliably upon contact with the human body, possessing sufficient mechanical strength to provide effective gait correction force, thus highly matching the material's performance with complex rehabilitation needs.

[0038] Furthermore, to achieve high-precision and robust gait data acquisition, the multi-sensor module specifically includes a pressure sensor array 4a and a gravity direction sensor 4b, which is also called an inertial measurement unit (IMU). The pressure sensor array 4a, for example, can be a flexible piezoresistive or capacitive sensor array, seamlessly embedded or integrated into the plantar region of the 4D-printed orthotic body 2, or integrated into a replaceable insole. To precisely capture the spatiotemporal distribution characteristics of the center of pressure (CoP) during gait, the array is laid out according to the key anatomical weight-bearing areas of the human foot, divided into a hindfoot (calcaneus) pressure acquisition area, a midfoot (arch) pressure acquisition area, and a forefoot (metatarsal head) pressure acquisition area, each area equipped with multiple independent sensor units. The gravity direction sensor 4b can specifically employ a MEMS chip integrating a three-axis accelerometer, a three-axis gyroscope, and an optional three-axis magnetometer, and is securely fixed to the lateral side of the lower leg segment of the 4D-printed orthotic body 2. This heterogeneous sensor layout design, which combines plantar pressure and lower leg posture, aims to simultaneously acquire information from two completely different physical dimensions: ground reaction force and lower limb kinematic posture. This provides a rich, redundant, and complementary data source for subsequent data fusion algorithms.

[0039] Furthermore, in an optional implementation, the gait recognition algorithm deployed within the main control module 1 performs fusion processing on the aforementioned multi-source data. Specifically, the main control module 1 uses pressure distribution data (e.g., total pressure in each region, CoP position) collected by the pressure sensor array 4a to highly sensitively detect gait events related to ground contact, such as heel strike, full foot strike, and toe lift. Simultaneously, based on angular velocity and acceleration data collected by the gravity direction sensor 4b, it calculates the attitude angles (pitch, roll, yaw) and motion state of the lower leg and ankle joint in three-dimensional space. Subsequently, the main control module 1 employs a fusion algorithm (e.g., Kalman filter, decision tree, or trained machine learning model) to perform weighted fusion and logical comparison of the data from these two sources to accurately identify the user's gait cycle and its sub-phases. For example, when the pressure signal in the hindfoot pressure acquisition area shows a steep upward peak, and simultaneously the gravity direction sensor 4b detects that the angular velocity of the lower leg in the sagittal plane changes from positive to negative (i.e., forward swing deceleration), it can be determined with high confidence that a heel strike event has occurred. This fusion comparison method can effectively filter out noise and interference caused by factors such as signal drift, changes in ground material, and sudden changes in walking speed from a single sensor, thereby greatly improving the accuracy of gait event detection and the robustness of overall gait staging, and ensuring the precise timing of electrical stimulation triggering.

[0040] In another alternative implementation, to achieve personalized electrical stimulation therapy, the main control module 1 incorporates an automated electrical stimulation calibration module. This calibration module is configured to perform a closed-loop dose-response calibration process before or periodically before treatment: First, with the user seated or standing, the functional electrical stimulation module 3 outputs a scanning electrical stimulation signal whose intensity (e.g., current amplitude or pulse width) increases linearly or stepwise over time. Simultaneously, the pressure sensor array 4a in the multi-sensor module collects and monitors subtle changes in plantar pressure distribution induced by this electrical stimulation in real time. This pressure change is a direct biomechanical manifestation of the dorsiflexion torque generated in the foot due to tibialis anterior muscle contraction. Then, the system algorithm continuously determines whether the amount of pressure change (e.g., decrease in forefoot pressure or CoP posterior displacement distance) reaches a preset biomechanical threshold representing effective functional contraction. Once the pressure change reaches this threshold, the system immediately records the corresponding electrical stimulation intensity parameter and determines it as the effective stimulation intensity for the user in the current state. During subsequent gait correction, the main control module 1 uses this individually calibrated effective stimulation intensity as a benchmark to output electrical stimulation signals. In this way, the electrical stimulation intensity is transformed from being estimated by the doctor's experience or a general setting to being customized precisely based on biomechanical feedback, ensuring that every stimulation is safe and effective, while avoiding discomfort caused by excessive stimulation intensity or ineffective treatment caused by insufficient stimulation intensity.

[0041] In an alternative implementation, to provide more comprehensive rehabilitation support, the 4D-printed orthotic body 2 also integrates an ankle hinge 2b. This ankle hinge 2b is precisely positioned corresponding to the anatomical ankle axis of the 4D-printed orthotic body 2 to provide adjustable, passive dorsiflexion assistance. Specifically, the ankle hinge 2b may include a hinge axis, an elastic adjustment element (such as a torsion spring, elastic band, or pneumatic / hydraulic cylinder) sleeved on the outside of the hinge axis, and a locking knob or regulating valve mechanically coupled to the elastic adjustment element. The user or therapist can rotate the locking knob to change the preload or damping coefficient of the elastic adjustment element, thereby achieving multi-level or stepless adjustment of the dorsiflexion assistance or resistance. In particular, the assistance can be adjusted to a zero-assistance state, allowing free movement of the ankle joint. The purpose of this design is to provide a hybrid, synergistic rehabilitation mode combining active electrical stimulation and passive mechanical assistance. In the early stages of rehabilitation, when the user's muscle strength is extremely weak, the mechanical assistance can be increased to provide sufficient support and ensure safe walking. As rehabilitation progresses and the user's muscle strength gradually recovers, the mechanical assistance can be gradually reduced to encourage the user to exert more effort actively and promote neuromuscular relearning. Ultimately, the assistance can be reduced to a state without assistance, allowing the user to walk entirely by relying on functional electrical stimulation and their own recovered muscle strength. This design greatly expands the product's applicable period and user group, seamlessly connecting the user's needs throughout the entire process from passive to active rehabilitation.

[0042] Furthermore, the functional electrical stimulation module 3 may specifically include one or more pairs of electrode pads 3a and a high-precision stimulation intensity adjustment unit ( Figure 1 (Not shown in the image). The electrode 3a can be a medical-grade flexible conductive silicone electrode or a fabric electrode. Its shape and size are optimized and it is securely fixed or integrated into the inner lining of the 4D-printed orthotic body 2 at the position corresponding to the movement point of the tibialis anterior muscle, ensuring good contact with the skin, low contact impedance, and uniform current density distribution. The stimulation intensity adjustment unit is electrically connected to the main control module 1. It is a constant current source or constant voltage source circuit that can precisely and dynamically adjust the intensity (current / voltage), frequency (Hz), pulse width (μs), and waveform (such as symmetrical / asymmetrical biphasic square wave) of the electrical stimulation signal output to the electrode according to the digital instructions of the main control module 1. This multi-parameter adjustable design allows the rehabilitation therapist to finely configure the electrical stimulation prescription according to the user's specific situation and treatment plan to achieve the best neuromuscular stimulation effect and the most comfortable treatment experience.

[0043] In another optional implementation, to enhance the product's intelligent interactive experience and remote management capabilities, the 4D-printed orthotic body 2 is equipped with elastic adjustment straps (such as elastic adjustment straps with Velcro 2a) for easy donning, doffing, and tightness adjustment. Simultaneously, the main control module 1 integrates a low-power wireless communication module, such as a Bluetooth Low Energy (BLE) module or a Wi-Fi module. This wireless communication module is configured to establish a bidirectional communication connection with an external mobile terminal (such as a smartphone, tablet, or dedicated handheld device). Through this connection, the user's real-time gait data (such as cadence, gait speed, gait symmetry index, plantar pressure map, etc.) and electrical stimulation parameters can be transmitted in real time and graphically displayed on an application (APP) running on the mobile terminal. More importantly, the APP also allows the user (within safe limits) or an authorized rehabilitation therapist to remotely adjust parameters, start / stop treatment, and set rehabilitation goals. This not only greatly facilitates daily monitoring and self-management of the rehabilitation process but also provides a powerful technical platform for cloud data storage, big data analysis, and remote rehabilitation guidance.

[0044] This application also provides a method for fabricating a functional electrical stimulation ankle-foot orthosis based on 4D printing and multi-sensor fusion. The specific process includes: First, using 4D printing technology (such as fused deposition modeling (FDM), selective laser sintering (SLS), etc.) and the aforementioned shape memory polymer-based composite materials, based on the user's personalized 3D digital model, the 4D printed orthopedic body 2 is prepared through integrated printing or separate printing followed by assembly. During the 3D modeling and printing path planning stage, precision chambers, wiring channels, fixing clips, and mounting interfaces for accommodating electronic components such as the multi-sensor module, functional electrical stimulation module 3, and main control module 1 are pre-designed and printed within the main structure.

[0045] Subsequently, the multi-sensor module, functional electrical stimulation module 3, main control module 1, power supply module and other electronic components are precisely assembled into the preset positions of the 4D printed orthotics body 2 by embedding, snapping, bonding and other methods, and the electrical connection between each component is completed according to the preset wiring channels.

[0046] Finally, after system testing and functional verification, the finished ankle-foot orthosis was obtained.

[0047] This fabrication method integrates the housing structure of electronic components with the orthodontic body through integrated design and printing, simplifying the cumbersome post-processing of slotting, wiring, and packaging of traditional orthodontics. It improves the product's integration, aesthetics, and production efficiency, while ensuring consistent product performance and long-term reliability.

[0048] In one specific embodiment, please refer to Figures 1 to 4 A functional electrical stimulation ankle-foot orthosis is disclosed. The overall structure of the orthosis is as follows: Figure 1 As shown, its system architecture includes a main control module 1 acting as the brain, a 4D-printed orthotic body 2 acting as the skeleton, a functional electrical stimulation module 3 acting as an active intervention device for muscles, a multi-sensor module acting as the senses (specifically including a pressure sensor array 4a and a gravity direction sensor 4b), and a power supply module (such as a rechargeable lithium battery) providing energy to the entire system. The 4D-printed orthotic body 2 uses the aforementioned PCL / PLA / HA composite material, which is personalized through CAD modeling and 4D printing after a three-dimensional laser scan of the user's affected limb. Its inner surface morphology closely matches the user's ankle and foot contour, and at bony prominences prone to pressure, such as the medial and lateral malleoli and the navicular bone, micron-level avoidance depressions are automatically generated by algorithms or filled with softer materials. The 4D-printed orthotic body 2 is equipped with elastic adjustable straps with Velcro for easy adjustment of the wearing tightness. An adjustable spring preload ankle hinge 2b is integrated at the anatomical ankle joint axis.

[0049] In this embodiment, the pressure sensor array 4a of the multi-sensor module is integrated into a removable insole and placed on the sole of the 4D-printed orthotic body 2. Its specific layout is as follows: Figure 2 As shown, the system is clearly divided into three pressure acquisition zones: forefoot, midfoot, and hindfoot. The forefoot zone has 10 densely distributed sensor units to monitor metatarsal head pressure, while the hindfoot zone has 8 sensor units to monitor calcaneal impact. This allows for precise capture of the anteroposterior and lateral migration trajectory of the plantar pressure center (CoP) during gait. The gravity direction sensor 4b is securely encapsulated and fixed to the lateral aspect of the lower leg, approximately 8 cm above the lateral malleolus, of the 4D-printed orthotic body 2.

[0050] The electrode pads 3a of the functional electrical stimulation module 3 are integrated into the lining of the lower leg portion of the orthosis, their positions anatomically calibrated to precisely correspond to the muscle belly movement point of the tibialis anterior muscle. Once the user wears the orthosis, simply fastening the straps allows the electrode pads 3a to adhere tightly and stably to the skin. The main control module 1 is electrically connected to all electronic modules via flexible printed circuits (FPCs) and is supplied with a stable 3.7V operating power by the power supply module.

[0051] The working process of this embodiment is as follows: Figure 3As shown, the process can be divided into an initial fitting and calibration phase and a daily use and real-time rehabilitation orthosis phase. In the initial fitting phase, the user puts on the orthosis for the first time. Under the influence of body temperature (approximately 37°C), the 4D-printed orthotic body 2 undergoes a preset adaptive deformation within 5-10 minutes, changing from a slightly loose fit to a fully fitted therapeutic state. The therapist sets an initial passive dorsiflexion assist value based on the user's current muscle strength and joint range of motion by rotating the locking knob of the ankle joint hinge 2b. Subsequently, an automatic calibration program for electrical stimulation intensity is initiated via a mobile app: the main control module 1 controls the functional electrical stimulation module 3 to output a test current starting from 0mA and increasing in 0.5mA increments, while the pressure sensor array 4a monitors changes in plantar pressure in real time. When the backward movement of the CoP caused by foot dorsiflexion exceeds a preset threshold (e.g., more than 10mm), the system automatically records the current value (e.g., 15mA) and sets it as the user's personalized effective stimulation intensity. Afterward, the user walks on flat ground for several minutes to collect initial data for training or fine-tuning the gait recognition model built into the main control module 1.

[0052] During daily use, when the user wears the orthosis and walks, the pressure sensor array 4a and the gravity direction sensor 4b continuously collect data at a sampling rate of 100Hz and wirelessly transmit it to the main control module 1. The gait cycle recognition algorithm within the main control module 1 (e.g., a pre-trained support vector machine (SVM) or lightweight neural network model) performs real-time analysis on the fused data, accurately identifying the gait flow as continuous gait events and phases. Figure 4 As shown in the figure, the triggering logic of the electrical stimulation is clearly illustrated: when the algorithm detects that the gait has entered the initial support phase (i.e., a steep peak appears in the heel pressure signal), the main control module 1 immediately triggers the functional electrical stimulation module 3, outputting a 100ms pulse sequence with a calibrated effective stimulation intensity to stimulate isometric contraction of the tibialis anterior muscle to control the foot's smooth landing and prevent foot slapping. Subsequently, the electrical stimulation remains off during the mid and late support phases. When the gait enters the mid-to-late swing phase (the IMU detects that the lower leg has crossed the body's vertical line and begins to accelerate forward), the main control module 1 triggers the functional electrical stimulation module 3 again, stimulating concentric contraction of the tibialis anterior muscle to raise the toes to a neutral or slightly dorsiflexed position, ensuring sufficient ground clearance between the toes and the ground, effectively preventing dragging and tripping. Through this precise electrical stimulation, which is highly synchronized with the gait cycle and supplied on demand, combined with the continuous passive assistance provided by the ankle hinge 2b, the two work synergistically to effectively correct foot drop gait and provide repetitive functional exercise training.

[0053] In another specific embodiment, the structure of the ankle hinge 2b is optimized. Instead of the spring-assisted mechanism in the above embodiments, the ankle hinge 2b in this embodiment employs a damping adjustment mechanism based on a viscous fluid or magnetorheological fluid. Internally, it consists of a sealed cavity, a piston, and a special fluid. The size of the throttling orifice for fluid flow or (for magnetorheological fluids) the applied magnetic field strength can be changed via an external electronic valve or adjustment knob. In this way, the damping torque generated during hinge rotation can be steplessly and smoothly adjusted. This design not only provides dorsiflexion assistance but also provides controllable resistance in the plantarflexion direction, simulating eccentric contraction training. It allows for more precise control of the ankle joint's movement speed and range of motion, making it particularly suitable for rehabilitation phases requiring flexible restriction of joint movement or advanced resistance training.

[0054] In another optional embodiment, the gait cycle recognition algorithm was upgraded. The algorithm deployed in the main control module 1 was upgraded from a traditional Support Vector Machine (SVM) model to a deep learning model based on Long Short-Term Memory (LSTM) networks. Compared to models such as SVM that only focus on features at the current time point, LSTM networks, as recurrent neural networks, are better able to learn and remember long-term temporal dependencies in gait data streams. In the initial adaptation phase, it is necessary to collect walking data from users over a longer period (e.g., 2-5 minutes) at different speeds and road conditions as training samples. After offline training of the LSTM model is completed on a cloud server or host computer, the trained lightweight model parameters are downloaded and deployed to the main control module 1. Thanks to the powerful ability of LSTM to capture complex temporal features, this embodiment significantly improves the accuracy and robustness of gait recognition in challenging scenarios such as unstable gait, large changes in walking speed, or walking on uneven surfaces, thereby ensuring the continuous accuracy of electrical stimulation triggering.

[0055] Furthermore, in an optional implementation, the calibration method for electrical stimulation intensity is improved to achieve higher physiological relevance. A pair of dry surface electromyography (sEMG) sensors are additionally integrated next to the electrode pads 3a of the functional electrical stimulation module 3 for directly acquiring electromyographic activity signals from the tibialis anterior muscle. During the electrical stimulation intensity calibration phase, the main control module 1 outputs incremental electrical stimulation pulses, while the sEMG sensors monitor the muscle compound action potentials (M-waves) generated by the tibialis anterior muscle in real time. When the peak value of the monitored M-wave reaches a preset electrophysiological response threshold representing a sufficient number of motor units recruited by the muscle (e.g., reaching 30% of the maximum M-wave amplitude), the main control module 1 records the current electrical stimulation intensity as the personalized effective stimulation intensity. Since electromyographic signals are the most direct and upstream electrophysiological response to muscle contraction, compared to indirect biomechanical parameters influenced by multiple factors (such as plantar pressure), this EMG feedback-based closed-loop calibration method is more direct, sensitive, and accurate, and can more realistically reflect the neuromuscular excitation state.

[0056] In summary, the embodiments of this application provide a highly intelligent, personalized, and comfortable new generation of ankle-foot orthotics by deeply and organically integrating a variety of cutting-edge technologies such as 4D printed adaptive structure, multi-sensor fusion gait recognition, personalized closed-loop calibration electrical stimulation, and adjustable mechanical assistance.

[0057] Those skilled in the art should understand that the above embodiments are merely optional embodiments of this application and are not intended to limit the scope of this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A functional electrical stimulation ankle-foot orthosis based on 4D printing and multi-sensor fusion, characterized in that, include: A 4D-printed orthotic body configured to wrap around and support the user's ankle and foot area; At least a portion of the 4D-printed orthotic body comprises a shape memory polymer-based composite material configured to change shape upon receiving a preset stimulus in order to conform to the user’s body contours and avoid bony prominences. A multi-sensor module is installed on the main body of the 4D printed orthosis to collect gait data of the user in real time during walking. A functional electrical stimulation module disposed on the main body of the 4D printed orthosis is used to output electrical stimulation signals to the user's anterior tibialis muscle; A main control module is connected to the multi-sensor module and the functional electrical stimulation module. The main control module is configured to identify the user's gait cycle based on the gait data collected by the multi-sensor module, and when it is identified that the user's gait cycle is in the early support stage or the middle and late swing stage, it controls the functional electrical stimulation module to output an electrical stimulation signal to stimulate the tibialis anterior muscle to contract and assist in ankle dorsiflexion.

2. The ankle-foot orthosis according to claim 1, characterized in that, The shape memory polymer-based composite material is a thermoresponsive shape memory polymer, and the preset stimulus is human body temperature; When the ankle-foot orthosis is worn on the human body, the 4D-printed orthosis body undergoes an adaptive shape change under the stimulation of the body temperature to conform to the user's ankle and foot contours and avoid bony prominences.

3. The ankle-foot orthosis according to claim 1, characterized in that, The multi-sensor module includes a pressure sensor array and a gravity direction sensor; The pressure sensor array is a flexible piezoresistive sensor array. The pressure sensor array is embedded in the sole area of ​​the 4D printed orthotic body. The pressure sensor array is divided into a forefoot pressure acquisition area and a hindfoot pressure acquisition area according to the anatomical structure of the human foot. The forefoot pressure acquisition area and the hindfoot pressure acquisition area are respectively provided with multiple sensor units. The gravity orientation sensor is a MEMS triaxial accelerometer, which is fixed to the outside of the ankle of the 4D printed orthotic body.

4. The ankle-foot orthosis according to claim 3, characterized in that, The main control module is used to determine the foot landing state based on the pressure distribution data collected by the pressure sensor array, and to determine the ankle flexion and extension state based on the posture data collected by the gravity direction sensor. The pressure distribution data and the posture data are fused and compared to identify the user's gait cycle. The gait cycle includes at least the heel strike phase, mid-stance phase, heel lift-off phase, and swing phase; The main control module is configured to control the functional electrical stimulation module to output electrical stimulation signals when it detects that the user is in the initial stage of support or the middle and late stage of swing.

5. The ankle-foot orthosis according to claim 3, characterized in that, The main control module has a built-in electrical stimulation calibration module. The electrical stimulation calibration module is configured to: control the functional electrical stimulation module to output an electrical stimulation signal with increasing intensity, collect the plantar pressure change caused by the electrical stimulation signal through the pressure sensor array, and when the plantar pressure change reaches a preset threshold, determine the electrical stimulation intensity corresponding to the plantar pressure change as the effective stimulation intensity. The main control module, based on the effective stimulation intensity, controls the functional electrical stimulation module to output an electrical stimulation signal when it detects that the user's gait cycle is in the early support phase or the middle to late swing phase.

6. The ankle-foot orthosis according to claim 1, characterized in that, The 4D-printed orthotic body includes an ankle joint hinge, which is located at the corresponding position of the ankle joint of the 4D-printed orthotic body to provide dorsiflexion assistance. The ankle joint hinge includes a hinge shaft, an elastic adjustment element sleeved on the outside of the hinge shaft, and a locking knob mechanically coupled to the elastic adjustment element. By rotating the locking knob, the preload of the elastic adjustment element is adjusted to achieve graded adjustment of the dorsiflexion assistance. When adjusted to the no-assist state, the ankle joint hinge does not provide dorsiflexion assistance.

7. The ankle-foot orthosis according to claim 1, characterized in that, The functional electrical stimulation module includes electrode pads and a stimulation intensity adjustment unit; The electrode sheet is a medical flexible conductive silicone electrode, which is fixed to the 4D printed orthopedic body at the position corresponding to the tibialis anterior muscle of the human body; The stimulation intensity adjustment unit is electrically connected to the main control module, and the stimulation intensity adjustment unit is used to adjust the intensity, frequency and pulse width of the electrical stimulation signal according to the instructions of the main control module.

8. The ankle-foot orthosis according to claim 1, characterized in that, The 4D-printed orthotics body is equipped with an elastic adjustment strap, which is used to adjust the tightness of the fit. The main control module also includes a wireless communication module, which is configured to establish a communication connection with a mobile terminal to realize real-time display and remote adjustment of gait data and electrical stimulation parameters.

9. The ankle-foot orthosis according to claim 1, characterized in that, The shape memory polymer-based composite material is made of polycaprolactone, polylactic acid and hydroxyapatite, wherein the mass ratio of polycaprolactone to polylactic acid is 7:3, the amount of hydroxyapatite added is 5%-8% of the total mass of the shape memory polymer-based composite material, and the shape memory response temperature of the shape memory polymer-based composite material is 20-70℃.

10. A method for fabricating a functional electrical stimulation ankle-foot orthosis based on 4D printing and multi-sensor fusion, characterized in that, include: Using 4D printing technology and shape memory polymer-based composite materials, a 4D printed orthotic body is prepared by integral molding or by printing and assembling parts separately, and a chamber is formed inside the 4D printed orthotic body to accommodate a multi-sensor module, a functional electrical stimulation module and a main control module. The multi-sensor module, functional electrical stimulation module, and main control module are assembled into the 4D-printed orthotic body to obtain the ankle-foot orthotic.

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