Assessing PET composite prosthetic solutions: A step towards inclusive healthcare

Abstract

The demand for affordable prostheses is particularly high in Low Middle-Income Countries (LMICs). Currently, sockets are predominantly manufactured using monolithic thermoplastic polymers, which lack durability and strength, or consumptive thermoset resin reinforcing with expensive composite fillers like carbon, glass, or Kevlar fibers. However, there exist unmet and demanding needs among amputees for procuring low-cost, high-strength, and faster socket manufacturing methods. We evaluate a socket made from a novel manufacturing technique utilizing an affordable and sustainable composite material called commingled PET (polyethylene terephthalate) yarn, along with a reusable vacuum bag, to produce custom-made sockets in a purpose-built curing oven. Our innovative fabrication methodology enables the production of complex-shaped patient sockets in under 4 h. To evaluate the efficacy and performance of the PET sockets, we conducted trials with both unilateral and bilateral amputees over a six-month period, in collaboration with Bhagwan Mahaveer Viklang Sahayata Samiti (BMVSS) in India. Utilizing a 6-min walking test, we measured various gait parameters, including ground reaction forces and flexion angle, for both unilateral and bilateral amputees.

The gait analysis conducted on amputees using our PET-based sockets demonstrated their ability to engage in daily activities without interruptions, reaffirming the functional efficacy of our approach. By combining self-reinforced PET with our novel fabrication technique, we offer a unique and accessible solution that benefits clinicians and patients alike. This study represents significant progress towards achieving affordable and personalized prostheses that cater to the needs of LMICs.

1. Introduction

In the realm of global healthcare, prosthetic accessibility stands as a daunting challenge, particularly in low and middle-income countries (LMICs). Of particular concern is the rising incidence of diabetes-related amputations, projected to affect over 590 million people globally by 2030 [1,2]. Humanitarian crises exacerbate the demand for prosthetic devices, especially in LMICs where delayed medical care often leads to extensive amputations and heightened need for rehabilitation services. Shockingly, the World Health Organization (WHO) reports that less than 3 % of individuals with disabilities in these regions have access to essential rehabilitation services [3]. This limited access is primarily attributed to the prohibitive costs associated with prosthetic devices and manufacturing technology in LMICs [2]. Consequently, individuals in these regions confront significant hurdles in social integration and daily functioning, thereby affecting their overall well-being and quality of life [4].

At the heart of this issue lies the prosthetic socket(PS), the crucial link connecting an amputee's residual limb with the rest of the prosthesis. Traditionally, methods like thermoforming and composite lamination have been employed for socket fabrication. However, these methods present challenges, particularly in LMICs, due to their cost and reliability limitations. Monolithic thermoplastic polymers such as High-Density Polyethylene (HDPE) or Polypropylene (PP), while cost-effective, lack durability and are prone to cracking, compromising the strength of the prosthetic socket. On the other hand, laminated composites with thermosetting resin reinforced with high-performance fibers offer superior strength but remain financially inaccessible for many LMIC amputees.

Due to high cost of advanced fillers, many studies investigated the use of natural fibres as an alternative material, as well as employing laser scanning and 3D printing for PS manufacturing [5]. Firstly, natural fibres typically possess low strength and are often blended with synthetic fibres to achieve the desired material properties. Secondly, natural fibres tend to absorb moisture more easily than synthetic fibres which causes a change in the shape of the PS [6,7]. Finally, due to less durability, lack of quality, and inconsistency of natural fibre composite, the repair and replacement of PS would be higher. On the other hand, there has been a surge of interest in exploring 3D printing for prosthetic socket manufacturing. While offering advantages such as customization and reduced labor costs, 3D printed sockets often fall short in terms of strength and require additional post-processing. Alternatively, the direct molding method developed by Icelandic company Össur shows promise, utilizing a novel pressure casting technique. The method involves directly laminating the prosthetic socket onto the residual limb using the braided composite fabric and uniformly transferring the resin from the distal end to the proximal end [8]. However, accessibility and affordability remain concerns.

Recognizing the persistent need for faster manufacturing methods with high-strength materials and ease of reshaping, this study, conducted in India, aims to develop an efficient procedure for manufacturing patient-specific sockets. By utilizing a self-reinforced Polyethylene terephthalate (srPET) composite material, the study seeks to create functional and accessible sockets tailored to individual patient needs. The self-reinforced polymers, both reinforcement and matrix from the same polymer family, offer superior advantages by magnifying the interfacial bonding and enabling better load transfer between the fibres and matrix, leading to improved strength and stiffness of the laminate [[9], [10], [11]]. The methodology involves the development of a reusable vacuum bag and an innovative manufacturing approach utilizing woven fabric made from PET commingle yarn. To ensure the quality and performance of the manufactured PET socket, a comparison with standard HDPE sockets is conducted. At BMVSS, High-Density Polyethylene (HDPE) is the standard material used for prosthetic sockets. The amputees participating in the study were using HDPE sockets provided by BMVSS. To evaluate the performance of PET sockets, we used HDPE sockets as the benchmark for comparison, given that HDPE represents the current standard of care (SOC) in this context. Performance evaluation of the novel sockets is carried out using a portable gait setup device during a standardized 6-min walking test, culminating in an assessment of their efficacy and performance compared to standard HDPE sockets. Insights gained from this study aim to inform efforts to enhance prosthetic accessibility and improve the quality of life for individuals in LMICs, with a focus on the Indian context.

2. Methodology

The proposal for the trial of patient-specific sockets utilizing novel self-reinforced PET was endorsed by the Bhagwan Mahaveer Viklang Sahayata Samiti (BMVSS) in India. Ethical approval was obtained from the relevant authorities before proceeding with the trials, ensuring adherence to ethical guidelines and patient safety protocols.

2.1. Ethical considerations

The study's protocol underwent a thorough review and received approval from the De Montfort University Ethical Review Committee (Research Ethics Application Approval: 1920/553). Before participation in the study, informed consent was obtained from the subjects, which also covered the publication of photos and data involving their use of prosthetic sockets.

The proposal for the trial of patient-specific sockets utilizing novel self-reinforced PET was endorsed by the Bhagwan Mahaveer Viklang Sahayata Samiti (BMVSS) in India. Ethical approval was obtained from the relevant authorities before proceeding with the trials, ensuring adherence to ethical guidelines and patient safety protocols.

Patient recruitment for the trials commenced following the receipt of ethical approval. Prior to participation, all recruited patients were provided with detailed information about the trial, including its objectives, procedures, potential risks, and benefits. Informed consent was obtained from each participant, ensuring their voluntary participation and understanding of the study's requirements.

The manufacturing process involved meticulous attention to detail and adherence to established protocols to ensure the safety and efficacy of the sockets. Throughout the manufacturing process, patients' comfort and specific needs were taken into consideration, with adjustments made as necessary. Any concerns or issues raised by the patients during the trial period were promptly addressed by the research team, ensuring optimal patient care and support throughout the duration of the study.

2.2. Patient enrollment

Fifty participants were enrolled in the study, consisting of 43 individuals with unilateral transtibial (TT) amputations and 7 with bilateral TT amputations. Recruitment took place at the Bhagwan Mahaveer Viklang Sahayata Samiti (BMVSS) in India, where eligible individuals were identified and invited to participate in the study. Table 1 provides the descriptive statistics of the amputees participated in the study.

Table 1.

Descriptive statistics of participants with limb amputations.

Parameter	Value
Age (years)
Mean	39.5
Median	37
Standard Deviation	16.86
Gender
Male	42
Female	8
Type of Amputation
Unilateral	43
Bilateral	7
Years Since Amputation (years)
Mean	6.68
Median	2.5
Standard Deviation	8.11
Causes of Amputation
Trauma	34
Vascular	15
Occupation
Unemployed	10
Office Worker	2
Driver	2
Homemaker	6
Student	9
Business	16
Farmer	5

Prior to the commencement of the study, all prospective participants underwent a comprehensive informed consent process. They were provided with detailed information about the study objectives, procedures, potential risks, and benefits. Informed consent forms were presented in a language understood by the participants, ensuring clarity and understanding of the study requirements. Participants voluntarily consented to their involvement in the study and agreed to the use of their data, photos, and videos for publication purposes.

To uphold ethical standards and protect participants' privacy and rights, anonymization of patient data was rigorously implemented. Participants were assigned unique identifiers to ensure confidentiality, and any personally identifiable information was securely stored and accessed only by authorized personnel.

During the screening process, patients with wounds on the residual limb or experiencing significant stump pain, rendering them unable to be fitted with a prosthesis at that time, were excluded from the study. This exclusion criterion aimed to prioritize patient safety and ensure that participants were in suitable condition to receive and evaluate the prosthetic interventions.

All enrolled participants received an information letter outlining the study details and requirements. Additionally, they signed an informed consent form, reaffirming their agreement to participate in the study and consenting to the use of their photos and study data in publications. The informed consent process adhered to ethical guidelines and ensured that participants were fully aware of their rights and responsibilities.

This comprehensive approach to patient recruitment and participation prioritized ethical considerations, ensuring transparency, confidentiality, and respect for participants' autonomy throughout the study duration.

2.3. Patient-specific cast for prosthesis development

To create custom prostheses tailored to each patient's unique anatomy, we embarked on a meticulous process of preparing individualized casts. Following the Patella-Tendon Bearing method, we carefully executed a series of steps to ensure accurate molding and accurate representation of the residual limb.

Beginning with the application of cling film, we meticulously moulded it around the residual limb to delineate regions sensitive to pressure and those tolerant to it, laying the groundwork for capturing the limb's contours and nuances (as depicted in Fig. 1a). Subsequently, we wrapped the limb with a wet plaster of Paris (PoP) bandage, meticulously forming a negative cast that intricately mirrored the limb's shape and dimensions (illustrated in Fig. 1b).

Fig. 1.

Sequential stages in the fabrication process of patient-specific sockets. (a) Biomechanical marking of the stump. (b) Application of a wet plaster of Paris (PoP) bandage to create a negative cast of the limb. (c) Precision rectification procedure on the positive cast by skilled prosthetists. (d) Draping of a thin vacuum bag to secure the cast for further processing. (e) Formation of the cast with three layers of woven Polyethylene terephthalate (PET) fabric. (f) Placement of adaptor and addition of three additional layers to reinforce the socket structure. (g) Draping of the vacuum bag over the assembly. (h) Transfer of the setup to a purpose-built curing oven for consolidation. (i) Top view of the curing oven showcasing the uniform heating process. (j) Final cured patient-specific socket ready for integration with the prosthetic limb. The images shown in Fig. 1d–f, g, i, and j have been adapted from Ref. [17].

Upon completion of the negative cast, a transformative process ensued, converting it into a positive cast. Skilled prosthetists meticulously adjusted the positive cast by strategically adding or removing material in alignment with biomechanical landmarks, meticulously addressing the load-bearing areas of the stump (as demonstrated in Fig. 1c). Through this painstaking modification process, we ensured optimal weight distribution and comfort for each individual, laying the groundwork for the subsequent fabrication of bespoke prosthetic sockets, laying the groundwork for the subsequent fabrication of bespoke prosthetic sockets. Fig. 1D represents the final positive cast of an amputee after the cast rectification process.

Utilizing these refined positive casts as templates, we expertly crafted patient-specific sockets from Polyethylene terephthalate (PET) and High-Density Polyethylene (HDPE). These sockets were meticulously designed to offer a tailored fit, enhancing overall comfort, functionality, and mobility for the wearer. The systematic and precise nature of this composite-based socket preparation procedure played a pivotal role in seamlessly integrating the prosthetic sockets with the amputee's residual limb, ultimately fostering successful adaptation and utilization of the prosthetic devices.

2.4. Manufacturing patient-specific prosthetic sockets

The manufacturing process for patient-specific sockets (PSS) paralleled the methodology employed in creating test sockets, elaborated elsewhere [17]. We used Vacuum-Assisted Consolidation (VAC) at elevated temperatures for fabricating the SrPET sockets. To initiate this process, patient-specific casts underwent a meticulous oven-drying procedure at 200 °C for 1 h to eradicate moisture content, ensuring optimal performance and durability of the vacuum pump. Concurrently, six layers of woven fabric underwent a drying regimen inside the oven at 50 °C for 24 h, mirroring the protocol applied in the fabrication of PET laminates or ISO test sockets. For the sake of brevity, some information on this is given here. The SrPET and HDPE shows the high tensile strength of 132 ± 5 MPa and 17.17 ± 0.22 MPa. Also, the six layers of SrPET test sockets was fabricated, and tested based on ISO 10328 [[12], [13], [14], [15]] loading condition. The socket withstands the high compressive strength of 5781N surpassing the A125 loading criteria. Full detail on procedure on socket fabrication and its testing is given in our studies [16,17].

In order to avert any potential chemical reactions between the cast and resin, thin Reusable Vacuum Bags (RVBs) were strategically employed to isolate the cast from the central arrangement. The detailed preparation of the Reusable Vacuum Bag (RVB-PSS) for manufacturing patient-specific sockets has been outlined in a companion paper [17]. In summary, the RVB-PSS was meticulously fabricated using Acetone and Ease Release™ 200 Aerosol spray coating to ensure pristine surfaces and enhance release properties. The socket mould was generated using a Prusa i3 MK3S printer, with design parameters determined based on data from the Bhagwan Mahaveer Viklang Sahayata Samiti (BMVSS), an Indian non-profit organization. Analysis revealed key dimensions, including a maximum residual limb circumference of 250 mm at the patella and 120 mm at the distal end, with stump lengths ranging from 200 mm to 440 mm. To accommodate the high elongation properties of silicone, two conical frustums with varying mould heights (300 mm and 400 mm) were 3D-printed. Both moulds shared a consistent bottom diameter of 250 mm and a top diameter of 120 mm. The RVB-PSS assembly, utilizing the same test socket vacuum base plate, was meticulously assembled with the 3D-printed moulds. Comprehensive cleaning with Acetone and coating with Ease Release™ 200 Aerosol spray facilitated effortless demolding. Utilizing 1000 g of resin for the 300 mm height mould assembly and 1200 g for the 400 mm height mould assembly, a 2 mm thick RVB-PSS was successfully produced through meticulous mixing and pouring of the two-part resin (1A:1B). To prevent any undesired wrinkling in the socket and ensure proper isolation from the PoP cast, two thin RVBs (600 g resin each) were produced in proportion to the size of the 3D-printed mould. The RVB-PSS was meticulously fabricated to ensure resilience and reusability throughout the study, thereby facilitating accurate and reliable testing and validation of the socket manufacturing process.

The fabrication commenced with the application of three layers of fabric over the cast, with a pyramid-type connector strategically placed at the distal end's apex. Subsequently, an additional three layers of PET fabric were meticulously wrapped around the cast, with aerosol spray adhesive ensuring secure bonding of the fabric edges (as demonstrated in Fig. 1e and f). A tightly drawn RVB was overlaid on the fabric to prevent wrinkling and unravelling. Finally, the primary RVB was positioned over the entire assembly and firmly secured using stainless steel pipe clamps. This comprehensive setup, shown in Fig. 1g, was subsequently transferred to an autoclave (Fig. 1h and i), following the temperature profile used during the curing process for test sockets, to solidify the patient-specific socket. The temperature was increased at a rate of 5 °C per minute until it reached the material consolidation temperature of 200 °C, which was maintained for 20 min before being gradually cooled down to 100 °C. The manufacturing processes remained consistent across all patients recruited for this study.

In contrast, the manufacturing technique employed by the BMVSS clinical center diverged, utilizing a vacuum-assisted thermoforming approach to fabricate exoskeleton-based High-Density Polyethylene (HDPE) sockets. Practitioners utilized HDPE pipes measuring 100 mm in diameter, 300 mm in length, and with a thickness of 6 mm. These HDPE pipes underwent pre-heating in an oven at 200 °C for 40 min. Subsequently, the pre-heated HDPE pipe was draped over the rectified Plaster of Paris (PoP) cast while applying negative pressure at the base of the setup. Following cooling, the HDPE sockets were meticulously removed from the cast. A comprehensive description of the fabrication process for HDPE sockets can be found in a previous study authored by one of the researchers [18].

2.5. Walking performance evaluation

To comprehensively assess the functionality of the developed prosthetic sockets, we employed the Six-Minute Walking Test (6MWT), a widely recognized measure of walking ability in prosthetic users. Following protocols established by BMVSS, participants, both unilateral and bilateral amputees fitted with PET and HDPE sockets, underwent the 6MWT.

As mentioned earlier, thorough examinations of participants' residual limbs were conducted to ensure there were no pre-existing injuries or blisters that could compromise safety during the trial. Practitioners meticulously assessed socket, pylon, and foot alignment using a Laser posture device to optimize functionality and comfort. To mitigate any potential fatigue effects, participants were afforded a minimum of 30 min of rest between trials involving PET and HDPE sockets.

During the assessment, participants wore a portable gait setup device, as depicted in Fig. 2a, to monitor their gait parameters. In Fig. 2b and 2c unilateral amputees are shown using PET and HDPE sockets, respectively, demonstrating the real-world application of the developed prostheses. Fig. 2d illustrates the specific task assigned to each participant during the socket trials.

Fig. 2.

(a) Bilateral amputee utilizing a wearable gait setup; (b) Unilateral amputee wearing a PET socket; (c) Unilateral amputee equipped with an HDPE socket; (d) Experimental protocol during the field trial. The images shown in Fig. 2a, b, and d have been adapted from Ref. [17].

Throughout the 6MWT protocol, participants traversed a 12m-long hallway demarcated with cones, walking back and forth at their preferred pace. Participants were instructed to rest if they experienced fatigue or discomfort during the trial. In addition to prosthetic users, three healthy subjects, including one unilateral and one bilateral amputee, participated in the evaluation to provide a comprehensive assessment of the walking performance and endurance of both PET and HDPE sockets.

2.5.1. Wearable gait monitoring system

To assess gait performance during the 6-Minute Walking Test (6MWT), we developed two wearable footbeds equipped with Force-Sensitive Resistors (FSR) using 3D printing technology. These footbeds were strategically positioned under the toe, first metatarsal, fourth metatarsal, and heel regions to capture vertical Ground Reaction Forces (GRF). Additionally, knee flexion angles relative to the hip were measured using a burster potentiometer to determine angular displacement. This device continuously capture angular changes and joint movements during clinical trials, enabling precise tracking of gait dynamics and ensuring accurate data collection throughout the study.

Participants wore the footbeds securely attached to their legs using velcro or adhesive straps at designated locations, as illustrated in Fig. 2a, b, and 2c. Each footbed was connected to a controller box containing a Bluetooth transceiver, microcontroller, voltage divider, and battery. This setup enabled the real-time extraction of data on vertical GRF and sagittal knee flexion angles from each leg, facilitating comprehensive analysis of gait performance.

To account for variations in individual height and anatomical differences, the shank length of the footbeds was adjusted accordingly. Prior to assessing amputee participants, the reliability of the gait setup was validated using three healthy individuals who underwent the 6MWT. Once validated, the same setup was utilized to evaluate the gait performance of both unilateral and bilateral amputees. The wearable gait monitoring system demonstrated its reliability and effectiveness, as evidenced by a previous study conducted by the authors involving a significant number of subjects [19].

3. Results

In this section, we present the findings of our study, beginning with the gait analysis of healthy subjects. The results provide insights into the biomechanical parameters measured during the Six-Minute Walking Test (6MWT) and shed light on the performance of the wearable gait setup. Following the analysis of healthy subjects, we delve into the gait performance of amputees fitted with patient-specific sockets, comparing the outcomes between Polyethylene terephthalate (PET) and High-Density Polyethylene (HDPE) sockets.

3.1. Gait analysis of healthy subjects

Utilizing the gait setup, we extracted vertical ground reaction force (VGRF) data from each force-sensitive resistor (FSR) and measured sagittal knee flexion angles to analyze gait performance. The total GRF was calculated by summing up the data from four FSRs at each leg. Fig. 3a illustrates the typical vertical GRF response during the gait cycle of a healthy subject, depicting the M curve representing various gait cycle phases. The vertical GRF data, normalized to the subject's body mass, revealed a stance phase of 60 % and a swing phase of 40 %. An early peak, reaching 40%–50 % of body weight (BW), occurred at the onset of the stance phase, followed by a mid-stance valley of 20 % BW. As the load progressively transferred to the front of the foot during push-off, it reached higher than 100 % BW at the late terminal stance.

Fig. 3.

Evaluation of the wearable gait setup with a healthy individual depicting (a) schematic representation of gait phases in relation to ground reaction force (GRF) and body weight, (b) measured knee flexion angle, and (c) GRF response normalized to body weight.

Fig. 3b presents the degree of knee flexion for both legs throughout the gait cycle, displaying the mean angular displacement with standard error captured at 12 instances during the 6MWT. Typically, higher knee angular displacement occurs during the swing phase, facilitating foot clearance and forward progression, reaching a maximum of 60° at the mid-swing phase. However, a delayed response in reaching the peak was observed in the right leg compared to its counterpart.

The measured total GRF response, as depicted in Fig. 3c, demonstrated symmetrical outcomes in both legs of the healthy subjects. These findings regarding the healthy subject GRF were consistent with other gait performance interpretation studies, affirming the overall integrity of the setup with accommodating healthy subjects.

3.2. Gait analysis of the amputees

3.2.1. Gait characteristics

During the 6MWT, both unilateral and bilateral amputees completed the test without any interruptions or pain, showcasing the feasibility and comfort of the prosthetic sockets. Utilizing MATLAB R2022a (MathWorks, USA), the data collected from the gait setup underwent meticulous analysis to compute various gait parameters. The descriptive characteristics of the 6MWT for healthy, unilateral, and bilateral amputees are presented in Table 2 and Fig. 4, allowing for a comprehensive comparison. Table 2 presents data related to the Six-Minute Walking Test (6MWT), velocity, cadence, stance time, swing time, single support duration and percentage, and double support duration and percentage for both types of amputees.

Table 2.

Gait characteristics of the participants including unilateral and bilateral amputees fitted with PET and HDPE sockets.

		Unilateral amputees			Bilateral amputees
	PET socket		HDPE socket
	Amputated limb	Intact limb	Amputated limb	Intact limb	PET-L	PET-R	HDPE-L	HDPE-R
6MWT (m)	196.3636		214.8449		271		264
Velocity (m/s)	0.545455		0.596791		0.75		0.73
Cadence (steps/min)	40		39		38		36
Stance time (%GC)	60.7	71.1	61.5	70.69	62.19	59.29	62.91	61.87
Swing time (%GC)	39.23	28.89	38.49	29.30	37.80	40.70	37.08	38.12
Single support (s)	0.52	0.47	0.54	0.65	0.62	0.43	0.57	0.44
Single support (%GC)	28	39	29.20	38.39	39.16	36.26	36.77	35.73
Double support (s)	0.23		0.43		0.2		0.18
Double support(%GC)	31		32.29		23.02		26.13

Fig. 4.

Comparison of different gait phases for (a) Unilateral amputee and (b) Bilateral amputee.

For the unilateral amputee, noteworthy differences were observed between the PET and HDPE prostheses. The HDPE prosthesis enabled the amputee to cover a longer walking distance of 214m, compared to 196m with the PET prosthesis. While most parameters exhibited similarity between the two prostheses, Fig. 4a illustrates that the intact limb displayed a higher stance time with both PET and HDPE prostheses. Additionally, the PET amputated limb exhibited a longer stride time than the intact limb, whereas the opposite was observed with the HDPE prosthesis, showcasing varied gait characteristics influenced by the prosthetic material.

Notably, a significant variation was observed in the duration of double support time for the HDPE prosthesis (0.43 s) compared to the PET prosthesis (0.23 s), indicating potential differences in gait stability and weight distribution between the two materials. Conversely, the gait characteristics of the bilateral amputee, as shown in Fig. 4b, demonstrate consistent implications for both prostheses. . Despite achieving a similar walking distance of 271m with both PET and HDPE prostheses, the amputee exhibited a longer single support time on the left leg compared to the right for both prostheses, suggesting asymmetrical weight-bearing patterns during ambulation.

These findings underscore the influence of prosthetic material on gait characteristics and highlight the importance of personalized socket selection to optimize gait performance and mobility for amputees. Comparisons between healthy subjects and amputees further elucidate the impact of limb loss on gait dynamics and underscore the need for tailored interventions to address individual needs and preferences.

3.2.2. Gait kinetics - Unilateral amputee

In our investigation of PET and HDPE-based prostheses, we meticulously monitored the vertical ground reaction force (VGRF) at each Force Sensitive Resistor (FSR) and recorded data on sagittal knee flexion angles. These metrics were essential for assessing gait dynamics in unilateral amputees. Aggregating individual FSR VGRF data provided insights into the total vertical GRF exerted by each leg. The real-time data, normalized to a subject's body mass of 68 kg, is illustrated in Fig. 5a.

Fig. 5.

Comparison of ground reaction responses during the clinical trial relative to amputee body weight. (a) Unilateral amputee with PET prosthesis, (b) Unilateral amputee with HDPE prosthesis, (c) Bilateral amputee with PET prosthesis, and (d) Bilateral amputee with HDPE prosthesis.

Fig 5a and Fig 5b and illustrates a discernible disparity in GRF between the PET and HDPE-based prosthetic limbs and the intact limb. The PET-based prosthesis displayed notably higher GRF on the prosthetic limb, exceeding 100 % BW, whereas the intact limb exhibited a lower GRF, with distinct peaks reaching up to 50 % BW. Conversely, the HDPE-based prosthesis demonstrated lower VGRF in the prosthetic limb compared to the intact limb, with both legs exhibiting similar responses.

Examining the plantar force distribution over the foot during a 6MWT provided further insights into gait characteristics. Fig 6a and Fig 6b present the percentage contribution of VGRF from each FSR at 12 different instances, revealing nuances in gait performance. The intact limb showcased greater engagement of the heel and toe regions, while the amputated limb displayed varied performance with different prostheses. Interestingly, the amputee relied more on the metatarsal head region with the PET prosthesis, while the HDPE prosthesis showed consistent engagement of the metatarsal 1 and 4 regions.

Fig. 6.

Plantar force distribution (PFD) at the four FSR locations for (a) right unilateral amputee with PET prosthesis, (b) right unilateral amputee with HDPE prosthesis, (c) bilateral amputee with PET prosthesis, and (d) bilateral amputee with HDPE prosthesis.

3.2.3. Gait kinetics - bilateral amputee

Gait kinetics analyses were conducted for bilateral amputees fitted with PET and HDPE prostheses, similar to those performed for unilateral amputees. Total VGRF data, normalized to a subject's body mass of 60 kg, revealed comparable responses in both legs, as depicted in Fig 5c and Fig 5d. Interestingly, the PET prosthesis exhibited marginally lower VGRF compared to the HDPE prosthesis, indicating subtle differences in gait dynamics.

Further examination of plantar force distribution, presented in Fig 6c and Fig 6d highlighted consistent patterns between the PET and HDPE prostheses. Notably, both legs displayed significant engagement with the high metatarsal four regions, with variations observed in the pressure distribution between the metatarsal one and four regions.

Moreover, in both unilateral and bilateral cases, discrepancies in pressure distribution were observed between legs, emphasizing the influence of prosthetic materials on gait dynamics. These findings underscore the importance of tailored prosthetic solutions to optimize gait performance and comfort for amputees.

3.2.4. Gait kinematics - Unilateral amputee

During the 6MWT, we measured the sagittal knee flexion angle of the unilateral amputee in relation to the hip. As depicted in Fig. 7a, the mean and standard deviation responses of PET and HDPE-based prostheses were recorded at 12 different instances of the walking test. Despite noticeable differences in magnitude, the pattern of angular displacement remains consistent across various limb locomotions, albeit with some temporal phasing differences.

Fig. 7.

The measured mean and standard deviation of the knee flexion angle (a) unilateral amputee; (b) bilateral amputee.(IL - Intact Limb; AL - Artificial Limb).

The amputated limb fitted with the HDPE prosthesis displays a notably high flexion angle of 75° during the swing phase. In contrast, the angular displacement of 30° at the stance phase indicates that the amputated limb does not fully extend in the gait cycle. Conversely, the limb utilizing the PET socket exhibits a maximum mean angle of 51°, with a longer duration required to attain the swing phase.

It is noteworthy that minimal differences are observed in the intact limb when an amputee utilizes PET and HDPE prostheses, suggesting the significant influence of the prosthesis on the amputee's biomechanics, particularly affecting the gait pattern of both the intact and amputated limb.

3.2.5. Gait kinematics - Bilateral amputee

The knee flexion angle of the bilateral amputee, as demonstrated in Fig. 7b, shows that the right leg of both prostheses follows a similar pattern in the initial impact peak, albeit with a significant difference in magnitude. The HDPE prosthesis exhibits high knee flexion angles of 65 and 69° in the right and left legs, respectively, with identical swing times.

In contrast, the PET prosthesis reveals a lower knee flexion angle in the right leg, accompanied by a more substantial deviation in the swing phase of the gait cycle. Additionally, the plot clarifies that the amputee exhibits incomplete leg flexion with both prostheses during the stance phase.

These results highlight the differences in knee flexion angles and gait patterns between unilateral and bilateral amputees, underscoring the critical role of prosthesis selection in influencing gait kinematics and overall walking performance. Comparing these kinematic patterns with the kinetic responses discussed earlier provides a comprehensive understanding of the interplay between biomechanical factors and prosthetic design in amputee gait analysis.

3.3. Additional physical parameters of PET socket

The fabrication process of the PET socket involved several meticulous stages, with an approximate total manufacturing time ranging from 230 to 250 min. Initially, 50–60 min were dedicated to casting and rectification, followed by drying the cast in an oven at 200 °C for 60 min to ensure proper curing. Subsequently, the PET fabric layup required an additional 30 min, and the socket underwent final curing in a purpose-built oven for 80–90 min. The average weight of the socket, recorded for all seven unilateral (UL) and three bilateral (BL) amputees, was approximately 630g. For a more comprehensive understanding, Table 3 presents specific insights into the weight and other physical parameters of one UL and one BL subject who participated in the 6MWT.

Table 3.

Descriptive statistics and specifications of prostheses for amputees participated in gait studies.

Parameters	Unilateral Amputee	Bilateral Amputee
Age	37	50
Occupation	Clerk	Hotel Receptionist
Years Since Amputation	11 years	22 years
Reason for Amputation	Truck Accident	Train Accident
Continuing the Same Work After Amputation	No	No
Volume of the Stump (mm^3)	2.36E6	1.58E6
Weight of the PET Socket (g)	681	548
Weight of the Pylon and Adaptor (g)	452	404
Weight of the HDPE Socket (g)	883	840
Weight of the Foot (g)	805	805
Total Weight of PET Prosthesis (including pylon and foot) (g)	1938	1757
Total Weight of HDPE Prosthesis (no pylon, only foot) (g)	1688 (Right leg)	1645

Furthermore, the woven PET fabric utilized in the socket manufacturing process was procured at a cost of 13 USD per kilogram, making it an economical and viable material option for prosthesis fabrication. These additional physical parameters offer valuable insights into PET socket preparation, including time, weight, and cost considerations. The data underscores the practicality and feasibility of utilizing PET composites in developing prosthetic sockets, ultimately contributing to improved mobility and comfort for amputees.

4. Discussion

The realm of prosthetics has witnessed notable advancements, yet challenges persist, especially concerning material accessibility and efficient manufacturing, particularly in LMICs. Our study presents a novel technique for crafting personalized sockets utilizing recyclable composite materials. Collaborative clinical research with the Indian organization BMVSS underscores the potential of inclusive manufacturing in catering to diverse patient needs in LMICs. The method employed for crafting patient-specific PET sockets, leveraging reusable vacuum bags, demonstrates promising outcomes in gait analysis. These results suggest that PET-fibre-reinforced composite could emerge as a viable alternative to conventional socket materials, particularly in resource-constrained settings.

During the six-month field trial, amputees emphasized specific parameters such as the weight, aesthetics, and durability of their artificial limbs. While the cost of HDPE sockets at BMVSS is not a concern for amputees due to the provision of free artificial limbs, advanced composite material sockets elsewhere might pose financial barriers. While previous studies have explored alternative socket materials, few have comprehensively fabricated patient-specific sockets and analyzed their gait performance. Gait analysis unveiled differences in gait patterns between healthy subjects and the intact limb of unilateral amputees. The PET prosthesis favoured mid-foot landing, whereas the HDPE prosthesis exhibited a higher knee flexion angle. Force distribution and knee flexion angle in bilateral amputees remained similar for HDPE and PET prostheses. Notably, the prosthesis type, exoskeleton (HDPE) versus endoskeleton (PET), appears to influence force distribution.

This pioneering study validates the efficacy of low-cost, sustainable PET material and a purpose-built oven for socket fabrication in both unilateral and bilateral amputees. However, the study's limitation to only two amputees for gait performance analysis necessitates further research with a larger cohort. The variance in prosthesis type may have impacted force distribution, highlighting the need for future exploration into textile architecture of composites to streamline socket fabrication and produce near-net shapes of residual limbs.

In summary, our study provides valuable insights into alternative materials and manufacturing techniques for prosthetic sockets, with potential significant benefits for amputees in less-resourceful regions. The findings endorse the development of self-reinforced thermoplastic polymer-based recycled PET composites, reusable vacuum bags, and thermal curing for socket consolidation. Furthermore, the study introduces a novel approach for fabricating patient-specific sockets and quantifying gait performance. Future clinical trials involving a broader participant base are imperative to further advance this technology.

5. Limitations

While this study provides valuable insights into the development and application of patient-specific prosthetic sockets, it is important to acknowledge several limitations that may impact the interpretation of the findings. Firstly, the sample size of participants included in the gait analysis phase was relatively small, which may limit the generalizability of the results. Additionally, the study duration was constrained, and the inclusion of a larger and more diverse participant pool over an extended period could provide a more comprehensive understanding of the efficacy of the prosthetic sockets.

Furthermore, the reliance on a single assessment tool for gait analysis, albeit comprehensive, may overlook nuances in gait dynamics that could be captured by employing additional measurement techniques or tools. Variability in individual gait patterns and preferences may also influence the observed outcomes, emphasizing the need for personalized approaches in prosthetic socket design and evaluation.

Moreover, while efforts were made to standardize the fabrication process of the PET sockets, variations in manufacturing techniques or materials could introduce confounding factors that impact the performance and durability of the prostheses. Future studies could benefit from rigorous quality control measures to minimize such variability and enhance the reliability of the findings.

6. Future work

Building upon the insights gained from this study, several avenues for future research and development emerge that could further advance the field of prosthetic socket design and implementation. Firstly, longitudinal studies involving larger and more diverse cohorts of amputees could provide valuable insights into the long-term performance and user satisfaction with patient-specific sockets. Longitudinal data collection would also enable researchers to assess changes in gait dynamics and functional outcomes over time, facilitating a more comprehensive understanding of the impact of prosthetic interventions on amputee mobility and quality of life.

Additionally, further investigation into the biomechanical properties of alternative materials and fabrication techniques could expand the range of options available for prosthetic socket design. Advanced manufacturing technologies, such as additive manufacturing and computer-aided design, offer exciting possibilities for customizing prosthetic sockets to individual patient needs and preferences. Integrating these technologies into clinical practice could improve the accessibility and affordability of personalized prosthetic solutions, particularly in resource-limited settings.

Furthermore, interdisciplinary collaborations between biomechanical engineers, prosthetists, clinicians, and end-users are essential for developing holistic approaches to prosthetic care. By fostering collaboration and innovation across multiple disciplines, future research endeavors can address the complex challenges associated with prosthetic socket design, fabrication, and evaluation, ultimately enhancing the quality of life for individuals living with limb loss.

7. Conclusion

The study marks a significant advancement in the pursuit of affordable and durable prosthetic sockets tailored for LMICs. By leveraging self-reinforced PET (polyethylene terephthalate), a sustainable and cost-effective composite material, as a viable alternative for manufacturing functional sockets, our research underscores the potential for transformative change in prosthetic care accessibility. Our innovative manufacturing technique, which integrates a reusable vacuum bag and a purpose-built curing oven, enables the production of patient-specific sockets with intricate shapes, enhancing manufacturing efficiency and ensuring optimal fit for amputees. Gait analysis conducted on amputees using our PET-based sockets affirm the functional efficacy of our approach, demonstrating their ability to engage in daily activities without interruption. By combining self-reinforced PET with our novel fabrication technique, we provide a practical and accessible solution that benefits both clinicians and patients alike. The reduction in socket manufacturing time not only minimizes patient waiting periods but also improves the accessibility and affordability of prosthetic care in LMICs. Our study serves as a catalyst for further exploration and refinement of our methodology. Future endeavors should involve additional clinical trials to validate our findings and optimize the socket fabrication process further. Committed to inclusive and sustainable prosthetic solutions, we anticipate that our research will significantly enhance the quality of life for amputees in resource-constrained regions and contribute to a more equitable global healthcare landscape.

Funding

This research was funded by the Academy of Medical Sciences under Global Challenges Research Fund (GCRF) Scheme (grant number: GCRFNG\100125). The UK's Royal Academy of Engineering also financially supported this research work for the project "Up-cycled Plastic Prosthetics" (grant reference: FF\1920\1\30).

Data availability statement

The data presented in this study are available on request from the corresponding author.

CRediT authorship contribution statement

Yogeshvaran R. Nagarajan: Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization. Farukh Farukh: Writing – review & editing, Supervision, Resources, Funding acquisition. Karthikeyan Kandan: Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization. Amit Kumar Singh: Software, Project administration, Funding acquisition. Pooja Mukul: Investigation, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.