Application of additive technologies in the field of prosthetics

Introduction

Medicine has required prosthetics almost since its inception, and the use of modern technologies and engineering approaches is critically important in this field. This is primarily because a prosthesis compensates for functional deficits caused by the loss of a limb. Despite their apparent simplicity, human limbs have a complex mechanical structure and perceive a vast amount of environmental information essential for full-fledged daily activity. Merely restoring the visual appearance of a limb is insufficient; it is crucial to restore both mechanical and sensory functions [1]. Modern technologies now enable the creation of artificial limbs capable of movements similar to those of healthy ones. The latest prostheses can exchange information with the human body, which is vital for addressing numerous challenges associated with limb loss.

Over the past 50 years, the global population has doubled, as shown in Figure 1. According to UN projections, by 2050 the world population will reach 9.7 billion — approximately 20% more than today. Technological progress does not always reduce injury rates; both positive and negative factors are at play. While population size is not directly proportional to the number of people needing prostheses, a clear correlation exists. Consequently, demand for prosthetics continues to grow.

Background

From the earliest days of human history, prosthesis manufacturing has demanded cutting-edge technologies and materials due to the complexity of the human body. As prosthetic technology evolved, individuals who lost limbs were increasingly able to regain functionality. In ancient civilizations, attempts were made to create limb prostheses from basic materials such as leather, wood, and metals. Most often, these served primarily aesthetic purposes, though there are examples of low-functionality mechanical devices [3].

In the 20th century, the development of artificial limbs underwent radical changes driven by global conflicts, industrial revolutions, and breakthroughs in electronics, materials science, and biomechanics. This period marked the transition from mass-produced standardized designs to the integration of digital technologies, significantly enhancing functionality and comfort. Wars dramatically increased demand for prostheses. For instance, World War I (1914–1918) catalyzed the industrialization of prosthetic production. In the United Kingdom, over 41,000 amputations were recorded, leading to the establishment of specialized rehabilitation centers. In Germany, Otto Bock developed a modular prosthetic system with standardized mass-produced components, ensuring quality and process efficiency — except for the individually fitted socket [4]. World War II (1939–1945) further accelerated prosthetic advancements. Innovations at the time included the use of Bowden cables for force transmission (1948), which improved mobility and accessibility. Early myoelectric systems emerged in the 1940s: Reinhold Reiter’s patent in Germany laid the foundation for controlling prostheses via muscle electrical signals [4].

From the 1950s onward, the digital revolution — with transistors, processors, and computers — enabled device miniaturization. Prostheses evolved from passive or body-powered to myoelectric systems. In the 1980s, computer-aided design and manufacturing (CAD/CAM) systems, borrowed from other industries, were introduced to improve socket accuracy and production efficiency. Myoelectric prostheses, especially for upper limbs, used electrical signals from muscle contractions to control multiple grip patterns with high precision.

Materials also transformed: carbon fiber (introduced in the 1990s) offered an excellent strength-to-weight ratio, reducing user fatigue and improving comfort. Silicone liners, developed around the same time, enhanced fixation and minimized friction between the residual limb and socket.

Microprocessor-controlled joints became a key innovation: the "Intelligent Knee" (1990) was the first microprocessor knee joint, followed by the C-Leg (1997) and similar systems that improved safety and function [4]. Biomechanics advanced through 3D motion analysis, force sensors, and gait analysis systems.

By the end of the century, osseointegration techniques emerged, allowing direct skeletal attachment of prostheses, improving stability and proprioception — albeit with infection risks. These achievements laid the groundwork for 21st-century "smart" prostheses.

In recent decades, researchers in prosthetics and orthopedics have been addressing challenges arising from rapid technological progress, increasingly integrating advanced sensors and devices into prostheses. Each improvement raises costs for end users, so even today, simple cosmetic prostheses remain the most common [5]. Statistics show that patient satisfaction with cosmetic prostheses is relatively high, whereas myoelectric ones often lead to reduced comfort and lower satisfaction in surveys.

Current Limitations of Prostheses

The main limitations are as follows. First, robotic limbs equipped with electric motors and batteries significantly increase weight, causing discomfort and limiting long-term use [6]. For example, modern myoelectric upper-limb prostheses can weigh several kilograms, leading to muscle fatigue and reduced mobility — especially in patients with weakened musculature. The size of electronic components negatively affects aesthetics and functionality, making it difficult to mimic the natural shape and dynamics of a human limb, resulting in discomfort and visual unnaturalness.

Second, limited battery life (typically 8–12 hours) prevents all-day use and restricts daily activities. Third, the critical drawback remains the lack of full sensory feedback: patients cannot feel contact with objects, complicating precise tasks such as grasping fragile items or assessing surface texture. This stems from difficulties in integrating sensors capable of transmitting tactile and proprioceptive signals through neural interfaces without interfering with myoelectric control signals.

Combined with high production costs, these factors severely limit the widespread adoption of advanced robotic prostheses, making them accessible primarily to a small group of patients in developed countries. Current research therefore focuses on modular design, advanced materials (carbon composites, biocompatible polymers), and efficient technologies — including 3D printing — to reduce costs and enable personalization, resulting in more affordable, adaptable, and versatile prostheses. These innovations aim to overcome existing barriers through energy recuperation, lightweight design, and flexible sensory systems to improve feedback and quality of life for amputees.

Prosthetic Manufacturing Technologies

There are two main approaches to prosthetic production: traditional (subtractive) and additive manufacturing (Figure 2). Traditional methods still dominate the market due to skepticism toward additive technologies. However, subtractive manufacturing has significant drawbacks: it cannot produce truly unique, custom-shaped prostheses due to low process flexibility, and material waste is extremely high — up to 10 times more material may be removed than remains in the final product [7].

Experts in prosthetics and orthopedics [8] have highlighted the shortcomings of traditional methods. Reported costs include: upper-limb prostheses ($6,000–$10,000), below-knee prostheses (CAD 4,000–12,000), and above-knee prostheses (CAD 6,000–18,000). These high costs are largely due to labor-intensive and wasteful processes.

The primary advantage of additive manufacturing, regardless of application, is extremely low material waste — typically under 10%.

According to a survey [9], the main benefits of additive manufacturing are speed, cost-effectiveness, and low material consumption. The main drawbacks cited are anisotropic strength and inability to print transparent parts. Transparent sockets are used during testing to monitor skin contact; however, this issue is solvable — transparent and strong polymers like PETG already exist. Anisotropy can be mitigated through proven techniques: optimal part orientation, improved interlayer bonding, etc. Currently, strength limitations restrict most 3D-printed prostheses to cosmetic hands. Nevertheless, some companies already produce fully functional 3D-printed upper-limb prostheses, albeit on a limited scale. A major barrier to industrial adoption is the absence of qualification standards for structural safety of 3D-printed devices — largely due to insufficient research in this area. Further development of materials, designs, generative design, and AI will make additive prostheses lighter, stronger, and more comfortable, significantly improving user experience in the future.

3D printing has revolutionized many industries, including robotic prosthetics [10]. Various 3D printing technologies are used depending on patient needs, prosthesis type, and materials (Figure 3). The most popular are Stereolithography (SLA), Selective Laser Sintering (SLS), and Fused Deposition Modeling (FDM/FFF). Table 1 summarizes their advantages and disadvantages.

FDM/FFF is the most accessible and widely used technology, producing lightweight yet sufficiently strong upper-limb prostheses (especially hands and fingers) [8]. A key requirement is the balance between structural rigidity and joint flexibility. Though cheaper than alternatives, FDM suffers from anisotropy and lower mechanical strength.

SLA offers the highest surface quality and precision, making it ideal for facial prostheses and components requiring fine detail [11]. High resolution enables complex joints and ergonomic features that improve comfort and acceptance.

SLS excels in producing load-bearing components (e.g., lower-limb sockets) due to high material density. It has also been used in tissue engineering: for example, [12] successfully applied SLS to create personalized, strong bone scaffolds from polymers, metals, ceramics, and composites.

Material selection for 3D-printed implantable devices is critical for biocompatibility, safety, and load-bearing capacity. Common materials include titanium alloys, cobalt-chrome, and medical-grade polymers (PEEK, PCU). PEEK is widely used in spinal implants due to its radiolucency (no MRI/CT artifacts) [13]. PCU is applied in cardiovascular devices. Biodegradable and bioresorbable materials are also being explored to reduce long-term complications.

Material choice directly affects printing technology, strength, comfort, and durability. PLA and ABS are popular but insufficiently strong for high-load applications. Advanced materials like PEEK and Ti-6Al-4V offer superior properties but are expensive and difficult to process (especially PEEK via FDM).

Multi-material printing (particularly SLA and SLS) enables combining rigid and flexible regions — essential for mimicking bone-to-soft-tissue gradients. Table 3 lists key properties of materials used in additive prosthetics.

The integration of smart technologies (sensors, advanced control, nanoelectronics) is a relatively new and rapidly evolving field. Pressure, motion, and temperature sensors restore sensory feedback absent in traditional prostheses. Electroactive polymers (e.g., polypyrrole-based artificial muscles [15]) show promise for lightweight, responsive actuation. Myoelectric sensors enhance integration and control precision.

Expanding Prosthetic Functionality

Additive manufacturing enables unprecedented design freedom. Generative design is often used to optimize strength, ergonomics, and weight. In [16], generative design combined with topology optimization produced a lightweight, load-bearing lower-leg prosthesis (Figure 4).

Additive technologies also allow embedding therapeutic or comfort-enhancing devices. For example, the Russian company “Motorica” (Skolkowo resident) integrates neuromodulation systems into prostheses to treat phantom limb pain [17, 18]. Clinical trials have confirmed efficacy. Parallel research develops tactile feedback systems transmitting temperature, shape, and grip force.

Beyond limb prosthetics, 3D printing and machine learning enable personalized joint replacements [19]. One study designed an optimized hip prosthesis with theoretically infinite fatigue life under normal gait loads.

According to WHO, 1.4% of the world population are amputees, with 33% being upper-limb cases [20]. Traumatic amputation prevalence rose 49.2% from 1990 to 2019 [21]. Only 5% of traumatic amputees have access to prostheses due to cost and complexity. 3D printing could dramatically improve accessibility. Patients using 3D-printed prostheses report reduced phantom pain, improved psychological and physical well-being, and restored basic mobility and self-care ability — especially when fitted early post-amputation [22].

Additive manufacturing has already revolutionized medical device production, enabling complex, personalized in-vivo implants. However, several challenges remain:

1. Biocompatibility — implants must not cause inflammation or rejection.
2. Material properties — mechanical, thermal, and chemical characteristics must meet daily-load requirements.
3. Quality control — reliable standards and testing methods are needed.
4. Standardization — standardized processes and materials are essential for broader adoption.

Conclusion

A prosthesis is a highly complex device; traditional manufacturing is extremely time- and resource-intensive. 3D printing dramatically reduces production time (typically ~10 hours with minimal operator involvement) and cost (by approximately 20%). It enables unprecedented customization of all limb types. At the current stage of prosthetic development, 3D printing technologies are attracting enormous interest. The combination of speed, affordability, low waste, and customization accelerates their adoption and societal acceptance, marking a new era in medical technology.