4D Printed Surgical Devices: Current Capabilities and Challenges

Rodrigo Zapata Martínez

1,* a

, Carlos Aguilar

1,** b

, William Solórzano-Requejo

1,2 c

Oscar Contreras-Almengor

, Carlos Polvorinos Fernández

, Jon Molina-Aldareguia

and Andrés Díaz Lantada

ETSI Industriales, Universidad Politécnica de Madrid, Madrid, Spain

Department of Mechanical and Electrical Engineering, Universidad de Piura, Piura, Peru

IMDEA Materials Institute, Tecnogetafe, Getafe, Spain

Keywords: Additive Manufacturing, 4D Printing, Personalized Biomedical Design, Smart Materials and Structures.

Abstract: The concept of 4D printing refers to the ability of a 3D printed material or device to change shape in a

predefined manner controlled from the design stage. Currently, 4D printing research is performed by

employing various additive technologies and materials, whose special design features or functional properties

allow for these shape transformations or metamorphoses after printing. This smart shape-morphing behaviour

is already providing innovative concepts for biomedical engineering and healthcare technologies, although

important advances are still needed towards impactful transfer to society. This study presents different

polymeric additive manufacturing technologies: stereolithography, digital light processing and selective laser

sintering, that can be employed towards shape-morphing or 4D printed medical devices, in some cases at

prototyping level, in others for final production. Through the prototyping of different joints and kinematic

chains, configured as potential surgical actuators, the potentials and limitations of these resources are studied

and good design practices and future applications for 4D printed biodevices are provided. The applicability

of polymeric 4D printing to emulate and predict 4D printability with high-performance alloys is discussed.

1 INTRODUCTION

Industry and consumers already benefit from a wide

set of additive manufacturing or 3D printing

technologies, capable of processing polymers, metals,

ceramics, composites, biomaterials and even living

materials, such as stereolithography (SLA), selective

laser sintering (SLS), fused deposition modeling

(FDM), selective laser melting (SLM), electron beam

melting (EBM), bioprinting, lithography-based

ceramic manufacturing, to cite a few. Due to their

usual ability to produce products directly from the

raw materials, without involving costly production

tools, 3D printing technologies have sparked a lot of

interest among academic institutions and major

corporations.

The nature of 3D printing is highly

interdisciplinary, especially in the healthcare arena,

and involves the collaboration of materials scientists,

https://orcid.org/0000-0002-2611-7050

https://orcid.org/0000-0003-0291-3041

https://orcid.org/0000-0002-8166-4161

mechanical engineers, software developers, data

scientists, product designers, biomedical engineers,

healthcare professionals among many others.

Besides, as regards the biomedical industry, the

remarkable geometrical complexity achievable by 3D

printing technologies through layered manufacturing

processes is of special relevance for achieving

medical devices capable of interacting with the

complex morphologies of nature, human organs and

tissues. This enables biomimetic design approaches

towards medical devices with enhanced performance,

and the toolless production routes achieved through

additive manufacturing can importantly promote

personalized healthcare strategies.

Indeed, the developments in 3D printing in recent

years have enabled researchers to create complex

shapes that were impossible to produce using the old

traditional techniques. For instance, researchers have

been successful in creating remotely actuated robots,

https://orcid.org/0000-0003-3508-6003

https://orcid.org/0000-0002-0358-9186

**carlos.avega@upm.es (C.A.)

Zapata Martínez, R., Aguilar, C., Solórzano-Requejo, W., Contreras-Almengor, O., Polvorinos Fernández, C., Molina-Aldareguia, J. and Díaz Lantada, A.

4D Printed Surgical Devices: Current Capabilities and Challenges.

DOI: 10.5220/0011744300003414

In Proceedings of the 16th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2023) - Volume 1: BIODEVICES, pages 157-163

ISBN: 978-989-758-631-6; ISSN: 2184-4305

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

157

designs using algorithms and machine learning,

multi-material bioinspired designs, micro-

environments cell culture processes, special

biointerfaces for interacting with human tissues, and

drug delivery systems using the 3D printing

technologies.

However, because of the lack of regulations or the

slow production cycle, additive manufacturing

technologies are not yet transforming several sectors

as expected. Along with the limited printing volume

and certain typical defects, like warping or high

porosity ratio, that may occur if the printing process

is not perfectly performed, the limited number of

high-performance materials processable through 3D

printing is also a current barrier. In many cases

though, the advantages of additively manufactured

components outweigh the disadvantages.

Recently, contributions from materials scientists

have led to a remarkable increase of the range of

printing materials, including the use of some smart or

stimuli-responsive options like shape memory

polymers and alloys, piezoelectric ceramics,

electroactive polymers, to cite a few families, through

which several smart devices, even for the medical

field, can be straightforwardly designed and created.

Thanks to the possibility of printing with smart

materials and to the capability of creating functional

geometrical gradients, it is possible to obtain

structures with controlled geometrical modifications

after printing, which led to the concept of “4D

printing”, as recently reviewed (Aamir Ahmed, 2021).

In short, the term “4D printing” describes the

single-material or multi-material printing of a device

or an item that can change from a 1D strand into

another pre-programmed 1D, 2D or 3D shape, from a

2D surface into another pre-programmed 1D, 2D or

3D shape, and to morph between 3D and other

dimensions. Such transformations are facilitated by,

e.g., heating, light, or swelling in a liquid,

electrochemical reactions and by programming

differential behaviors across the printed geometry

through functional gradients of materials or

structures. These 4D printing methods open new

possibilities for non-electronic based materials to

incorporate programmability and clear decision-

making. They also provide flexibility and dynamic

responses for structures and systems of varied sizes

and herald important healthcare transformations.

The shape-morphing behavior of these smart

products, including shape-shifting and evolutive

medical devices, relies mostly on five fundamental

factors that must be kept in view while performing

design for 4D printing. These are: 1) the AM process,

2) the material used for printing, 3) the triggering

stimuli, 4) the mechanism of interaction, and 5) the

shape-morphing modeling (Farhang Momeni, 2017).

The first aspect is the AM process used for

printing. Numerous AM techniques exist, as already

mentioned. Almost all of them can print a 4D material

or device as long as the printing method and material

are suitable for the printer. The second factor is the

printing material which needs to respond to stimuli,

in some exceptional cases during printing or, in most

cases, after printing. These materials are frequently

referred to as smart materials (SMs) or programmable

materials. The kind smart material employed defines

the triggering stimulus, and the material’s reaction to

the triggering stimulus determines the self-

transformation ability. The third aspect, the actual

triggering stimuli, can be physical, chemical, and

biological. Physical stimuli include light, moisture,

magnetic and electric energy, temperature, UV light,

etc. Chemical stimuli include the use of chemical

reagents, the pH level, the employment of oxidizing

or reducing conditions, among many others. Among

biological stimuli it important to highlight the use of

enzymes and glucose or even the employment of

living cells and tissues during printing. In 4D

printing, when a stimulus is introduced, the structure

undergoes physical or chemical changes, such as

relaxation of stresses, molecular motions, and phase

changes, which cause the structural deformation. The

mechanisms of interaction and modeling are the

fourth and fifth factors. Not all materials can perform

the necessary transformation when a stimulus is

applied to smart material. We should offer an

interaction method that will plan the sequence of form

change, such as mechanical loading or physical

movement. The modeling is necessary to determine

how long the stimulus will affect the smart material

after providing the interaction mechanism.

Our team, within the iMPLANTS-CM project, is

focused on the development of biomedical devices

with shape-morphing properties. These are achieved

through 4D printing using a wide range of additive

technologies and materials and special design

features for empowering the shape changes. In this

study and introduction to 4D printing with polymers

is presented and illustrated through a set of rapid

prototypes designed as concepts for innovative

surgical actuators. Through their design and 4D

printing different good practices are reported.

2 MATERIALS AND METHODS

This section details the materials and technologies

used in the iMPLANTS-CM project with the

BIODEVICES 2023 - 16th International Conference on Biomedical Electronics and Devices

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objective of validating the design and polymeric 4D

printing of shape-morphing concepts of medical

devices. Both conventional and shape-memory

polymers are employed, and the design of printable

kinematic chains or mechanisms is utilised for

enhancing the metamorphic properties. The materials

used to this end correspond to each of the additive

manufacturing processes employed: photosensitive

resins and photosensitive resins with shape memory

for additive photopolymerization processes like laser

stereolithography (SLA) and digital light processing

(DLP) and nylon (PA12) and thermoplastic

polyurethane (TPU) for selective laser sintering

(SLS). Details are provided below.

2.1 Materials

2.1.1 Photosensitive Resins

Photosensitive materials are those that upon receiving

an amount of energy, typically from an ultraviolet

light source or laser beam, photopolymerize and lead

to solid components through additive or layered

photopolymerization procedures. They usually

consist of three components: the core composed of

different monomers; the photoinitiators, molecules

that react to ultraviolet light and initiate the

polymerization process; and, finally, the additive

additives that add color and some special properties

to the resin (Min Hong, 2015). In this study, Somos

epoxy resin is employed for laser stereolithography

and Anycubic resin compatible with the used digital

light processing is selected for printing purposes.

2.1.2 Nylon (PA12)

Polyamide 12 is one of the many materials belongings

to the group of aliphatic polyamides, also known

commercially as nylons. Although PA12 has slightly

inferior mechanical properties than those of PA6 or

PA6-6, it has become the most common material in

polymeric SLS 3D printing mainly for two reasons:

its lower melting point that facilitates processing and

its quite low hygroscopicity. The one used here,

provided by Sinterit, has an ultimate tensile strength

of 41 MPa with an elongation at break of 13%, as well

as an impact strength of 15 KJ/m2, making it a highly

versatile material for a wide set of applications

(Benjamin Shaw, 2016) and for rapid prototyping.

2.1.3 TPU

Urethane-based thermoplastic linear elastomers, also

known as TPE-U or TPU, are a group of block

copolymers of polyols and diisocyanates. The ratio

between the two polymers determines the final

properties of the material, ranging from semi-rigid

materials to materials with high elasticity. In general,

urethane-based elastomers stand out for their high

resistance to wear and abrasion, high tensile strength,

good cushioning capacity, good toughness and

resistance to grease and oils. In addition, it is

compatible with skin and has a high resistance to

fungi, which makes it suitable even for medical or

orthopedic applications. The one used here, provided

by Sinterit, has a tensile yield strength of 1.8 MPa and

a compressive yield strength of 3.5 MPa and an

ultimate tensile strength of 3.7 MPa with a strain at

break of 137 % (Tao Xu, 2020). It is selectively

melted using a laser and constitutes a good

complement or alternative to PA12 for soft devices.

2.2 Methods

2.2.1 DLP

Digital light processing (DLP) is a 3D printing

technology used to rapidly produce photopolymer

parts. The light is reflected on a Digital Micromirror

Device (DMD), a dynamic mask consisting of

microscopic-size mirrors laid out in a matrix on a

semiconductor chip. Rapidly toggling these tiny

mirrors between lens(es) that direct the light towards

the bottom of the tank or a heat sink defines the

coordinates where the liquid resin cures within the

given layer. Because the projector is a digital screen,

the image of each layer is composed of square pixels,

resulting in a three-dimensional layer formed from

small rectangular cubes called voxels (Jiumeng

Zhang, 2019). In this study Anycubic M3 and M3

Plus DLP printers are employed. (Formlabs, s.f.)

2.2.2 SLA

Laser stereolithography (SLA) is the foundational 3D

printing technology. It works by using a high-

powered laser to harden liquid resin that is contained

in a reservoir to create the desired 3D shape. In a

nutshell, this process converts photosensitive liquid

into 3D solid plastics in a layer-by-layer fashion using

a low-power laser and photopolymerization. In this

study a 3D Systems “legacy” SLA-3500 SLA printer

is employed.

2.2.3 SLS

SLS operation principle is powder sintering with the

help of infrared laser, working within an elevated

temperature chamber, which helps the grains of the

powder to consolidate before being bound with the

4D Printed Surgical Devices: Current Capabilities and Challenges

159

laser beam. In the conventional SLS printer there is a

so called “bed” on which the roller spreads a thin

layer of powder followed by sintering according to

the layers sliced from a 3D model file. Afterwards the

platform moves down by a small increment and the

process repeats until the last layer is formed. After the

process comes the post-processing part, which

requires removing the model from the un-sintered

powder suspension and sandblasting it.

Probably the most interesting advantage of SLS,

as compared with polymeric SLA and DLP or with

metallic selective laser sintering or melting (SLS /

SLM) is the fact that 3D printing is performed without

any supporting structures, as the complex-shaped

models are supported by the powder during the

printing process. This constitutes a very remarkable

aspect in 4D printing, as moveable objects,

interwoven elements and mechanisms can be printed

with great accuracy (Abishek Kafle, 2021). In this

study a Sinterit Lisa Pro SLS printer is employed.

Table 1 below provides a comparative study of the

features of the different printing technologies and

materials used, which provide a varied selection of

resources usable for polymeric 4D printing. (Piszko,

s.f.)

Table 1: Summary of polymeric 4D printing tools

used in this

study.

Technology

DLP

SLA

SLS

Machine

Anycubic

Photon M3

Plus

3D Systems

SLA3500

Sinterit Lisa

Pro

Build volume

245x197x122

350x350x400

110x160x245

Layer height

0.02-0.200

0.05-0.150

0.075-0.175

Resolution

6K screen

(44 µm/pixel)

0.250-0.300

mm beam

diameter

0.350-0.400

mm beam

diameter

Materials

Photo

sensible resin

Photo

sensible resin

PA, TPU, PP

3 RESULTS AND DISCUSSION

3.1 Differences Found in Technologies

The central objective pursued by the aforementioned

iMPLANTS-CM project is the fabrication of

biomedical devices using shape memory materials,

specifically NiTi, to obtain final products. As this

technology has a high cost and is currently under

development, other 4D printing technologies with

polymers are employed for rapid prototyping

purposes.

These more accessible technologies and

materials, already presented in section 2, support

designers during the conceptual, design and

prototyping phases, before resorting to the printing in

high-performance materials such as NiTi. For

instance, in the example shown in figure 1 below,

laser stereolithography with shape-memory epoxy is

employed to obtain an articulated mechanism. 4D

printing is illustrated by heating the mechanism after

its printing and performing the training of the shape

memory effect (opening of the actuator). Once cooled

down, a subsequent heating leads to shape recovery.

Figure 1: Example of 4D printing in a photosensitive shape

memory resin. The previously heated and deformed device

returns to its original position when heated. The shape-

memory is empowered by the printing of an articulated

mechanism. Upper images: training process. Lower images:

trained and recovered geometry after activation of shape-

memory effect.

Prior to redesigning for SLM and to analysing the

applicability of these rapid prototyping tools

(polymeric 4D printing processes) to emulate the final

SLM with special alloys, it is crucial to consider the

variations and similarities between each of the

methods used.

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First, some of the analogies between

photopolymerization -as possible rapid prototyping

technology- and SLM -as final production method-

are discussed. According to the screen resolution, the

size of the pixels utilized in DLP technology is

comparable to the laser hatch used in SLM

technology. Some similarities are also detected, as

regards the utilization of supports throughout the part

construction process. The analogies with SLA are the

same as with DLP, but DLP is in general a quicker

and less expensive process than SLA, although

printing volume is compromised. In consequence,

depending on the design and part size under

evaluation, the more adequate is chosen.

Figure 2: Collage of different prototypes of joints and

lattices for 4D printed surgical actuators; from up to down:

a spheric joint printed in DLP, examples of superelastic unit

cells printed in TPU, gripper 3D with hyperbolic joints

printed in nylon (PA12).

Second, the common features of polymeric SLS

and metallic SLM are analysed. Although the SLS

printer’s laser precision is less precise than the SLM

printers, using both powders enable us to study how

the concentration of heat affects the powder during

the printing process, enabling us to determine which

printing direction can produce the best results but

taking into account that SLS technology is self-

supporting and SLM technology is not, so we cannot

establish more similarities between these printing

processes.

In summary, each of the technologies employed

provides relevant information when planning an SLM

print using NiTi, as a shape memory material, to

enable 4D applications.

To better illustrate the interest of polymeric 4D

printing for the rapid prototyping of shape-morphing

devices, some additional joints, lattices and kinematic

joints or mechanisms are designed and manufactured

employing the different thermoset photopolymers,

thermoplastics and elastomers and the polymeric

processing technologies described in section 2.

By means of example figure 2 presents a set of

selected prototypes are obtained employing digital

light processing and selective laser melting. Among

them, different links, joints and structures for

conceptual surgical actuators or manipulators are

shown.

3.2 Difficulties During the Polymeric

4D Printing Processes

First, the need to use supports in SLM technology,

like in DLP and SLA, restricts the usable printing

direction and leaves aesthetic defects at the contact

points between supports and model that need to be

post-processed for enhanced interactions with the

skin, tissues and organs of interest. In this regard, the

employment of DLP and SLA as rapid prototyping

tools can help to emulate the expected surface finish

and to plan the required supporting structures, when

printing using other more high-performance materials

and processes, such as SLM of NiTi.

Second, although polymeric SLS was initially

expected to be much more adequate for emulating

metallic SLS and SLM, the lack of supports in

polymeric SLS makes it quite different from a design

perspective.

3.3 Validated Concepts

In any case, it is important to point out that the

experience gained with all the technologies

mentioned above has allowed us to additive

manufacture actuators and conceptual devices that

validate the shape memory or superelastic properties

of polymers employed for additive manufacturing

using laser stereolithography, digital light processing

and selective laser sintering.

In this regard, according to prototypes shown in

figure 1 and 2, the employment of designs involving

kinematic chains and joints forming mechanisms has

been found to enhance the shape-morphing or

4D Printed Surgical Devices: Current Capabilities and Challenges

161

metamorphic ability of these devices. Lightweight

design is promoted by means of topology

optimizations as shown in the actuator’s structure.

Current research trends include the exploration of

the possibility of printing these kinds of designs in

NiTi using SLM technology, for which the various

polymeric 4D printing technologies and materials

used here provide interesting insights.

Furthermore, for supporting researchers working

in the field, table 2 provides a summary of tolerances,

results and proposed good practices for different

printed joints and polymeric additive manufacturing

technologies applicable to the 4D printing of

conceptual medical devices. Among them, the

various applicable tolerances for reaching adequately

movable links in 4D printed mechanisms, for the

different geometries, materials and technologies

used, are highlighted, and constitute relevant design

guidelines.

4 FUTURE LINES

There is room for improvement in the world of 4D

printing, and some challenges should be overcome

before these procedures make a real impact in the

medical arena:

Further progress must be made in the knowledge

of the mechanisms that stimulate the extra dimension

added by this type of technology, as well as the

control of the displacements generated with the

intention of obtaining a better programmability of the

materials.

Additionally, the usage and development of new

processes that enhance the current manufacturing and

surface finish restrictions, so that the finished

products can be used in sectors with strict regulatory

requirements, like the medical industry.

In this study some preliminary designs of joints,

mechanisms and structures for medical actuators, for

example for surgical practice, but with potentials for

biomedical robotics and artificial limbs, have been

presented. Polymeric 4D printing has verified their

manufacturability and serves as a set of technologies

for planning the creation of similar geometries with

higher-performance materials and technologies, as

has been discussed.

Towards the future, other designs linked to shape-

morphing or evolutive prostheses, fostering

minimally-invasive surgical procedures and capable

of evolving with patients, according to their healing

and growth processes, should be explored. Some

applications in the emergent area of 4D bioprinting,

within the fields of tissue engineering, regenerative

medicine and biofabrication, are also foreseen.

5 CONCLUSIONS

The increase in demand for customized devices has

led to an impressive growth in 3D printing over the

last few years. The ability of certain materials to add

a new dimension, 4D printing, also allows the

possibility of extending the functionality of devices,

as well as their useful life. It is crucial to consider

aspects like printing direction, design tolerances, and

Table 2: Summary of tolerances, results and proposed good practices for different printed joints and polymeric additive

manufacturing technologies applicable to the 4D printing of conceptual medical devices.

BIODEVICES 2023 - 16th International Conference on Biomedical Electronics and Devices

162

surface finish in order for our 4D printed product to

be effective because these elements collectively

determine whether a product is valid or not.

The printing direction influences the device to

show better mechanical properties if the chosen

direction is the right one. In addition, this direction

influences the surface finish, another of the factors

mentioned, since, depending on the printing direction,

the layer-by-layer effect will be different, having

relevance in final parts as well as in parts that need

post-processing to obtain the desired finish.

Finally, tolerance control is vital in the design

phase, being a relevant factor in the performance of

actuators, mechanisms and joints that may be

integrated into a final product.

This article tries to show that 4D printing is useful

and a reality today, but it also demonstrates that a

proper design phase, if possible, is more relevant than

in conventional manufacturing methods, since a

number of factors that affect the quality and

performance of the final product are brought to light,

but once they are successfully controlled, they allow

us to squeeze the most out of additive manufacturing

using new innovative materials, taking advantage of

the benefits from the point of view of customization

of the final product, making this technology being

used in leading sectors such as medicine, automotive

or aerospace industry.

ACKNOWLEDGEMENTS

The research presented has been supported by the

following research and innovation project:

“iMPLANTS-CM: impresión de metamateriales

empleando aleaciones con memoria de forma y

gradientes funcionales para una nueva generación de

implantes inteligentes”, funded by the “Convocatoria

2020 de ayudas para la realización de proyectos

sinérgicos de I+D en nuevas y emergentes áreas

científicas en la frontera de la ciencia y de naturaleza

interdisciplinar” funded by Comunidad Autónoma de

Madrid (ref. del proyecto: Y2020/BIO-6756).

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