1

Key challenges for the next generation of

wearables

Mohammad Amin Rezaei

The material in this tutorial is based in part

on IEEE Spectrum and my own research.

For more information, please write to

m.amin.rezaei@email.kntu.ac.ir

© 2020 K. N. Toosi University of Technology

1.Introduction

Wearable sensors have received

considerable attention since they enable

continuous physiological monitoring

toward maintaining an optimal health

status and assessing physical

performance. Different wearable sensors

have gained significant interest over the

past few years. Wearable sensors can be

defined as devices which can be worn by

humans to track human health. Based on

the advances in fabrication of sensors,

wearable health sensors have been

developed for several purposes. In this

regard, various wearable sensors have

been fabricated to recognize different

disease. Wearable sensors can be divided

to two main groups: (a) physical, and (b)

electrochemical [1].

Wearable health monitoring systems

are considered as the next generation of

personal portable devices for

telemedicine practice. These systems are

based on monitoring different kinds of

biological signals released by human

beings through saliva, urine, breathing

and epidemic skin perspiration.

Scientists identified three engineering

problems that must be tackled for an

emerging class of biochemistry

wearables: flexibility, power, and

treatment delivery. In this report, the

problems mentioned in the following

sections are discussed, in the second

section discussed material characteristics

of this devices, third section introduce

biofuel cell as power that said challenges

and solution and section four, More

advanced biochemistry wearables could

also deliver drugs or chemicals

themselves in addition to sensing, where

one device would be capable of

performing delivery the drug needed

subcutaneously through microneedles

that this section mentioned microneedles

delivery strategies.

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2. Desired features for wearable

healthcare devices and Materials design

strategies

wearable healthcare devices design

concepts have several ideal

characteristics, e.g., stretchability,

ultrathin, biocompatibility,

biodegradability, and self-healing all of

which will be highlighted in this section.

2.1 Stretchability

In recent years, the application of brittle

and hard like metals and silicon in wearable

device requiring large deformation has been

limited. A stretchable sensor with high

performance and elastic mechanical

response is an ideal choice for the next

generation of health care applications. Good

tensile properties make the conformal

contact between the device and the dynamic

complex structure skin have high spatial and

temporal resolution, which enhance

collecting the signal from the skin interface.

In the past decade, the rapid development of

new concepts of structural design, new

nanomaterials, manufacturing technology

and applications have facilitated to great

advances in stretchable electronic

technology. At the device level, there are two

main forms of retractable design, i.e.

essentially retractable materials or through

appropriate geometric layout of

conventional materials.

Recent advances in the development of

stretchable materials have enabled many

inherent stretchable devices to be realized.

For instance, Bao et al. proposed a design

concept for scalable semiconductor

polymers, including the introduction of

chemical groups to facilitate dynamic non-

covalent crosslinked of conjugated polymers.

The results show that the high field-effect

mobility of the polymer can be recovered

even under 100 % strain after 100 cycles.

Figure 1.Schematic of the conjugated polymers under

stretching and releasing

The above research provides a general

platform for integrating other intrinsically

stretchable polymer materials and makes it

possible to manufacture the next generation

of stretchable wearable device.

Another way to keep good strain-resistant

conductivity and stretchability is to fabricate

the conductive composites. Metal

nanoparticales (NPs), nanowires (NWs),

nanosheets and carbon nanotubes (CNTs)

can be combined with stretchable

elastomers. Cao et al. reported a composite

ink consisting of adhesive rubber and soluble

silver salts. This ink can be placed directly on

a ballpoint pen and written on a stretchable

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substrate, thus making stretchable sensors

and interconnects with the wearable

electronic devices. However, for intrinsically

stretchable devices, a challenge is to find

sealed, stretchable packaging materials.

2.2 Ultrathin and conformality

the traditional real-time monitoring

equipment has some serious defects, such as

rigid structure, high power consumption and

limited function, which brings inconvenience

to daily life and limits the medical

application. An attractive way to solve these

problems is to develop flexible biomedical

sensors that provide conformal and ultra-

sensitive properties for invisible links

between humans and electronic devices. In

this regard, a variety of concepts have been

adopted to achieve large-scale and close

contacts with conformal skin, including the

development of active sensing matrices and

the fabrication of ultra-thin, low modulus,

light weight, flexible and stretchable

membranes for conformal lamination on

human skin surfaces. Among these, it is

necessary to reduce the thickness of devices

so as to make the electronic system more

imperceptible, flexible and conformable,

which is particularly conducive to the skin

and implantable electronic products. Despite

these advantages, the polymer properties

and ultra-thin geometry of the substrates still

pose enormous challenges to fabrication. In

order to assist this process, rigid support

substrates with sufficient adhesion are

usually added to adapt to harsh

microprocessing circumstances.

However, after fabrication the stripping of

ultra-thin and fragile polymer films from the

supporting substrates will bring new

problems to the integrity and performance

maintenance of wearable systems.

Therefore, in order to solve this problem,

sacrificial layer can be lead-in between rigid

support and polymer matrix. Then, the last

layer of the sacrificial layer is removed by wet

etching, and the flexible electronic system on

the polymer substrate is obtained. For

instance, Lee et al. studied an ultra-thin,

comfortable, vibration-responsive e-skin

that can detect skin acceleration, which is

highly linear with the sound pressure of Ti/Cu

film as a sacrificial layer. Compared with

commercial speech recognition equipment,

the device has higher sensitivity and flat

frequency response in the speech frequency

range, which can clearly recognize human

speech in the presence of environmental

noise or masks. Because of its low stiffness

and ultra-thin structure, the device shows

comfortable fit on the skin. Their research

demonstrates great potential as the next

generation of speech recognition devices for

human-machine interface (HMI) and internet

of things (IoT) applications, which need

accurate voice information even in harsh

acoustic environments.

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Figure 2.(a) Schematic of the ultrathin vibration-

responsive devices. (b) Optical image of the sensor

attached on the neck skin.

2.3. Biocompatibility and biodegradability

In real-time medical applications, there is

a need to be closely related to the biological

interface, so it is hoped that wearable

medical equipment will not cause additional

health threats and comfort, and avoid

restrictions on daily activities. The

biocompatibility between human body and

wearable sensors is the key to avoid

triggering immune response. In general,

compared with the matrix materials, active

materials have higher risk. It is reported that

due to the needle-like shape and small size of

CNTs (carbon nanotubes), injecting large

quantities of them into the lungs of mice can

cause asbestos-like pathogenicity.

Therefore, for future healthcare application

of nanostructured materials, there is a great

need for more in-depth understanding of

immune response and a definition of

exposure criteria under various

circumstances, such as skin contact, intake,

inhalation and injection. It is found that GaN

has good biocompatibility and is an ideal

choice for the application of high-speed

electronic and optoelectronic devices. Many

biocompatibility biosensors based on GaN

are proposed. Recently, Rogers et al.

proposed a silicon-based multifunctional

brain sensor. The sensors for medical

monitoring are completely biodegradable.

Multiple immunohistochemical studies of

brain tissue after implantation (2, 4 and 8

weeks) showed that the sensor and its

byproducts dissolved in intracranial space

were biocompatible. This traditional

semiconductor is biocompatible and can be

used for biomedical implants and health

monitoring. At the same time, a number of

organic active substances, like

polyethylenedioxythiophene (PEDOT) and

polypyrrole (PPy), have been widely found to

be biocompatible which can be used for

monitoring cytoactive. Carbonized

commodities such as tissues and cotton also

have great potential in building

biocompatible wearable sensors.

Recently, biomaterials have been

extensively studied as substrates and active

components of various biocompatible

electronic devices. The highly conductive

carbon nanofibers films extracted from silk

fabrics which were treated by simple thermal

treatment without any chemical treatment

were reported by Wang et al. This kind of

carbon material has the characteristics of

large production capacity, natural renewable

resources, and is beneficial to the

environment and human beings. Although

the electronic performance of these devices

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will require considerable optimization to be

useful for wearable sensor applications, they

show a promising biocompatibility route.

Transient electronics technology is a new

emerging technology in recent years. Its goal

is to develop systems and equipment that

can be absorbed by human organisms or

environmental microorganisms at

programmed speed. Biodegradable and

Water-soluble materials are often

investigated to be used in transient

implantable devices. For example, Bao et al.

designed and manufactured a highly

sensitive flexible pressure sensor made

entirely of biodegradable materials. The

sensor has good sensing performance and

can be used as a wearable medical device for

continuous cardiovascular monitoring,

including pulse signal acquisition of radial

artery, carotid artery and human femur. This

is the first step in sensor manufacturing.

More complex and degradable sensors can

be used in human biomedical applications to

avoid further surgical intervention and

reduce waste generation.

Figure 3.(c) Illustration of the biodegradability healthcare

sensing systems. (d) Photograph of the as-fabricated

system.

2.4. Self-healing

At present, wearable healthcare device is

often limited by robustness because its

components are easily scratched and

damaged on the human body, which may

damage its function and reduce its

perception performance. Ideal wearable

equipment can not only retain its electronic

function, but also heal itself after slight

mechanical damage by restoring the

electrical and mechanical characteristics.

Very recently, great progress has been

obtained in the development of self-healing

chemistry, but only a few self-healing

polymer systems have been used in

electronic applications. Composite materials

are commonly used as self-healing materials,

which are filled with drug-loaded capsules or

conductive particles. The composite

hydrogel has strong and fast self-healing

ability. The self-healing efficiency induced by

reversible Ag-thiolate bond is as high as 93 %,

which provides a good electromechanical

performance in deep impression for the self-

healing of hydrogels. In addition, by

combining the conductive one dimensional

network with the tensile self-healing

polymer, nano-materials/ polymer

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composites can reconstruct and restore the

mechanical and conductive properties.

Figure 4.Schematic illustrations of the self-healing

mechanism for composite polymer matrix.

2.5 Flexible substrates

For traditional rigid medical sensors, glass,

Si or SiO2 are used as substrates, which is not

the key factor to determine the properties of

the device. Yet, the substrate material is a

key consideration in the development of

wearable device structure. Low roughness

and high mechanical flexibility are the high

requirements of substrate materials.

Generally, metal foils, rubber and elastic

polymers are extensively choose as

substrates because of their great mechanical

elasticity, good chemical resistance and

thermal stability. Many commercial

polymers and elastomers can be used as

substrates for flexible and stretchable

electronic products. PDMS

(polydimethylsiloxane) is a commercial

silicon elastomer with good processability,

hydrophobic, non-flammable, non-toxic and

high tensile strength (up to 1000 %). It has

been used in wearable sensors, prostheses

and microfluids. The inherent flexibility and

expansibility of PDMS make the

corresponding devices respond well to

compression strains, tension and torsion. In

addition, the extensibility of PDMS can be

improved by geometric structure to meet the

application requirements of coplanar

devices. Therefore, PDMS matrix has good

biocompatibility, soft, curved and extensible

skin surface, which is suitable for biological

health applications. The sensitivity of

microstructured thin films is much higher

than that of unstructured elastomer thin

films, and can be adjusted by different

microstructures. These advantages expand

its applicability as a large area substrate for

wearable sensing systems. In certain

specially appointed applications, some other

substrates like Ecoflex or PU may also be

beneficial. Although silicon elastomer films

have been widely used as substrates for

wearable sensing systems, they still face

some key challenges: (1) Compare with

silicon wafers commonly used in electronic

industry, the surface quality of silicon

elastomers is difficult for integrated

technique; (2) The problem of long-term

stability of silicon elastomer is widespread

due to their high permeability; (3) The poor

thermal stability of the silicon elastomer

seriously limits the preparation temperature

of the electrode.

In addition to synthetic substrates, some

natural materials have also been opened for

manufacture of wearable system substrates.

Biomaterial is the largest material system in

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nature. It has good biocompatibility,

biodegradability, versatility, sustainability

and low cost. Fibers and textiles are ideal for

wearable sensing systems because they are

supposed to be the closest natural materials

to human skin. For instance, natural silks are

not only an abundant and attractive

biomaterial, but also satisfy the mechanical

requirements of irregular deformation.[2]

3.Power

As the demand for wearable sensors

grows, so too does the demand for relevant

power sources. Wearable sensors are

increasingly becoming “energy-hungry” in

order to meet the increasing demands of

detecting multiple parameters

simultaneously, performing complex data

analysis, communicating with other sensors,

devices and data transmission. The scientific

community that has interest in solving this

issue has broadly focused on three aspects –

develop low-power energy efficient devices,

compact, energy dense wearable power

sources and adaptive algorithms for

intelligent, low-power consuming

electronics. Advances in the field of wearable

energy devices have not been able to cope

with the speed at which wearable sensor

technology has progressed. Limitation

caused by inefficient wearable power

sources is a common roadblock for effective

adaptation of wearable sensors. It is thus

imperative to develop wearable power

sources that are in the vicinity of the target

wearable electronic device in order to

obviate the need for long wires and ease of

complete device integration with the body.

In this regard, several avenues, such as

wearable batteries, supercapacitors, solar

cells, biofuel cells, thermoelectric, and

piezoelectric/triboelectric, have been

explored by researchers to realize wearable

power devices. In biochemical wearable

batteries refer to biofuel cells.

3.1 biofuel cells challenges

Biofuel cells that harvest biochemical

energy using biological components

represent an attractive “green” alternative

for various wearable and implantable

applications. Biofluids, such as sweat, tears,

interstitial fluid and blood, are rich with

metabolites that can be exploited as fuels by

wearable biofuel cells to generate usable

electrical energy. Moreover, biofuel cells act

as selfpowered sensors and hence these

systems mandate lower energy

requirements as compared to conventional

chemical sensors. This is attractive for

wearable applications where energy supply

is usually a critical challenge. Although, these

attributes sound striking, today’s wearable

biofuel cells face several daunting challenges

that hamper their use as viable energy

sources for various wearable applications. To

begin with, enzymes that convert the fuels

into energy are quite labile and lose their

catalytic properties in presence of harsh

conditions that may be encountered in

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wearable electronics, e.g. varying

temperature, humidity, or presence of

certain chemical species. The operational

lifespan of the wearable biofuel cells is also

short compared to the energy demands of

the wearable electronics. In addition,

continuous reproducible flow of high

concentration of biofuel to the wearable

biofuel cells is not possible, since the flow of

these biofuels is regulated by the human

physiology. Thus, continuous generation of

constant power is quite challenging. Also, the

power generated by these devices is low with

the output voltage changing gradually with

time. Furthermore, wearable biofuel cells

must possess stretchability to endure

mechanical deformations common for

wearables applications. Developing body

compliant stretchable biofuel cells which

protect the enzymes and other chemical

reagents from degrading/leaching under

changing conditions is certainly a challenge.

A concoction of these challenges currently

impedes the utilization of wearable biofuel

cells to directly power electronic devices.

3.2 solution

By properly addressing the challenges facing

wearable biofuel cells, these devices could

become a viable energy source for on-body

applications. A handful of researchers are

attempting to address the issue involved in

developing soft, stretchable biofuel cells by

combining advances in materials science. For

example, our group recently demonstrated a

highly stretchable glucose biofuel cell that

could be repeatedly stretched by 300%

without much effect on its power generation

ability (Figure 5). Similarly, Ogawa et al

demonstrated a stretchable textile-based

biofuel cell that could be repeatedly

stretched by 50% with the small impact on

the power output (20-30% loss). Similar

efforts must be made to develop body-

compliant biofuel cells. The problem of long-

term stability of wearable biofuel cells under

diverse conditions could be addressed by

incorporating enzyme stabilizing agents or

nano/microsized hybrid materials and by

providing biocompatible microenvironments

to the immobilized enzyme. Researchers

could also look into developing multi-fuel

biofuel cells, complete oxidation of fuels and

rechargeable biofuel cells to address the

issue of continuous supply of energy. At the

same time, researchers should leverage

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nanomaterials to further enhance the power

conversion efficiency of the biofuel cells.[3]

Figure 5.highly stretchable biofuel cells

4. Treatment delivery

4.1 Microneedles: delivery strategies

Microneedle arrays (MN) are minimally

invasive devices that by-pass the SC barrier,

thus accessing the skin microcirculation and

achieving systemic delivery by the

transdermal route. MN (50– 900 µm in

height, up to 2000 MN cm2) in various

geometries and materials (silicon, metal,

polymer) are produced using

microfabrication techniques. MN are applied

to the skin surface and painlessly pierce the

epidermis, creating microscopic aqueous

pores through which drugs diffuse to the

dermal microcirculation. MN are long

enough to penetrate to the dermis, but are

short and narrow enough to avoid

stimulation of dermal nerves or puncture of

dermal blood vessels.

Solid MNs are normally employed in the

so-called ‘poke with patch’ approach. Solid

MN are applied to the skin and then

removed, creating transient aqueous

microchannels are created in the stratum

corneum. Subsequently, a conventional drug

formulation (transdermal patch, solution,

cream or gel) is applied, creating an external

drug reservoir (Fig. 6A). Permeation through

these microchannels occurs via passive

diffusion. The main limitation of this

approach is the requirement for a two-step

application process, which may lead to

practicality issues for patients. The materials

used to produce solid MN are typically

silicon, metals and polymers.

Coated MNs are prepared by coating solid

MN with a drug formulation prior to skin

application. After insertion of coated MN

arrays into the skin, the coated drug

formulation will be dissolved and deposited

in the skin (Fig. 6B). This delivery strategy is

typically referred to as ‘coat and poke’.

Coated MNs have been employed for the

rapid cutaneous delivery of macromolecules,

such as vaccines, proteins, peptides and DNA

to the skin. This type of MN allows a simple

one-step application process, but its main

limitation is the restricted amount of drug

that can be coated onto the finite surface

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area of the MN structures. Accordingly, the

use of coated MNs is restricted to potent

molecules/drugs. Various techniques have

been developed to efficiently coat the

individual MN shafts in MN arrays.

The third type of MN are dissolving MNs.

They are made by micro-moulding soluble

matrices, generally a biocompatible polymer

or sugar, including the active substance. The

skin insertion of the array is followed by

dissolution of the MNs tips upon contact with

skin interstitial fluid. The drug cargo is then

released over time (Fig. 6C). The release

kinetics of the drug depends upon the

constituent polymers’ dissolution rate.

Therefore, controlled drug delivery is

achievable by adjusting the polymeric

composition of the MN array, or by

modification of the MN fabrication process.

Hollow MNs allows the delivery of a

particular medication into the skin via the

injection of a fluid formulation through the

inserted hollow needles (Fig. 6D). This type of

MNs allows continuous delivery of molecules

across the skin through the MN bore using

different methods: diffusion or pressure- or

electrically driven flow. Such systems are

possibly capable of delivering larger amounts

of drug substances in comparison to solid,

coated and dissolving MNs. Hollow MNs are

made from a range of materials, including

silicon and metal, glass, polymers and

ceramic. The main limitations of hollow MNs

are the potential for clogging of the needle

openings with tissue during skin insertion

and the flow resistance, due to dense dermal

tissue compressed around the MN tips

during insertion. The first limitation can

possibly be overcome by using an alternative

design to locate the bore-opening at the side

of the MN tip. Partial needle retraction

following insertion may also enhance fluid

infusion, due to relaxation of the compressed

tissue around the tips. However, use of liquid

drug formulations will require a suitable,

possibly complex, reservoir and liquid

formulations are notoriously unstable,

particularly at the elevated ambient

temperatures found in the developing world.

relatively new type of MN arrays are

prepared from hydrogelforming matrices.

Such systems were first described recently by

Donnelly et al. This novel strategy involves

integrated systems consisting of crosslinked

drug-free polymeric MN projecting from a

solid baseplate to which a patch-type drug

reservoir is attached. After application of the

MN array to the skin, the inserted needle tips

rapidly take up interstitial fluid from the

tissue, thus inducing diffusion of the drug

from the patch through the swollen

microprojections (Fig. 6E). These systems are

manufactured using aqueous blends of

specific polymeric materials, namely

poly(methyl vinyl ether-co-maleic acid)

crosslinked by esterification using

poly(ethyleneglycol). Garland et al. showed

that drug delivery can be tailored by

modulating the crosslink density of the

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hydrogel matrix. Importantly, hydrogel-

forming MNs are removed intact from skin,

leaving no measurable polymer residue

behind. However, they are sufficiently

softened to preclude reinsertion, thus

further reducing the risk of transmission of

infection. Other polymers that can be used to

prepare hydrogel-forming MNs are chitosan,

PLGA and poly(vinyl alcohol). With these

alternative polymer systems, however, the

drug is included inside the hydrogel-forming

MN patch rather than in an external patch,

thus limiting the quantity of drug that can be

delivered.

Figure 6. A schematic representation of five different MN types used to facilitate drug delivery transdermally. (A)

Solid MNs for increasing the permeability of a drug formulation by creating micro-holes across the skin. (B)

Coated MNs for rapid dissolution of the coated drug into the skin. (C) Dissolvable MNs for rapid or controlled

release of the drug incorporated within the microneedles. (D) Hollow MNs used to puncture the skin and enable

release of a liquid drug following active infusion or diffusion of the formulation through the needle bores. (E)

Hydrogel-forming MNs take up interstitial fluids from the tissue, inducing diffusion of the drug located in a patch

through the swollen microprojections.

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References

[1] Nasiri, Sara, and Mohammad Reza Khosravani. "Progress and challenges in

fabrication of wearable sensors for health monitoring." Sensors and Actuators A:

Physical (2020): 112105.

[2] Lou, Zheng, et al. "Reviews of wearable healthcare systems: Materials, devices and

system integration." Materials Science and Engineering: R: Reports 140 (2020): 100523.

[3] Bandodkar, Amay J., Itthipon Jeerapan, and Joseph Wang. "Wearable chemical

sensors: Present challenges and future prospects." Acs Sensors 1.5 (2016): 464-482.

[4] Larraneta, Eneko, et al. "Microneedle arrays as transdermal and intradermal drug

delivery systems: Materials science, manufacture and commercial development."

Materials Science and Engineering: R: Reports 104 (2016): 1-32.