Navigating the future of healthcare with innovations and challenges in implantable battery technology for biomedical devices

Abstract

Human Machine Interfaces and biomedical prosthetics are advancing rapidly, merging human and machine capabilities. These innovations offer tremendous benefits, but the effectiveness of implantable medical devices (IMDs) hinges on the reliability of their batteries. This article explores the various battery technologies used to power IMDs. The review focuses on the unique characteristics, identifies current challenges and future opportunities in the design and enhancement of batteries for IMDs. The review delves into different battery technologies, emphasizing advancements in electrode materials, biocompatible electrolytes, innovative power delivery systems, and novel energy harvesting techniques. It explores the potential of incorporating new nanomaterials, wireless charging solutions, and bio-energy harvesting methods in battery design. Furthermore, the review discusses recent progress in AI-powered implantable battery health monitoring. The study identifies key challenges in existing battery technologies, such as issues with energy density, cycling stability, and longevity, and points out possible enhancements facilitated by introducing advanced materials and cutting-edge technologies. The review also highlights the promise of AI techniques in improving the health monitoring of implantable batteries. The review highlights the critical need to address the stringent requirements of implantable battery design to drive the advancement of healthcare technologies. By adopting novel materials, innovative charging, and energy harvesting methods, along with AI-driven health monitoring, substantial improvements in implantable battery performance can be achieved, thereby enhancing the reliability and effectiveness of biomedical prosthetics and implantable devices.

Article Highlights

New energy-harvesting techniques could power IMDs without needing frequent battery replacements.

Use of novel nano materials could propel advancements in implantable batteries enabling IMDs last longer and work more efficiently.

AI-powered monitoring predicts battery health, improving the reliability and safety of medical implants.

Introduction

Battery powered IMDs are a boon to the human community, as they could provide an enhanced quality of life for patients in many cases. Implantable devices say, cardiac devices and neural implants, drug delivery systems, biosensors or patient monitoring systems function as life-supporters, which highlights their importance in medical field [1]. The performances of such implants are determined by their uninterrupted and safe service to patients which demands reliable and long-lasting batteries for powering medical implants. Such a persistent service with inherent biocompatibility and safety has also kindled researches to search for efficient power sources to drive these devices sustainably [2]. An optimal solution that resulted from this search space is the integration of energy harvesting technologies with IMDs. Sustainable energy harvesting is a break-through in the field of IMDs that opened up avenues for self-powered implantable devices [3] [4]. Such self-powered devices rely on piezoelectric, triboelectric or ultrasonic principles for scavenging biomechanical energy from human motion and internal organ functioning in order to power biomedical implants sustainably [5]. Examples of such energy harvesting principles extend from utilizing muscle contractions and pressure fluctuations in cerebrospinal fluid respectively [6] [7]. Even there are researches that suggest powering pacemakers from heartbeats [8]. For instance, piezo-electric energy harvesters fabricated by introducing single crystal, lead indium niobate-lead magnesium niobate-lead titanate (PIN-PMN-PT) which has higher piezoelectric charge coefficient along with Ni stressor and adapting a metal–insulator-metal (MIM) structure were demonstrated to generate 20µA, with a current density of 3.08 µA/mm3 [9]. Electromagnetic energy harvesters are emerging as a promising alternative for triboelectric, piezoelectric energy harvesters [10]. Adaptable electromagnetic energy harvesters based on dynamic coil switching and excited by swaying movement of moving arm and simple axial rotation were demonstrated to increase the average power generated between 63.3 and 79.6% as compared to non-adaptable conventional counterpart [11]. Even levitation based electromagnetic energy harvesters capable of generating energy from mechanical vibration was developed and experimentally verified. It was demonstrated to produce 635 mW of power using a dynamically adaptable coil-array architecture [12].

However, self-powered device developments are not devoid of challenges and the requirement of wireless data transmission by certain devices like drug delivery microchips and multi-functional implantable sensors that aid in real-time monitoring [13, 14], demand reasonable amount of power [15] and thus, bio-energy harvested is not sufficient in those cases. Researcher striving hard to utilize bio-energy for implantable devices, recommend optimising microelectronic device design for low power consumptions. However, Wireless neural interfaces and closed-loop neuromodulation devices consumes reasonable power to record and stimulate therapeutic interventions [16, 17]. In addition, review of transdermal, skeletal and micro-chip drug delivery systems emphasizes the importance of sustainable power sources while designing implantable devices [18–20]. All these studies related to implantable drug delivery and monitoring devices reveal the insufficiency of bio-harvested energy and demand for, compact and biocompatible batteries to act as power sources. These demands necessitate developing batteries with good reliability [21]. Thus, the development of power sources for implantable devices is two-fold with (i) researches searching for biocompatible and reliable batteries and (ii) researches that strive to harvest bio-energy for powering up devices [22]. Thinking out of the box, the integration of energy harvesting technologies and biocompatible power sources could essentially advance the capabilities of these implantable devices. Summary of recent advancements in battery technologies towards advancing implantable batteries is shown in Fig. 1.

Fig. 1 [Images not available. See PDF.]

Implantable batteries—current scenario

In conclusion, the development of IMDs relies on the continuous innovation of power sources to enable sustainable and reliable operation. Energy harvesting technologies, wireless power transmission, and biocompatible power sources play a crucial role in advancing the capabilities of implantable devices, enabling new possibilities for drug delivery, monitoring, and therapeutic interventions. Motivated by the significance of medical implants and the need for sustainable power sources for their reliable operation, this study is focused to present the recent advancements in batteries and novel energy-harvesting techniques for powering IMDs. An exhaustive search of Google Scholar and Scopus database is carried out to identify research articles pertaining to the topic. Article were selected by searching the databases for self-powered biomedical implants, energy harvesting for biomedical devices, novel implantable batteries, self-powered biosensors, neural implants, wireless power transfer, self-powered drug delivery, battery less drug delivery, implantable medical devices and challenges, implantable batteries and challenges. This review presents the recent, novel battery technologies and energy harvesting techniques introduced for powering biomedical implants, along with the future battery technologies, and challenges in transferring technology from research to clinical use. And thus, could act as a valuable source of information for researchers and academicians.

Battery technologies and their suitability for implantable devices

Because of their innate significance in driving the sustainability of IMDs, research and development related to implantable battery technologies present innumerable opportunities to explore. Suitability of lithium-ion, lithium-polymer, thin-film, solid-state battery technologies in terms of their energy density, size, lifespan, rechargeability, and biocompatibility were widely studied. Due to their intrinsic high energy density and long lifespan, lithium-ion batteries lead the chart in powering medical implants. However, rigidness and heaviness induced discomforts have necessitated the importance of incorporating flexibility and stretchability to Li-ion batteries and this is made feasible by using electrically conducting carbon nanotube fiber springs [23]. However, such flexibility resulted in relatively low capacity and poor cycle stability of the cathode compared to that of the anode [24]. As safety and biocompatibility issues rose, researchers started focusing on solid-state lithium batteries with a solvent-free composition [25], biocompatible ionic liquid–biopolymer electrolyte-enabled thin and compact magnesium–air batteries and biodegradable thin-film magnesium primary batteries with silk fibroin–ionic liquid polymer electrolyte [26]. Moreover, nature inspired interfaces for realising sewable, weavable, and washable fiber zinc batteries were investigated for wearable power textiles [27]. Silk-fibroin ion-exchange membranes could generate power from salinity differences [28] and those that depend on piezo-electric principles [29] were proposed as good power source for implantable devices. This reveals that, the limitations posed by conventional batteries say limited lifetime, rigid architecture, and inferior energy density are addressed by recent developments in order to improve the efficiency of batteries towards biomedical use [30]. In addition, most recent developments enable auto-charging techniques and wireless power supply options for implantable batteries [31, 32]. Moreover, electrochemical, mechanical properties, degradability, biocompatibility are emphasized while developing implantable batteries for aqueous environment [33]. This collective reveals the advancements under battery technologies in order to improve the performance of batteries that power biomedical implants.

Electrical characterizations of batteries

Electrical characteristics of novel batteries and power harvesting techniques were verified by conducting different experiments ranging from simulations & modelling, in vivo, ex vivo, bulk proto-types. In the case of piezoelectric ultrasonic energy harvester (PUEH) resonant frequency is found by immersing the diaphragm in water and passing burst-mode sinusoidal signals. Signals in the range 100 kHz to 1 MHz is applied and the response of PUEH is monitored. Thin-diaphragm design resulted in a wide bandwidth of 170 kHz to 820 kHz resulting from the overlapping 1st and 3rd resonant peaks. In addition, to eliminate the interference of near-field, a sinusoidal signal of amplitude 10 Vpp and frequency 200 kHz is applied between two PUEH and distance is varied in range (1 – 4 cm) and the optimal distance for different maximal output is determined. Practically, it is difficult to implant device at precise location which often results in a generated voltage deviated from experimental results. This issue is rectified by adjusting frequency within the broad band of PEUH. The energy harvesting capability with varied frequency is demonstrated by recreating the device with bulk lead zirconate titanate (PZT) transmitter and a PUEH receiver in water tank. The power output for different frequency inputs is quantified and it is found that a power of 84.3 nW is achieved for 370 kHz. In addition, it is found that by applying an Ultrasound (US) intensity of 700 mW/cm² power as high as 59.01 μ W with an intensity of 2.9 mW/cm² can be generated by the design. Experimental test results match well with COMSOL simulation results obtained using finite element analysis (FEA) model [3]. In another experiment, triboelectric energy harvester was investigated in vivo by simulating the human body internal environment by using a Yorkshire pig. It was found that the energy harvester can generate 10 V and 4 μA output without external signal conditioning. In addition, fluctuations in output voltage with respiratory motion was recorded, which signify its capability to monitor respiratory functions [4]. When PZT energy harvesters are tested in animal models, different consciousness states are used to determine the device characterisation say with adequate anaesthetic control, recovering state and conscious state. The device could respond well in proportion to heart function under these three states producing 2.3 V, 2.2 V and 0.3 V peak-to-peak voltages respectively [5]. A schematic of the piezo-electric energy harvesting device is depicted in Fig. 2.

Fig. 2 [Images not available. See PDF.]

Piezo-electric bio-mechanical energy harvesting device schematic (Reproduced from [5] Under the Creative Commons CC BY license, http://creativecommons.org/licenses/by/4.0/). a Piezo-electric capacitor, PI encapsulate stacked over Silicon substrate b Interconnection of the layers c device with flexible cable d Electrical circuit equivalent

Electromagnetic induction-based energy harvesters are characterised for output voltage generated over time for different input signal frequency, and different coil connections. And experimental results demonstrated that by switching coil connections autonomously, output power generation can be varied. A practical electromagnetic (EM) generator could produce 5.1 mW and 4 mW power by using 16 and 8 coils respectively [11]. The wireless charger Integrated circuit (IC) used with wirelessly powered neural interface has an efficiency of > 85% over a wide range of supply voltage say, (4.35 to 16) V and current say, 0.1 A to 2 A [16]. In an experiment based on witricity based wireless power transfer, coupled mode theory (CMT) was used to analyse interaction between resonators. Theoretical and numerical results confirm efficient energy exchange at resonant frequency is feasible provide there exists a strong coupling between the resonators [20]. Primary limitation with wireless and witricity-based power induction are the requirement of an even distribution of electromagnetic field for strong coupling and researches towards addressing this demand could advance wireless power transfer-based implantable.

Integrated solid state batteries with chitosan choline nitrate CS-[Ch][NO₃] polymer electrolyte film was characterized using an optical microscope and results depict that addition of [Ch][NO₃] in weight ratio 1:1 to chitosan improved the electrolyte film’s ionic conductivity significantly 7.3 × 10^–4 S cm⁻¹ whereas, pure Chitosan films had ionic conductivities less than 10^–8 S cm⁻¹ [21]. Whereas, a silk-fibroin choline nitrate (SF-[Ch][NO₃] (1:3)) composite electrolyte exhibited an ionic conductivity of 3.4 mS cm⁻¹and the battery exhibited a capacity of 0.06 mAh cm⁻² at a current density of 10 μA cm⁻² [26]. Chitosan and silk-fibroin electrolyte-based batteries exhibit good biodegradability and thus are recommended for powering short-time transient biomedical implants, used for delivering drugs and monitoring tissues under regeneration. Rechargeable solid-state fiber batteries designed using Zn/MnO₂ was demonstrated to have a volume energy density of 91 Wh·L⁻¹ maintaining a capacity of 98% even after 1000 charge-recharge cycles [34]. A summary of the advancements with respect to implantable batteries is presented in Table 1.

Table 1. Advancements in battery technologies powering IMDs

Ref. no	Battery principle	Method/innovation	Research significance/application	Battery performance
Energy harvesting
[3]	Piezoelectric ultrasonic energy harvester	Adjusting frequency of input US	Improved Coupling efficiency	84.3 nW for 370 kHz of US
[4]	Triboelectric energy harvesting	Triboelectric layer active sensor	Self-powered intracorporeal sensor	10 V, 4µA
[5]	Piezoelectric energy harvesting	PZT stack on Kapton substrate using transfer printing	3.95% reduced bending stiffness than present commercial patch	3 V
[6]	Electromagnetic induction generator	Stimulated muscle contraction and linear to rotational motion conversion	Alternate for implantable battery	111 µW
[7]	Piezoelectric energy harvester	Pressure fluctuations of the cerebrospinal fluid	By increasing area and size of energy harvesters power up to 26 nW can be generated	0.62 nW and energy density of 12.6 nW/cm2
[8]	Elastic skeleton with two piezoelectric composites	Natural energy of a heartbeat	Powering cardiac pacemakers	Output current of 15 μA
[9]	Piezoelectric single crystal	Use of excellent piezoelectric charge coefficient materials. PIN-PMN-PT and optimized Ni stressor	Self-powered cardiac monitoring	20µA, current density of 3.08 µA/mm3
[15]	Piezoelectric energy harvester	Single-crystalline Lead Magnesium Niobate (PMN-PZT)	IMDs	17.8 V, 1.74 µA
[22]	Piezoelectric energy harvester	Indium modified crystalline Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 PIMNT	High current output that can power deep neural stimulation	0.57 mA, 11 V
[29]	Porous flexible piezoelectric cantilever	Polyvinylidene fluoride-tri-fluoroethylene thin film with a dual-cantilever structure	Sustained pacemaker operation	0.5 V and 43 nA
Wireless charging
[16]	Medical grade rechargeable 200 mAh Li-ion battery	inductively coupled wireless recharging, sapphire window to prevent physical electrical connection	Wireless neural interfaces	7 Hrs continuous operation between recharges, 2 MHz power link
[17]	Rechargeable 500 mAh Li-ion battery pack	–	Neural interface for recording patient’s neural state	11.3 h of continuous, wireless operation
[20]	Witricity – based, hexagonal mat	Power transfer by two objects with same intrinsic resonant frequency	Higher efficiency and longer operating distance for wireless power transfer	3.84 V to 1.24 V
[32]	Near-field wireless power transfer	Air coupling and alternating magnetic field of 1 MHz	Battery less stimulator	Transmitter can power the implant from 12 cm
Novel battery technologies that have the potential to drive IMDs
[18]	Disposable reverse electrodialysis (RED) battery – Iontophoresis	electroconductive hydrogel to facilitate electron transfer and electrically mobile drug nanocarriers	Transdermal drug delivery system	–
[21]	Polymer electrolyte-enabled biocompatible magnesium-air battery	Choline nitrate embedded in chitosan	Device thickness of approximately 300 μm	1.80 V, 118 µW, power density of 3.9 W L−1
[23]	Flexible and stretchable spring like electrodes for flexible supercapacitors and Li-ion batteries	Aligned multi-walled carbon nano tubes (CNTs)	Flexible electronics and wearable textile	300% more flexible
[24]	Metallic fabric cathode for flexible Li-ion batteries	V2O5 hollow multi-shelled structures	Energy storage in flexible and wearable electronics	Capacity of 222.4 mA h g−1 even after 500 cycles
[34]	Rechargeable solid-state Zn/MnO2 fiber battery	Graphene oxide–embedded polyvinyl alcohol hydrogel electrolytes	Energy storage in flexible and wearable electronics	Maintaining 98.0% capacity even after 1000 charging-recharging cycles
[26]	Bio-degradable Mg batteries, Biodegradable electrolyte	Silk fibroin—choline nitrate Enzymatic degradation of the device in buffered protease XIV solution after 45 days	Transient medical bionics	specific capacity of 0.06 mAh cm−2
[27]	Aqueous zinc batteries (AZBs), polydopamine polymers as organic redox-active cathodes in AZBs	Removal of soluble monomers or oligomers and the boosted ratios of the quinone groups	Fiber electrodes as the threads for next-generation wearable energy-storage devices	Specific capacity of 372.3 mA h g−1 and 80% retention over 1700 cycles
[28]	Reverse electrodialysis devices (RED) batteries	Silk fibroin nanofibril (SNF) membranes regulates ion diffusion to generate electric power	Tissue-integrated batteries for future IMDs	1.58 V, power density of 0.59 mW/m2
[30]	Flexible Symmetric Na-ion micro battery – hetero-nano-mat electrode & biocompatible electrolyte	Simultaneously electrospinning and electro-spraying followed by carbonization. For uniform incorporation of active materials/Carbon spheres into the carbon nanofiber matrix	New implantable power source for bioelectronics	High-capacity retention approximately 98%

Device specific requirements

Cardiac implantable devices

Pacemakers, implantable cardioverter defibrillators (ICDs), and cardiac resynchronization therapy devices are the major cardiac implantable devices [35]. Playing an important role of regulating heart functions, these devices demand tailored requirements from batteries that power them. Longevity is an important factor to be considered while designing cardiac resynchronization therapy [36] and ICDs. However, longevity of battery varies between manufacturers [37, 38] and thus, consistent design has become the need of the hour. Another important factor that affects the longevity of an implantable battery is the amount of ventricular pacing provided by the implant which in turn is patient specific and increases with time [39]. In spite of careful designing, there are some inherent uncertainties that cause a battery failure in ICDs which includes (i) mismatch in patient life expectancy and service life of the cardiac implant [40] (ii) pre-mature battery depletion due to inclusion of low voltage capacitors [41] and (iii) discrepancy between estimated and actual life time of batteries which necessitates reliable predictions [42]. These studies reveal that batteries with good longevity, reliability and energy efficiency could ensure uninterrupted functioning of cardiac implantable devices.

Neural implants and brain machine interfaces (BMI)

Neural implants play an important role in improving the standard of living in many patients, by replenishing neural signals, which includes low-frequency signals from neuron populations to high-frequency action potentials from individual neurons [43]. Being integrated with BMI for proper monitoring and control, neural implants should be capable of recording signal from brain and modulating the signals to be delivered to neurons [44]. The difficulties related to device placement and its trans-receiving abilities demand specialized battery design in terms of both the battery’s physical features and electrical quantities. Biocompatibility, miniaturization, and targeted power delivery to specific regions of the brain [45, 46] are some of the important requirements to be addressed by batteries powering neural implants and BMI devices. The intricacies of the target tissues demand highly biocompatible materials to be used in neural implants which include the battery material too [47]. Further, flexible and miniaturised devices are highly recommended as they could provide conformal contact with the curved cortex surface which aid in stable neural signal processing. When it comes to batteries for neural implants, the prima facia requirements delve around miniaturisation and biocompatibility for ensuring safe delivery of power to the device. With this said, wireless power transmission and biological energy harvesting [48] techniques are heavily explored as they show good bio-compatibility compared to other techniques. Whereas, explorations in terms of using nature-derived materials are also investigated to a vast extend as conventional power sources are the only reliable sources when power demand is high. Thus, when it comes to neural implants’ flexibility, miniaturisation and biocompatibility are primary requirements of conventional batteries design. However, emerging techniques say wireless power transmission and bio-energy harvesting are in infancy and thus, provides opportunities for exploration.

Drug delivery systems and biosensors

Recent developments in implantable drug delivery systems and biosensors have changed the scenario of chronic treatments and rehabilitation setups. Thanks to nanoscience, it is only with their inherent nanoscale bio-sensors, implantable physiological monitoring devices have come to reality [49]. Even drug delivery systems have been designed using nanoscale materials in order to harvest the benefits of miniaturisation [50, 51]. These studies signify the growth of nano-biosensor as IMDs [52]. Drug delivery systems are capable of providing precise and controlled delivery of drugs whereas, biosensors made real-time monitoring of physiological parameters a reality. Such systems in turn depend on efficient powering sources for their proper functioning which demands efficiency, reliability, miniaturisation and safety.

Intrinsic to implantable drug delivery systems are their multi-reservoirs which carry certain amount of medicine which has to be delivered on a predefined schedule and thus such units occupy certain amount of device volume which imposes stringent size reduction to be adapted for every other component attached to it and batteries are not an exception [53]. As size restrictions are the major roadblock in designing batteries for implantable drug delivery systems and monitoring systems, micro-battery technologies were introduced for implantable [54]. Energy harvesting from glucose fuel cells were demonstrated to be capable of generating energy up to a peak power density of 43 µA were recommended for powering implantable drug delivery devices [55, 56]. On the other hand, drug delivery systems made from biodegradable polymers capable of delivery drugs on-demand, by sensing a biomarker from biological environment, are emerging as a promising alternative to conventional reservoir-based drug delivery systems [57]. Biomedical implants such as cardiac stents loaded with drugs were demonstrated to prevent restenosis and achieve long-term patency [58]. Such systems were capable of working battery less and achieve desired goal of delivering precise drugs only over a short duration of time. One of the limitations with conventional implantable drug delivery system is their inability to alter drug release rates as and when required. Such customisability demands signal trans-receiving capabilities to be added to the device and again, size and power restrictions come into play. Considering all these factors into accounts, wireless power transfer to drug delivery devices instead of traditional batteries are explored to a large extend [59].

Inherent to biosensors and monitoring devices, wireless power transfer as well as energy harvesting techniques were vastly researched. Ultrasensitive low-power implantable biosensors capable of monitoring wound and bone fracture healing phases require minimal power for their reliable operation, which could be harvested from biological sources [60, 61]. For instance, recent bio-medical orthopaedic implants were equipped with energy harvesters capable of providing power for monitoring implant health therein. Finite element analysis of such modified hip implant, incorporated with 3-piezoelectric energy harvesters was demonstrated to have a potential to produce 1.76 V and 55 J/s of power [62]. However, in the case of implantable drug delivery and biosensor-based monitoring system, a major constraint on battery design is their size, which could be effectively addressed by wireless power transfers techniques. And this stringent design restriction opens up opportunities for use of nano technology.

Implantable battery–health monitoring

AI-powered battery health monitoring techniques have emerged as a boon for implantable batteries, as they are capable of providing reliable early information about battery replacements. Implantable batteries are broadly classified into two categories say (i) primary batteries that are not rechargeable and have fixed years of services and (ii) secondary rechargeable batteries developed from lithium-ion cells. In addition to ensuring biocompatibility, longevity and sustainability of implantable batteries, means of battery monitoring and management should be ensured [63]. A battery management unit capable of determining the battery state-of-health (SOH) as well as predicting its remaining useful life (RUL) ensures proper battery management. Battery SOH and RUL prediction plays a major role in estimating the probable need for battery replacement and hence, has attracted research interest in recent days [64, 65]. Figure 3 presents the various methods adapted for SOH and RUL prediction. The methods used for battery SOH and RUL prediction is classified as.

Fig. 3 [Images not available. See PDF.]

Methods adapted for SOH and RUL prediction

Model based approaches

Model based approaches that use empirical methods, equivalent circuits in order to estimate battery health.

Adaptive filtering

Extended Kalman filters, adaptive multi-parameter estimators are used for the estimation of battery status parameters.

Data-driven methods

Data-driven methods include machine learning, deep learning estimators, along with statistical estimators like Bayesian processes, Gaussian processes.

A hybrid model utilizing data driven approach and adaptive filtering approach is developed for predicting RUL of implantable batteries [66]. Cycle synchronization along with dynamic time warping is employed with long-short term memory (LSTM) based SOH predictions in order to handle uneven degrading cycles of batteries [67]. Similarly, in another research LSTM with Bayesian optimisation is used for the prediction of SOH and RUL [68]. All these approaches aid in learning the battery health status and predicting RUL of a battery.

Challenges and advances

Major challenges to be addressed while designing batteries for IMDs includes the longevity of the device, size miniaturisation, biocompatibility of the materials used and addressing the safety regulations for commercialisation of the device [69]. All these challenges pose lot of opportunities for research and development. An important limitation to the developments in implantable batteries is inherent to itself i.e., slow advances in battery technologies. However, recent developments in material and nano sciences are addressing this challenge to an extent by discovering new biocompatible electrolytes, electrodes and miniaturising implantable device size. Exploration of different battery technologies with integrated novel nanomaterials, bio-inspired energy harvesting techniques and wireless charging abilities were proposed as alternatives to conventional batteries. In addition, exploration of alternative power sources such as glucose fuel cells also aid in advancing implantable battery technologies. Enhanced longevity demands tuning battery parameters and hence use of novel materials for electrodes and electrolytes were explored for investigating battery performance [70].

In addition to longevity, biocompatibility and safety factors demand the exploration and use of novel biodegradable materials and hence use of materials with enhanced biodegradability are explored [71, 72]. Mg-ion, Zn-ion and Na-ion batteries constitute the bio-degradable group whereas certain Zn-ion, Li-ion, biofuel and other batteries form the non-degradable group of batteries that have the potential to drive future battery technologies [73]. Exclusive and special testing of batteries to study their degrading natures is recommended as a measure to improve safety [74, 75]. Especially wireless charging technologies were recommended to address these challenges effectively. Thus, a detailed discussion of the limitations posed by current battery technologies is made and solutions to challenges with proposals for advancing battery qualities for powering IMDs are highlighted. Summary of the challenges in designing implantable batteries is presented in Table 2. Battery fabricated from novel material is shown in Fig. 4.

Table 2. Challenges to be addressed while designing implantable batteries

Si. no	Ref. no	Research significance	Material used
1	Longevity and Sustainability
	[23]	Flexible and stretchable batteries	Lithium-ion batteries
	[24]	Improving cathode performance	Lithium-ion batteries
	[25]	Block copolymer electrolytes	Rechargeable Li-ion batteries
2	Studying degradation mechanism for safety improvement
	[74]	Degradation mechanism study	Commercial Li-ion batteries
	[75]	Capacity fade analysis	Sulphur cathodes in Li-ion batteries
3	Enhancing biocompatibility
	[21]	Biocompatible and Biodegradable thin-film	Magnesium–air batteries
	[27]	Washable fiber	Zinc batteries
	[30]	Biocompatible symmetric sodium-ion micro batteries	Sodium-ion
	[71]	Biodegradable battery	For bio-energy harvesting devices
	[72]	Programmable electro-cross-linked electrolyte	Biocompatible zinc battery

Fig. 4 [Images not available. See PDF.]

An example of novel material for implantable batteries – Green and programmable Zn-alginate polymer electrolyte (Reproduced from [72]. Under the Creative Commons CC BY license, http://creativecommons.org/licenses/by/4.0/). a Step by step preparation of Zn-Alg-5 electrolyte. b Variation of electro-cross-linking time vs thickness of the polymer c Photographs of Zn electro-cross-linking at different time intervals

Regulatory landscape and safety considerations

Being part of a bio-implantable device, commercialisation of implantable batteries demands stringent regulatory and safety rules to be satisfied [76]. Though such regulations are mandatory for medical applications, in most of the cases they act as hurdles to implementation of a technology. For instance, the safety regulations differ across demographic landscapes and this result in delaying the availability, or in some cases unavailability of a valuable technology to certain parts of the world [77]. Generally, medical equipment and devices are classified into three classes say class I to class III based on their type of usage, complexity and risk factors associated, with the riskiest one occupying class III. Cardiac pacemakers occupy class III as they are implanted invasively. Humans with active implantable medical devices (AIMD) fall under a category of sensitivity risky group and have to be protected against dangers of electromagnetic interferences (EMI) [78] and this necessitates considerations from engineering perspective in order to ensure safety while designing implantable devices [2].

There were concerns regarding security of such implantable devices especially with health monitoring and controlled drug delivery systems, as they are enabled with wireless controlling facilities [79]. As such devices are susceptible to manipulation, which in turn, on a worst case become a victim of targeted attacks leaving the patient in a life-threatening situation [80]. These vulnerabilities showcase the importance of ensuring security of such devices and thus regulatory codes includes aspects related to such security too. Post-market surveillance by using unique device identification systems is recommended to ensure safety and security of implantable devices [81].

Stringent regulatory and safety codes are coined and made mandatory to be satisfied by medical equipment and devices in order to ensure patient safety. These regulations strive to classify devices based on their vulnerability as well as propose standards to be followed while designing medical devices [82]. Recently security considerations were also included in them and post-market surveillances of medical devices are recommended to ensure the same in the case of implantable monitoring devices. Addressing these considerations become necessary for advancing battery technologies. List of literatures detailing the safety recommendations to be followed in designing implantable batteries is presented in Table 3.

Table 3. Literatures related to regulatory and safety codes

Si. no	Ref. no	Regulatory measures	Associated implantable device	Implementation strategy
1	[76] [77]	Recommendations for safe working of implantable devices	All	By testing requirements
2	[78] [2]	Classification of medical equipment	Cardiac AIMD exposure to EMI	By proper knowledge transfer to patients
3	[79] [80] [81] [82]	Recommendations to avoid security threats	Implantable drug delivery and Health monitoring devices	By post-market surveillance and unique device identification

Future directions

Investigating the opportunities and challenges in implantable batteries design gains significant importance due to its inherent role in powering life-saving medical devices. Biomedical implants can be broadly categorised into three say (1) implants used for stimulating internal organs for their proper functioning, (2) implants capable of sensing an internal condition and delivering required drugs and (3) implants for monitoring organ functions. Implants belonging to category (1) and (3) demand considerable power for their functioning and thus novel power sources were the need of the hour. In the case of category (2) implantable devices which includes on-demand drug delivery, flexible and biodegradable nano technology-based battery-less, nano-bio-sensors, are a potential solution. However, these devices are still in infancy and their real-time clinical use and commercialization are highly hindered by regulatory and safety requirements [83].

Non-rechargeable Li-Ion batteries has been the sole player powering bio-medical implantable devices for more than 3 decades. Researches in search of new electrode materials for Li-ion batteries are proposed for enhancing longevity issues [84]. Micro- and nano-structured vanadium pentoxide (V₂O₅) electrodes when introduced as electrodes for Li-ion battery exhibited good electrochemical performance and cycle stability [85]. In addition, using pie-shaped electrodes achieved balanced gravimetric and areal energy densities in lithium sulphur batteries [86]. Moreover, biocompatibility issues are addressed by using biocompatible ionic liquid–biopolymer electrolytes for magnesium-air batteries [87]. However, limitations related to bio-compatibility, longevity of such Li-ion batteries demand search for new batteries technologies. Novel, hybrid energy harvesting and wireless electrical stimulation, power transfer techniques are emerging as potential solutions for powering IMDs [88]. Even wireless power transmission to implantable systems using ultrasound signals were vastly studied [89]. However, a major limitation with such energy harvesters is their meagre output.

Thus, the future scope for implantable battery design rest in the hands of (i) search for novel materials and energy harvesting methods for powering IMDs, (ii) biocompatible nano-bio-sensors for effective drug delivery and (iii) innovative power delivery methods pave the way for reliable and long-lasting implantable devices.

Conclusion

The hunt for reliable and sustainable power sources for biomedical implantable devices is a hot topic of research at present and is going to shape the future for biomedical innovations. Thus, this review discussed the recent advancements in battery technologies specific to the major implantable devices say cardiac implants, neural implants, drug delivery and health monitoring systems followed by the challenges surrounding the advancement of implantable batteries. Meanwhile, biocompatibility and safety related challenges are studied along with the safety considerations posed by regulatory bodies in order to deliver safe and secure medical implants.

Thus, innovations in battery technologies in terms of novel energy harvesting, biocompatible materials and wireless power delivery methods will pave way towards promising horizons. Which in turn, holds great potential in transforming the healthcare industry by providing reliable biomedical devices and advancing patient’s standard of living.

Author Contributions

All authors contributed to the study, conception and design. Data collection and analysis were performed by Priya Lakshmipathy and Hady Habib Fayek. The first draft of the manuscript was written by Umapathi Krishnamoorthy. Review and editing done by Umapathi Krishnamoorthy, Manohar Ramya. All authors read and approved the final manuscript.

Funding

Open access funding provided by Manipal Academy of Higher Education, Manipal. The authors did not receive any funding support from any organization for this work.

Availability of data and materials

No datasets were generated or analysed during the current study.

Declarations

Competing interests

The authors declare that they have no conflicts of interest to report regarding the present study.

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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