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Electroactive Materials for Tissue Engineering

Teresa Marques-Almeida1, Estela O. Carvalho1, Unai Silvan2,3, Senentxu Lanceros-Méndez1,2,3, and Clarisse Ribeiro1

1University of Minho, CF-UM-UP-Physics Centre of Minho and Porto Universities and LaPMET-Laboratory of Physics for Materials and Emergent Technologies, Campus de Gualtar, 4710-057 Braga, Portugal

2BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, 48940 Leioa, Spain

3Basque Foundation for Science, Ikerbasque, 48009 Bilbao, Spain

1.1 Introduction

Tissue engineering (TE) is a branch of regenerative medicine that aims to repair and restore damaged or lost tissues using biological substitutes [1]. TE combines three key factors to seek this ultimate goal: cells, scaffolds, and biochemical and/or biophysical cues [2]. Scaffolds are used to provide a supportive setting that encourages cell growth and matrix synthesis, thereby promoting the formation of new tissue. In turn, biochemical and biophysical cues are employed to create the optimal microenvironment for long-term communications between cells and surrounding tissues/organs. The ultimate objective is to closely mimic the native microenvironment of the impaired tissue [2, 3].

Besides the well-established biochemical cues provided by bioactive factors, electrical signals play a crucial role in cell activity as a biophysical cue. Electrical signals regulate a variety of typical physiological processes, from brain activity to heartbeat [2, 4]. In this context, electroactive biomaterials have emerged as one of the most promising scaffolds for TE application in recent decades. These materials promote conduction of electrical charges and exchange of ions with the surrounding environment. Therefore, the incorporation of electroactive materials into scaffolds for TE can provide a platform for delivering electrical signals to cells, promoting cell proliferation or differentiation, and consequently tissue regeneration [5]. The characteristics of these materials can be modified by adjusting their composition, morphology, and processing conditions to optimize their performance for specific TE applications.

An in-depth understanding of how the cellular microenvironment evolves over time and the pathways that might be used to impose electricity on cells/tissues via electroactive materials is critical for the development of active scaffolds. Additional investigations in this field aim to develop tunable electroactive materials with enhanced biocompatibility and improved electrical properties, opening up new possibilities for TE and regenerative medicine [5, 6].

1.2 Relevance of the Electrical Signals in the Human Body

Human body functions and homeostasis are influenced by physical stimuli [7]. Electromagnetic radiation, temperature, and mechanical forces are external physical stimuli known to present a significant impact on biological events. On the other hand, intrinsic electrical signals and mechanical forces (compressive loading, hydraulic pressure, shear stress, and tensile forces) are the internal physical stimuli that also have been demonstrated to naturally control cell fate by enhancing cell target functions such as adhesion, migration, proliferation, and differentiation [8]. Although all physical stimuli are important and have relevance on the cellular level, it is widely accepted that electrical signals are the most prominent physical stimuli for controlling many physiological processes and may even outperform other physical cues [2, 4, 9].

In the eighteenth century, Luigi Galvani and coworkers demonstrated the presence of an intrinsic form of electricity responsible for nerve conduction and muscle contraction. For the first time, they reported the possibility of causing muscle twitches in freshly killed animals using electric signals and then demonstrated the existence of the injury potential [10]. Since then, many researchers corroborated those findings and clearly defined phenomena such as membrane potentials (Vm) and later action and resting potentials. Currently, it is stablished that Vm are fundamental features of all cells and are caused by the movement of ions across the cell membrane, which results in charge separation and the generation of an electrical potential difference. This occurrence is both a by-product and a regulator of a wide range of essential properties at multiple biological organization levels, intrinsic to the normal function of all cells, organelles, and molecules [9, 11]. For instance, these signals drive processes such as respiration, influence pH levels, and modulate the redox state. Additionally, they facilitate cell-to-cell communication, guide cell migration (in a process known as galvanotaxis), and contribute to tissue repair [12].

Electrical signals enable cells to communicate with each other at the tissue and organ levels, which in turn plays a crucial role in the organ's performance [13]. Various types of tissues exhibit specific electrically conductive and/or electrically responsive properties, such as piezoelectricity and ferroelectricity, and thus several functions are controlled by electrical signals. Figure 1.1 depicts tissues that inherently make use of these signals to regulate different physiological processes [2, 4], such as neural communication [14, 15], bone regeneration [16, 17], heartbeat activities [18, 19], muscle contraction [20, 21], and wound healing [22, 23]. Moreover, compelling evidence indicates that alterations in cellular and tissue excitability, along with more extensive electrical fields across tissues, may have implications for embryo development and tumorigenesis [12, 24].

Figure 1.1 Electric activity in the human body.

All these organs and tissues are known as electroactive tissues given their ability to generate and transmit electrical signals [13]. The nervous system exhibits the highest electrical activity, facilitating the transmission of signals from neurons through synapses to their respective destinations. The pacemaker cells in the heart, a specialized subpopulation of cardiomyocytes, generate rhythmic impulses that propagate through the entire heart and trigger mechanical activity, i.e., heart beating, in ventricular myocytes [6]. Bone exhibits piezoelectric properties, and it generates electrical charges in response to mechanical stress, which ultimately triggers cell growth and repair. Regarding skin, the normal electrical fields are disrupted after an injury, resulting in abnormalities. The Vm is severely disrupted, and a wound electrical field forms, driving cells to the wound for healing purposes, such as epithelial cells in skin injuries [25].

Given the importance of electrical cues in physiological tissue function, disease manifestation and progression, and regeneration, extensive research has been conducted to identify the ideal conditions for electrically stimulating tissues and modulating cell response in vitro, in order to understand the action mechanisms and develop functional therapeutic interventions, especially in the scope of TE. Table 1.1 provides a few examples of the influence of in vitro and in vivo electrical stimulation on different mammalian cell types. Notwithstanding, the given stimulation may vary depending on several factors, such as the strength and frequency of the electric field, its duration, and specific characteristics of the cells being stimulated.

Table 1.1 Overview of in vitro and in vivo electrical stimulation effects on different mammalian cells.

Cell type In vitro effect of electrical stimulation In vivo effect of electric stimulation Refs. Neurons Enhanced neurite outgrowth, improved myelinization, and increased synaptic activity Enhanced axonal sprouting, and improved functional recovery after spinal cord injury [14, 15, 26, 27] Skeletal muscle cells Enhanced myogenic differentiation and myotubes contraction Improved muscle function in mice with muscular atrophy [20, 21, 28, 29] Bone cells Increased osteoblast cell proliferation, suppressed osteoclast recruitment, and enhanced calcification Improved bone regeneration, and increased matrix formation around orthopedic implants [16, 30, 31] Skin cells Increased collagen production, stimulation of proliferation, and differentiation of keratinocytes, fibroblasts, and endothelial cells Enhanced epithelialization and improved wound healing [32] Cardiac muscle cells Enhanced stem cell differentiation, improved contractile function and maturation, and promoted cardiomyocyte alignment and synchronization Promoted angiogenesis, reduced apoptosis, and inflammation in ischemic myocardium [33-35] Cancer cells Reduced proliferation, apoptosis...