New-born infants are subject to several novel experiences that cause physiological, biochemical and behavioural indicators of stress; even routine and common events such as handling, changing a diaper or being bathed can induce a stress response. Tactile interventions such as skin-to-skin touch and 'still containment hold' are widely used in clinical care with apparent positive results, such as lower mean respiratory heart rate and pain measures, and higher oxygen saturation (Boundy et al., 2016; Pineda et al., 2017); yet, dynamic touch interventions have reported to be more beneficial than static touch interventions (Field, Diego, Hernandez-Reif, Deeds, \& Figuereido, 2006; Manzotti et al., 2019; Vickers, Ohlsson, Lacy, \& Horsley, 2004).
A distinct class of unmyelinated C-fibre sensory neurons called C-tactile afferents (CT), found exclusively in the hairy skin of the body, that respond optimally to gentle stroking touch at a velocity of \~3cm/s, are hypothesised to process the pleasant and social rewarding properties of touch (Olausson et al., 2010, McGlone et al., 2014). CT fibre activation also plays a role in pain inhibition (Gursul et al., 2018; Habig et al., 2017; Liljencrantz et al., 2017) and may also be linked to the development of self-regulation (Van Puyvelde, Gorissen, Pattyn, \& McGlone, 2019), thereby serving a neuroprotective function for the developing infant brain.
Touch induced-analgesia such as skin-to-skin contact and breastfeeding are also considered as standard care non-pharmacological interventions for procedural pain in infants (Campbell-Yeo et al., 2019; Chang, Filoteo, \& Nasr, 2020). However, dynamic touch is not currently acknowledged as a non-pharmacological intervention in the UK.
Touch is the first sense to develop in utero at approximately 8-weeks gestational age and is the most mature sense postnatally (Marx \& Nagy, 2015). The benefits of touch are now widely recognised in postnatal care with the World Health Organization (2019) recommending that term infants engage in skin-to-skin contact with their mother or carer for at least an hour after birth due to the physiological benefits for both infant and mother.
There is limited research into the underlying mechanism behind the physiological benefits of touch, either in-utero or post-partum; however, it is hypothesised that foetal movements cause the amniotic fluid to elicit movement of lanugo (fine hair), which activates specific nerve fibres - C-Tactile afferents (CTs) to facilitate growth regulation (Bystrova, 2009).
CTs signal the affective component of touch (Löken, Wessberg, Morrison, McGlone, \& Olausson, 2009; McGlone, Cerritelli, Walker, \& Esteves, 2017). Found exclusively in hairy skin they project to the insular cortex (Olausson et al., 2002), part of the brain's self-regulation system and associated with emotional processing (Dalgleish, 2004). Microneurography, a method employed to visualise and record the traffic of nerve impulses, from CTs find that they respond optimally to low force and velocity dynamic touch delivered at \~ 3cm/sec at skin temperature (Löken et al., 2009), supporting the hypothesis of the social function for the nerve in nurture, grooming and affiliative behaviours.
A recent study carried out with preterm infants in a Neonatal Intensive Care Unit (NICU) found that dynamic touch, delivered at CT optimal speed, reduced heart rate and increased blood oxygenation levels in neonates (Manzotti et al., 2019). Should the hypothesis be accepted in this study it will offer insight into some of the mechanisms behind the physiological advantages of touch, leading to the improvement of neurodevelopmental outcomes for newborns e.g., during painful procedures. However, there are instances where infants are deprived of touch, such as lack of skin integrity and risk of infections, lack of parental presence and disorders such as foetal alcohol syndrome or neonatal abstinence syndrome. This series of studies is aimed at individuals who are deprived of touch - it is not seen as a replacement for maternal/carer touch.
Here we will investigate the tolerability of the device-delivered tactile stimulation at CT-optimal velocity by measuring physiological indicators of stress in infants, as determined by heart rate and blood oxygenation, and assessment of stress behaviours.
